Building an emulator for my 65C816 computer

I’ve been working on adding text-based input and output to my 65C816 computer prototype, but it’s not yet working reliably.

To unblock software development, I decided to put together an emulator, so that I can test my code while the hardware is out of action.

Choosing a base project

There were three main options I considered for running 65C816 programs on a modern computer,

  1. Write a new emulator from scratch
  2. Extract the useful parts of an open source Super Nintendo or Apple IIgs emulator and adapt them
  3. Find a 65C816 CPU emulator, and extend it.

I found a good candidate for the third option in Lib65816 by Francesco Rigoni, which is a C++ library for emulating a 65C816 CPU. The code is GPLv3 licensed, and I could see where I would need to make changes to match the design of my computer.


After reading the code, I could see some places where I would need to modify the library, so I copied it in to a new folder, together with its logging dependency and sample program, and refactored it into one project.

I started by extending RAM up to 16 banks (1 megabyte), and mapping in a ROM device which serves bytes from a file. I also updated the handling of interrupt vectors, so that the CPU would retrieve them from the system bus.

Adding 65C22 support

My first test program was an example of preemptive multi-tasking from my previous blog post. At this point it ran, but did not switch between tasks, since that requires extra hardware support.

I implemented a minimal support for the 65C22 VIA used in my computer design, just enough to log changes to the outputs, and to fire interrupts from a timer.

The Lib65816 library does not support NMI interrupts, which are used in this test program. I added edge detection, and connected the interrupt output of the 65C22 to the CPU NMI input in code.

This screen capture shows the test program during switching between two tasks, with a different VIA port being accessed before and after the switch.

This gave me some confidence that the CPU emulation was good enough to develop on, so I moved on to the next test program.

Adding serial support

The second test program is also from an earlier blog post, and tests printing and reading characters from an NXP SC16C752 UART.

I added a very minimal emulation of the UART chip I’m using, suppressed all of the logging, and converted output to use the curses library. This last step enabled character-by-character I/O, instead of the default line-buffered I/O.

This screen capture shows the program running under emulation. It simply prints “Hello world”, then echoes back the next 3 characters that the user types.


I didn’t plan to write an emulator for my custom computer, but I am currently debugging some tricky hardware problems, and this detour will allow me to continue to make progress in other areas while I try to find a solution.

I plan to keep the emulator up to date with hardware changes, since it will make it possible to demonstrate the system without any hardware access. It’s also easier to debug issues now that I can choose two different execution environments.

The hardware design, software and emulator for this computer are online at mike42/65816-computer. I would like to again acknowledge Francesco Rigoni’s work on Lib65816. I’m very grateful that he chose to release this under the GPL, since it allows me to re-use the code for my project.

Let’s implement preemptive multitasking

I am currently working on building a computer based on the 65C816 processor. This processor has a 16-bit stack pointer, which should make it far more capable than the earlier 65C02, at least in theory.

I wanted to check my understanding of how the stack works on this processor, so I tried to build the simplest possible implementation of preemptive multitasking in 65C816 assembly.

What I’m working with

I am working with a breadboard computer. It has no operating system, and no ability to load programs or start/stop processes.

To get code running, I am assembling it on a modern computer, then writing the resulting binary to an EEPROM chip.

To see the result of the code, I am using a logic analyser to watch some output pins on the 65C22 VIA. The 65C22 chip also has a timer, so I connected its interrupt output to the NMI input of the CPU for this test.

Test programs

I wrote two simple programs which infinitely loop. These programs use ca65 assembler syntax.

The 65C22 has two 8-bit ports, so I made each test program write a recognisable pattern to its own output port, so that it’s easy to see which one is running at any given time, and check that it is not disrupted by the task switching.

The first program writes to Port A of the 65C22, and alternates the value between 0000 0000, and 0000 0011.

.segment "CODE"
; Task 1
  .a8                             ; use 8-bit accumulator and index registers
  sep #%00110000
  lda #%00000000                  ; alternate between two values
  sta PORTA
  lda #%00000011
  sta PORTA
  jmp @repeat_1                   ; repeat forever

The second program writes to Port B of the 65C22. This uses a ror instruction to produce a pattern across all digits of the output port.

.segment "CODE"
; Task 2
  .a8                             ; use 8-bit accumulator and index registers
  sep #%00110000
  lda #%01010101                  ; grab a start value
  ror                             ; rotate right
  sta PORTB
  jmp @repeat_2                   ; repeat forever

Context switching

I’m implementing round-robin scheduling between two processes.

My basic plan was to use a regular interrupt routine to save context, switch the stack pointer to the other process, restore context for the next process, then return from the interrupt.

.segment "CODE"
  rep #%00110000
  ; Save task context to stack
  pha                             ; Push A, X, Y
  phb                             ; Push data bank, direct register

  ; swap stack pointer so that we will restore the other task
  tsc                             ; transfer current stack pointer to memory
  sta temp
  lda next_task_sp                ; load next stack pointer from memory
  lda temp                        ; previous task is up next
  sta next_task_sp

  ; Clear interrut
  sep #%00110000                  ; save X only (assumes it is 8 bits..)
  ldx T1C_L                       ; Clear the interrupt, side-effect of reading

  rep #%00110000
  ; Restore process context from stack, reverse order
  pld                             ; Pull direct register, data bank
  ply                             ; Pull Y, X, A

Setup process

The above switch works while the two processes are up and running, that takes some setting up. Firstly, the two variables need to be referenced somewhere.

.segment "BSS"
next_task_sp: .res 2              ; Stack pointer of whichever task is not currently running
temp: .res 2

The boot-up routine then sets everything up. It’s broken up a bit here. First is the CPU initialisation, since the 65C816 will boot to emulation mode.

.segment "CODE"
  clc                             ; switch to native mode

The second step is to start a task in the background. This involves pushing appropriate values to the stack, so that when we context switch, “Task 2” will start executing from the top.

  ; Save context as if we are at task_2_main, so we can switch to it later.
  .a16                            ; use 16-bit accumulator and index registers
  rep #%00110000
  lda #$3000                      ; set up stack, direct page at $3000
  ; emulate what is pushed to the stack before NMI is called: program bank, program counter, processor status register
  phk                             ; program bank register, same as current code, will be 0 here.
  pea task_2_main                 ; 16 bit program counter for the start of this task
  php                             ; processor status register
  ; match what we push to the stack in the nmi routine
  lda #0
  pha                             ; Push A, X, Y
  phb                             ; Push data bank, direct register
  tsc                             ; save stack pointer to next_task_sp
  sta next_task_sp

  lda #$2000                      ; set up stack, direct page at $2000 for task_1_main

The nex step is to set up a timer, so that the interrupt routine will fire regularly. This causes interrupts to occur ~28 times per second.

  ; Set up the interrupt timer
  .a8                             ; use 8-bit accumulator and index registers
  sep #%00110000
  lda #%11111111                  ; set all VIA pins to output
  sta DDRA
  sta DDRB
  ; set up timer 1
  lda #%01000000                  ; set ACR. first two bits = 01 is continuous interrupts for T1
  sta ACR
  lda #%11000000                  ; enable VIA interrupt for T1
  sta IER
  ; set up a timer at ~65535 clock pulses.
  lda #$ff                        ; set T1 low-order counter
  sta T1C_L
  lda #$ff                        ; set T1 high-order counter
  sta T1C_H

Finally, “Task 1” can be started in the foreground.

  ; start running task 1
  jmp task_1_main

Linker configuration and boilerplate

This is the first time I’m blogging about code written in 65C816 native mode, so for the sake of completeness, I’ll also include the updated linker configuration. The most important change (compared with this blog post) to that 32 bytes are now set aside for interrupt vectors.

    ZP:     start = $00,    size = $0100, type = rw, file = "";
    RAM:    start = $0100,  size = $7e00, type = rw, file = "";
    PRG:    start = $e000,  size = $2000, type = ro, file = %O, fill = yes, fillval = $00;

    ZEROPAGE: load = ZP,  type = zp;
    BSS:      load = RAM, type = bss;
    CODE:     load = PRG, type = ro,  start = $e000;
    VECTORS:  load = PRG, type = ro,  start = $ffe0;

Note that this linker configuration is not quite complete, but it is good enough to get code running. The zero page is not relevant for the code I’ll be writing, so I might remove that at some point.

I also have definitions for the 65C22 I/O registers, which correspond to the location that it is mapped into RAM on my prototype computer.

PORTB = $c000
PORTA = $c001
DDRB = $c002
DDRA = $c003
T1C_L = $c004
T1C_H = $c005
ACR = $c00b
IFR = $c00b
IER = $c00e

The final piece of the puzzle then, are definitions for all those interrupt vectors.

.segment "CODE"

unused_interrupt:                 ; Probably make this into a crash.

.segment "VECTORS"
; native mode interrupt vectors
.word unused_interrupt            ; Reserved
.word unused_interrupt            ; Reserved
.word unused_interrupt            ; COP
.word unused_interrupt            ; BRK
.word unused_interrupt            ; Abort
.word nmi                         ; NMI
.word unused_interrupt            ; Reserved
.word irq                         ; IRQ

; emulation mode interrupt vectors
.word unused_interrupt            ; Reserved
.word unused_interrupt            ; Reserved
.word unused_interrupt            ; COP
.word unused_interrupt            ; Reserved
.word unused_interrupt            ; Abort
.word unused_interrupt            ; NMI
.word reset                       ; Reset
.word unused_interrupt            ; IRQ/BRK

The process for building this assembly code into a usable ROM are:

ca65 --cpu 65816 main.s
ld65 -o rom.bin -C system.cfg main.o

This outputs an 8KiB file. I need to pad this with zeroes up to 32KiB, which is the size of the ROM chip I am burning it to.

truncate -s 32768 rom.bin

Lastly, I write this to the ROM.

minipro -p AT28C256 -w rom.bin


I connected a logic analyser to two wires from each output port, the NMI interrupt line, and the reset signal, and opened up sigrok.

This clearly shows the CPU switching between the two processes each time an interrupt is triggered.

Zooming in on the task cut-over, I can also see that the patterns on each port are different, as expected.

Of course this did not work the first time, and I spent quite a bit of time de-constructing and re-constructing the code to isolate bugs.

As result, I shipped some improvements to my 6502 assembly plugin for JetBrains IDE’s, which I have blogged about previously. The plugin will now provide suggestions for mnemonics. There is a project setting for 6502 vs 65C02 vs 65C816 mode, and it will only suggest mnemonics which are available for the CPU.

It also provides a weak warning for binary or hex numbers which are not 8, 16 or 24 bits, which would have saved me some time.

Lastly, it has optional checking for undefined/unused values. This helps catch problems in the editor, and allows me to identify unused code and variables.


I already knew how multitasking works on a high-level, but it was quite interesting to implement it myself. I have been using this as a test program for my 65C816 computer, since it exercises RAM, ROM, I/O, and uses interrupts.

I don’t think I will be able to use this exact method for more complex programs, because it will break down a bit with multiple interrupt sources. After writing this code, I also found that it’s not common for modern systems to save register values to the stack when context-switching, and that a process control block is used instead.

The next step for my 65C816 computer will be the addition of an NXP UART, which I tested in an earlier blog post.

Interfacing an NXP UART to an 8-bit computer

I recently tested the SC16C752 UART from NXP with my 6502 computer, and I was able to use it to receive and send characters. This blog post shows the simple test program and circuit that I used.

Some background

I built a 65C02-based computer last year, which uses a serial connection for text I/O. I used the WDC 65C51N to provide a UART interface, and I found it to be quite limited.

I am now building a 16-bit computer, and I want to avoid the 65C51N, for two main reasons:

  • It runs at 5v, and I’m planning to convert the design to 3.3v later on. The other 65-series chips I’m using support a wide voltage range.
  • It can only store one byte at a time. I am planning to run at a higher baud rate, and the system will have multiple interrupt sources. It would take very careful programming to avoid losing bytes with a 65C51N in this setup.

I spent some time investigating alternative UART IC’s, and found some discussion on about replacing the 6551 with an NXP industrial (SC26 or SC28 series) UART. At the time of writing, many of the hobbyist-friendly PLCC versions of these chips are listed as end of life, though I’ve since found pin and software-compatible versions of this made by MaxLinear (eg. the XR88C92).

For my uses though, I chose the NXP SC16C752. It has two channels, 64-byte receive and transmit buffers, and supports a wide voltage range. I don’t mind that it’s a different series to these standard chips, because I plan to write software from scratch. The only downside is the packaging, which is LQFP48 with 0.5mm pin pitch. I needed to solder this tiny chip to an adapter board in order to use it.

The test circuit

I wired this up to my 6502 computer project as shown in the diagram below. IO3 is a spare address decoding output, which maps the chip to address $8c00, while the rest of the signals connect to the 6502 CPU directly.

I used a 74AC00 and 74AC04 to generate the active-high reset and separate read/write signals (as suggested on the same thread linked before). I plan to use a PLD to generate a qualified write signals on my 65C816 computer build.

Note that there are separate chip-selects for each UART, and I’ve only connected one here. I have also not connected the interrupt output for this test, and have some unused inputs which are left floating.

Developing a test program

I started writing some code, and connected a logic analyser to the UART output. The early tests did not work well, and I now know that this is because I had not enabled the FIFOs, causing characters to be lost.

This screen capture shows an attempt to send the entire alphabet to the UART, but only the letters “A”, “B” and “M” appearing at the output.

I came up with this test program, which does 8-N-1 communication at 115,200 bits per second.

; Test program for the SC16C752B dual UART from NXP
.import sys_exit

; Assuming /CSA is connected to IO3
UART_BASE = $8c00

; UART registers
; Not included: FIFO ready register, Xon1 Xon2 words.
UART_RHR = UART_BASE        ; Receiver Holding Register (RHR) - R
UART_THR = UART_BASE        ; Transmit Holding Register (THR) - W
UART_IER = UART_BASE + 1    ; Interrupt Enable Register (IER) - R/W
UART_IIR = UART_BASE + 2    ; Interrupt Identification Register (IIR) - R
UART_FCR = UART_BASE + 2    ; FIFO Control Register (FCR) - W
UART_LCR = UART_BASE + 3    ; Line Control Register (LCR) - R/W
UART_MCR = UART_BASE + 4    ; Modem Control Register (MCR) - R/W
UART_LSR = UART_BASE + 5    ; Line Status Register (LSR) - R
UART_MSR = UART_BASE + 6    ; Modem Status Register (MSR) - R
UART_SPR = UART_BASE + 7    ; Scratchpad Register (SPR) - R/W
; Different meaning when LCR is logic 1
UART_DLL = UART_BASE        ; Divisor latch LSB - R/W
UART_DLM = UART_BASE + 1    ; Divisor latch MSB - R/W
; Different meaning when LCR is %1011 1111 ($bh).
UART_EFR = UART_BASE + 2    ; Enhanced Feature Register (EFR) - R/W
UART_TCR = UART_BASE + 6    ; Transmission Control Register (TCR) - R/W
UART_TLR = UART_BASE + 7    ; Trigger Level Register (TLR) - R/W

.segment "CODE"
  lda #$80                  ; Enable divisor latches
  sta UART_LCR
  lda #1                    ; Set divisior to 1 - on a 1.8432 MHZ XTAL1, this gets 115200bps.
  sta UART_DLL
  lda #0
  sta UART_DLM
  lda #%00010111            ; Sets up 8-n-1
  sta UART_LCR
  lda #%00001111            ; Enable FIFO, set DMA mode 1
  sta UART_FCR
  jsr test_print
  jsr test_recv
  lda #0
  jmp sys_exit

test_print:                 ; Send test string to UART
  ldx #0
  lda test_string, X
  beq @test_done
  sta UART_THR
  jmp @test_char

test_string: .asciiz "Hello, world!"

test_recv:                  ; Receive three characters and echo them back
  jsr uart_recv_char
  sta UART_THR
  jsr uart_recv_char
  sta UART_THR
  jsr uart_recv_char
  sta UART_THR

uart_recv_char:             ; Receive next char to A register
  lda UART_LSR
  and #%00000001
  cmp #%00000001
  bne uart_recv_char
  lda UART_RHR

The program sends the text “Hello, world!”, then waits for input. The next three characters it receives are sent back to the user, and then the program terminates. The only thing in this test program which is specific to my 6502 computer is sys_exit, which a routine to transfer control back to the operating system.

It was very rewarding to see the fast, error-free text output on the logic analyser for the first time. This is a great improvement from the 65C51N.

I was also surprised at how short the delay is to echo back a character. It will be interesting to see how much the latency increases once there is some more complex software handling the input.

Wrap up and next steps

I plan to use the NXP SC16C752 for my 65C816 computer project, which will be controlled via text-based input and output.

I’m testing it on its own first, so that I can refer to a known-good setup if it doesn’t work. This is enough to get started, though there are some features which I wasn’t able to test yet, since I can’t use custom interrupt service routines on this setup.

65C816 computer – initial prototype

I am currently working on building a 65C816-based computer from scratch. I have some ideas for how I want to do this, but my first task is to put together a working prototype. I’ll be extending this in different ways to work towards the goals outlined in my last blog post.

This is about the simplest computer I can come up with which uses the 65C816 processor, but there is still quite a lot to go though.


All of the integrated circuits on this prototype are through-hole packages, suitable for use on a breadboard.

  • WDC W65C816S CPU
  • WDC W65C22S VIA I/O chip.
  • Alliance AS6C4008-55PCN 512k x 8 55ns SRAM
  • Atmel AT28C256 32k x 8 EEPROM
  • Atmel ATF22LV10 PLD for address decoding
  • 74AC04, 74AC245, 74AC573 for handling the multiplexed data bus and bank address bits
  • 1.8432 MHz oscillator
  • DS1813-5+ supervisory circuit

I also used the following equipment to program the chips, and view the result of the test program:

  • TL-866II+ EEPROM programmer
  • 8-channel Cypress FX2-based logic analyser

I’m working on Linux, and also use the following software tools:

  • ca65 – for assembling code for the CPU
  • minipro – for programming the EEPROM and PLD.
  • galette – for assembling the logic definitions for the PLD.
  • sigrok – for working with the logic analyser

Bank address latching

The 65C816 has a 24-bit address bus, and the top 8 bits are multiplexed onto the data bus. This is not a big problem, but it does mean that I need some extra glue logic compared to a plain 6502 system. A diagram in the 65C816 data sheet shows a suggestion for how to do this, and I’m starting with that implementation.

I used a 74AC04 to invert the clock, 74AC245 bus transceiver for the data bus, and a 74AC573 latch for the bank address (upper 8 bits of the address bus). This first page also shows the DS1813-5+ supervisory circuit on the reset line, which resets all of the components on power-up.

I’m aware that I should be generating a two-phase clock (there is some discussion about that here), though I can’t see any problems arising from the the delay on the inverted clock on this particular circuit.

Memory map and address decoding

The memory map for this computer is very simple. All of this will change, but I did need to start with something to get some code running.

Only the lower 19 bits of the address space are properly decoded, and mapped to one of three chips.

Address Maps to
010000 – 07FFFF RAM – 448 KiB
00E000 – 00FFFF ROM – 8 KiB
00C000 – 00DFFF I/O – 8 KiB
000000 – 00BFFF RAM – 48 KiB

I used a programmable logic device (PLD) to implement this. It’s easy enough to do this with discrete gates, but a PLD will make it much easier to keep chip count and propagation delays under control as I add more features. I’ve written about programming PLD’s in previous blog posts (here, here and here), and I plan to use them to implement several features.

The particular part I am programming here is an ATF22LV10. These are still produced the time of writing, and operate at either 3.3 volts or 5 volts, which is useful for this project. They can be also programmed with the common TL-866II+ chip programmer, which I also use for programming EEPROMs.

The definitions for decoding this are below. I called this file address_decode.pld.


Clock  RW     NC     NC     A19    A18    A17    A16    A15    A14    A13    GND
NC     /RAMCS /ROMCS /IOCS  /RD    /WR    NC     NC     NC     NC     NC     VCC

; ROM is the top 8 KiB of bank 0.
; prefix is 0000 111xxxxx xxxxxxxx
ROMCS = /A19*/A18*/A17*/A16*A15*A14*A13

; IO addresses are mapped below ROM in bank 0.
; prefix is 0000 110xxxxx xxxxxxxx
IOCS = /A19*/A18*/A17*/A16*A15*A14*/A13

; RAM is selected for any address does *not* match 0000 11xxxxxx xxxxxxxx
RAMCS = A19 + A18 + A17 + A16 + /A15 + /A14

; Qualify writes with clock
WR = Clock*/RW


Address decode logic for 65C816 computer.

ROM is top 8K of bank 0, I/O is 8k below that, everything else is RAM.

I assembled this with galette:

galette address_decode.pld

This generates a .jed file, which can be used to program the chip. The ATF22LV10 is not on the compatibility list for my programmer, but it works just fine if I identify it as an ATF22V10CQZ.

minipro -p ATF22V10CQZ -w address_decode.jed

Firmware and testing

I came up with a simple test program which would require working ROM, RAM, and I/O. The CPU boots to 6502 emulation mode, and this test program does not access memory outside bank 0.

I created a linker configuration file which mostly reflects the system’s memory map, and called it system.cfg.

    ZP:     start = $00,    size = $0100, type = rw, file = "";
    RAM:    start = $0100,  size = $7e00, type = rw, file = "";
    PRG:    start = $e000,  size = $2000, type = ro, file = %O, fill = yes, fillval = $00;

    ZEROPAGE: load = ZP,  type = zp;
    BSS:      load = RAM, type = bss;
    CODE:     load = PRG, type = ro,  start = $e000;
    VECTORS:  load = PRG, type = ro,  start = $fffa;

I then wrote this test program, main.s. The program runs from ROM, and writes a hard-coded value to one of the output ports. It then stores a 0 in RAM, then increments it and stores that to an output port in an infinite loop, which will show a counting pattern if RAM can be read and written.

PORTB = $c000
DDRB = $c002


.segment "CODE"

; Write 42 to VIA port B
  lda #%11111111 ; Set all pins to output
  sta DDRB

  lda #42
  sta PORTB

; Start at 0, and increment a value in RAM
  lda #00

  sta PORTB         ; Output the value
  jmp loop



.segment "VECTORS"
.word nmi
.word reset
.word irq

I assembled this with ca65, then linked it with ld65. As far as the assembler knows, it is generating code for a 6502 CPU, not a 65C816. I am not using any advanced features, so this should be fine for this test.

ca65 main.s
ld65 -o rom.bin -C system.cfg main.o

This outputs an 8 KiB binary file, which is the amount of ROM space mapped in the computer’s memory. The chip is actually 32 KiB though, so I filled the rest with zeroes.

truncate -s 32768 rom.bin

I then wrote the program to the EEPROM using minipro.

minipro -p AT28C256 -w rom.bin

I ran this program, with a logic analyser connected to port B of the 65C22 VIA. The capture below shows the initial hard-coded number, followed by the counting pattern, which indicates that everything is working as expected.


I’m starting simple here, so that I have a working base system to modify. Everything seems to be working fine on breadboards, so I’ll proceed with that for now. I may still need to re-build everything with shorter wires, since this is getting quite difficult to work with.

It is also worth noting that most of the components here should run at 3.3 volts, which is where this project is eventually headed. Only the clock, reset IC and EEPROM would need to be substituted at the moment.

This blog post is only a snapshot of the project. You can check the current status on GitHub here.

A first look at the 65C816 processor

The 65C816 is an interesting processor from the 1980’s. I recently wired one up on a breadboard, and I’ll be blogging here about my attempts to build something useful with it.

How I got here

I spent a couple of months last year designing and building a computer based on the 65C02, an old 8 bit processor. I wanted to create a hardware platform for running 6502 assembly programs. This was successful, and I learned a lot about how computers work while building and programming it.

This has got me interested in the 65C816, which is a later extension of the same architecture. The only well-known systems which used it were the Super Nintendo and Apple IIGS. From a programmer’s point of view, this chip seems far more capable than its pre-decessor, though developer tools are a bit scarce.

New project goals

My plan is to build a modern computer which uses the 65C816 CPU, so that I can really learn how it works.

Over a few revisions, I am aiming to build up to a system with a serial connection for simple text I/O, a modest clock speed, and 1 megabyte of static RAM.

I’ve researched some existing designs, and settled on two simple constraints to give this project its own character.

  1. Use only in-production parts.
  2. Don’t use an FPGA or microcontroller to bootstrap the system.

This will ensure that I am learning what it takes to build a computer around this processor, and not just offloading the tricky parts to more powerful device with better tooling. I also hope this increases the accessibility of the design, since anybody could build it without needing to obtain obscure retro parts.

I am not avoiding programmable logic entirely though. Where classic designs often used custom chips, I will be using ATF22V10 PLD’s. I will also leave the door open adding video output via a microcontroller-based terminal emulator in the future.

Lastly, I’m selecting parts with a view to converting everything from 5 volts to 3.3 volts part-way through the project, which will open up many possibilities.

Other than the Apple IIGS and Super Nintendo, there are three hobbyist designs which I’m using for inspiration:

A quick note about open source

It’s never been a stated goal of this blog, but wherever possible, I use open source tools, and produce open source software. I expect that to be difficult for this project. The 65C816 not widely used, and most hobbyist code for the CPU is licensed as source-available freeware, with restrictions on commercial use.

I would like to be able to release my code under an OSI-approved open source license, and have the option of incorporating copyleft code later on. Unfortunately for me, it seems I will need to write a lot of low-level code from scratch to get this kind of licensing certainty. I will be working primarily in assembly language, though C code would be very useful, so I will certainly be spending some time investigating compiler options along the way.

A smoke test

This is the first iteration of the design, which will live for now on a series of solderless breadboards.

This is simply a 65C816 processor, being fed a hard-coded NOP (no-operation) command. When it runs, the lights show the address bus counting upwards, as the CPU program counter increases.

The RDY, ABORTB, and BE inputs are connected to +5v through resistors, while RESB, NMIB, IRQB are connected to +5v directly. The NOP opcode is fed to the CPU over the data bus, by making the pattern 11101010 (0xEA) with 1k resistors. The schematic for this is below.

The clock is an LM555 timer, fed through a 74AC04 inverter, because the rise and fall times on a 555 are not fast enough. The clock speed is 6.9Hz (that is not a typo), since R1 is 10 kΩ, and R2 is 100 kΩ, and the capacitor is 1 µF.

Next steps

This is going to be a long project, which I will develop incrementally. The next step will be adding ROM, RAM, glue logic and a 65C22 I/O chip.

I am also testing alternatives to the 65C51N UART chip, which I found quite limited when I used it for my last project.

Rendering my 6502 computer project in Blender

For the past couple of months, I’ve been working on building a 6502-based computer from scratch.

In a previous blog post, I wrote about designing a 3D-printed enclosure for this project in Blender.

During this build, I needed to wait for some parts, so I had some time to see what I could do with the 3D models of the case and circuit board. I ended up animating the assembly process, and learned a couple of tricks along the way.

Splitting components and PCB

I already had a 3D model of the circuit board and components. This was initially exported from KiCad, in STEP format. I then used FreeCAD to convert it from STEP to STL, which I imported into Blender for designing the case. I also repeated this export with some different footprints, since I needed sockets and microchips.

It is a challenge to make this look good, because the circuit board and all components come in as one object, and have lost all of the detail from the KiCad 3D view.

I spent a lot of time splitting objects, then selecting faces so that I could assign different materials. I’m using a free BlenderKit account for materials, and using procedural materials where possible.

I also modeled the DC barrel jack and slide switches, which were the first of many components which were missing from the original KiCad export. Still, something was missing.

Adding missing details

I thought that the PCB in the previous image was too bare. The real PCB uses plated through-holes, and almost every hole in a real PCB has a shiny solder pad around it.

To fix this, I exported the front solder-mask layer from KiCad as an SVG, which looks something like this:

I imported this SVG into Blender, scaled it to fit the PCB, extruded it, then went through a series of boolean modifiers. This resulted in a new object which contained every part of the PCB which was not covered in solder mask. I assigned this a shiny metal material.

After this was applied, I subtracted this from the original PCB model, giving the parts of the PCB which are covered in solder mask.

Showing both of these at the same time, I have a PCB with solder pads and plated through-holes.

Importing the silkscreen

This still was not right. On the real PCB, everything is labelled.

I exported the front silkscreen layer from KiCad as an SVG. This layer looks something like this:

I then used InkScape to change this to white text on a transparent background, before finally exporting it as a PNG. I mixed this into the procedural texture which I was using for the PCB, using the transparency as the mix factor. As a final touch, I also exported the front copper layer, and used this as the displacement input for the final material. The nodes are shown in the screen capture below.

This allows me to render a PCB which looks a lot like the real thing.

UV unwrapping everything

In the earlier render, all of the chips had clear surfaces, while real chips are covered with text and logos.

I created small transparent/brown PNG images for each chip, using Inkscape, then UV unwrapped the chips. I mixed the logo into the texture, much the same as I did for the PCB.

I also UV unwrapped each resistor, in order to paint on the correct colour codes. This was perhaps the most egregious waste of time in this whole thing.

Adding final parts

There were plenty more parts which I couldn’t export from KiCad, and needed to model from scratch. I used the bolt factory add-on to create screws, nuts, and hex standoffs (both a screw and a nut, combined).

I also added power and reset buttons (which are partly threaded), plus the speaker. I decided not to add wires, since it would be beyond my skill at the moment.

When everything is displayed at once, a lot of this detail is hidden behind other objects.


With that last render, I decided to see if I could make an animation of all of the parts coming together. I’ve never animated anything in Blender, so I had a lot to learn.

I added a tracking constraint to the camera, which is panning around an invisible object on the table, and animated everything using key-framed movement. Parts are raised out of view with slightly randomised rotation/location before falling into place.


I exported the animation from Blender as 360 separate 1920 x 1080 images, to be played back at 30 frames per second. I would normally use ffmpeg to convert an image sequence into a video, but I wanted to try adding an audio track.

I imported the rendered animation into Kdenlive, which can process image sequences (the button is “Add clip” / “Import as sequence”), fetched a track from the Free Music Archive (Scanglobe – Current. CC BY-NC), and rendered the project.


I wanted to get a textured, 3D model of this entire project so that I could easily illustrate documentation and project pages. I definitely achieved this, though working with the exported circuit board and components in Blender was tedious.

The version of Blender used here is 2.93, and the files were exported from KiCad 5.1.10.

Assembling my 6502 computer

I recently finished assembling my 6502 computer, so that the whole project is housed in a 3D-printed case.

This is something that should be easy. I already blogged about making the case, and making the circuit board. All that is left is to put the board in the case, right? Well thanks to some mistakes on my part, things are not quite so simple.

More parts

I needed quite a few parts to finish this project, including a power button, reset button, hex standoffs, short screws, and rubber feet. Since this is the first time I’m hitting this stage of a project, I also needed to buy a Du Pont connector kit, stranded wire, heat-shrink tube, and glue.

I have been powering the project from a USB/serial adapter, so I also bought a new one to install permanently. A full parts list is available in the GitHub repository for this project.

I also needed to widen the hole for the reset button due to a mix-up. The specific part I used requires a 7mm hole, while I had designed the case for a button with a 6mm hole.

Making connections

The first step was to modify the board so that it would be compatible with the new UART, which involved de-soldering the female pin headers which I installed previously, and replacing them with male pin headers. I used only a soldering iron, solder wick, and flux for de-soldering, and it was not a fun process. The board survived, so I’ll call it a success.

Next, I soldered wires to the underside of the board for the power button, reset button, and speaker to connect. Each of these is a pair of wires, soldered at one end, and a 2-pin female Du Pont connector on the other.

I attached corresponding connectors on the buttons and speaker. The power button also has an in-built LED, and there are plenty of places to power this from the board’s expansion headers. I added a 2-pin female Du Pont connector, plus a resistor, neatly hidden by heat-shrink.

I checked everything with a multimeter before powering it on. The UART pins were connected correctly to the 65C51N chip and ground. No continuity between the board’s power and the external power unless the power button is pressed, no continuity between the reset line and ground unless the reset button is pressed, and so on.


When I powered on the computer, nothing came up on the terminal, which means that I broke something. I tested some voltages, and found the computer was stuck in reset.

The reset signal on the 6502 is active-low, and I’ve added a supervisory IC. Any time the reset signal is low, or the power drops below a minimum voltage, this IC holds it low for 150ms before returning it to +5v. I was confident that the reset buttons and power voltage were fine, so I suspected that one one of the other chips could be causing the problem.

Everything is socketed, so I removed the 65C51N and tested again. This chip is closest to the de-soldering work I was doing earlier, so I thought I may have damaged it.

There was no change, so I started removing other chips, and found that after removing the CPU, the computer was no longer getting stuck in reset.

Following that lead, I found that one of the data bus lines, D4, was connected to the reset line. This would cause activity on the data bus to reset the computer, which would cause this problem.

A quick look at the board in KiCad showed that at one of the points I had attached the reset button, D4 is 0.5mm away from the reset line.

I de-soldered that wire and found a tiny break in the solder mask on D4. This was easy enough to fix, I simply attached the wire for the external reset button elsewhere.

I also bent the CPU pins when re-installing, but after fixing that, I was back in business.

SD card changes

I have been writing some test programs which use a microSD card via one of the 65C22 I/O ports, and wanted to fit this in the case as well.

I swapped the Adafruit ADA254 microSD break-out for the SparkFun DEV-13743. It is slightly smaller, includes pull-up resistors, and still provides level-shifting.

I made a small adapter by attaching a female pin header to a Du Pont housing with superglue. I couldn’t get the “card detect” feature to work, so I wired it up differently to my previous blog post, and updated the code to match. Details of the new wiring is on GitHub.

I also added some masking tape when I installed this to prevent any short circuits. The metal on the outside of the microSD socket is connected to ground, and it could touch some IC pins.

Quick lessons

This is only a learning project for me, so I am trying to take some lessons away from each part of it.

The high-level lessons would be to plan for external connections before building the PCB, and to have all parts on hand before designing the case. That would have saved a lot of re-work, though adapting a design without starting from scratch is a valuable skill in itself.

If I were starting from scratch, I would add pin headers to the board for connecting the power and reset buttons instead of including buttons on the board, and remove the (now unused) DC barrel jack as a power input. I would also make the board larger, and the case taller, so that everything is less cramped. It takes a lot of force to remove chips from machined pin sockets, and the board is too compact to get a screwdriver under the chips.

SD card support is also a bit of a hack, and could be built-in to the board with some effort.

Final results

I now have a fully self-contained 8 bit computer, and it looks more like a finished project than an in-progress experiment. It boots to BASIC, or with the flip of a switch, launches a ROM which allows me to load machine code assembled on my PC.

With the lid removed, I can access the 40-pin expansion header, and one spare I/O port.

The hardware side of this project is now done, and I have a platform for testing any 6502 assembly code on real hardware.

Adding an SD card reader to my 6502 computer

I recently attempted to add SD card support to my home-built 6502 computer.

This was successful, but more challenging than I expected. This blog post is just a few things that I learned along the way.


SD cards are a popular way to add storage to electronics projects. In my case, I will be using the SD card in SPI mode. My home-built computer has an expansion port with some GPIO pins, which should be sufficient for this.

My basic plan was to:

  • write some test data to an SD card from a modern computer
  • interface the SD card to my home-built 6502 computer
  • write a 6502 assembly program to print out the test data

Since this is a learning project, I started from scratch, and avoided reading any existing implementations of SPI or SD cards before jumping in.

Test data

I started this project by generating a small test file, repeating each character in the alphabet 512 times.

echo abcdefghijklmnopqrstuvwxyz | fold -w1 | while read line; do yes $line | head -n512 ; done | tr -d "\n" > disk.img

It takes a one-line shell command to write this image to an SD card on all good operating systems, or a few clicks in the disk imaging utility.

dd if=disk.img of=/dev/my_sd_card

My laptop has an SD card slot, but I also have Windows on it at the moment, which turned this into a far larger problem than I could have imagined.

Driver support for this particular SD card reader was dropped in Windows 10, there are no built-in tools for imaging disks on Windows, and I had to try multiple tools before I could find one which would write a raw disk image (one which does not contain a valid filesystem).

In retrospect, this was the first of many lessons about not having the right tools for this project.

Physical interface

I connected an SD card module to my 6502 computer, via a breadboard. I used a module which could level-shift, because my computer runs at 5V, where SD cards usually run at 3.3V. This particular module is an Adafruit ADA254.

I also added pull-up resistors to all of the data lines, after noticing that there were several sitations where they would float. The wiring I used is:

The full schematic for my 6502 computer is on this blog post, while the schematic for the SD card module is online here.

Software: Part 1

With some test data and a hardware interface, I got to the third step, which was to read data from the SD card. I was completely unprepared for how tricky it is to do this from scratch, without the right experience or debugging tools.

As a quick re-cap, my development environment is still very limited: I am writing 6502 assembly on a modern computer, and I have the ability to send the assembled program to my home-built 6502 computer over a serial connection.

I started by using lecture notes from the University at Buffalo’s EE-379 course, available online here. This describes SPI, SD card command structure, and the sequence to initialise a card.

The two protocols I needed to implement are SPI, which transfers the bits, plus the SD card commands which run over that. I had to write quite a lot of code with no feedback, because it takes several commands before SD cards are ready to work with in SPI mode, and the exact details vary depending on the generation of card (SDHC vs SDXC, etc).

After days of debugging, I was unable to get a response from the SD card. With few other options, I started filling my code with the assembly-language equivalent of print statements, and found that every byte from the SD card was the default FF. This means ‘no data’ in this context.

Print statements also slowed the program down to a crawl, which allowed me to use a pizeo buzzer to listen to each line. This clicks each time a signal changes between 0 and 1, so I was able to check that each line had the expected activity. I found that the SD card was sending responses, but that the computer was not receiving them, because they were not connected to eachother on the breadboard.

Software: Part 2

After connecting the card correctly, I could see responses for the first time. The format here is byte sent / byte received, with line breaks between commands.

SD card detected
40/7f 00/ff 00/ff 00/ff 00/ff 95/ff ff/ff ff/01 ff/ff
48/7f 00/ff 00/ff 01/ff aa/ff 0f/ff ff/ff ff/09 ff/ff ff/ff ff/ff ff/ff
7a/7f 00/ff 00/ff 00/ff 00/ff 75/ff ff/ff ff/01 ff/00 ff/ff ff/80 ff/00 ff/ff ff/ff

The correct initialisation sequence depends on the type of SD card, and I spent a lot of time going through each possibility, looking for one one which worked. When this failed, I found this summary online, which includes more information about decoding the responses: Secure Digital (SD) Card Spec and Info.

As it turned out, I was not sending the correct CRC for CMD8, and a CRC error is indicated by the 09 (binary 0000 1001) response to the second command above. Each bit in the response has a different meaning, and I had mistaken this for indicating an unsupported command, which is expected for some card types.

It’s a good thing that I had this problem, because searching for the CRC algorithm turned up this collection of AVR C tutorials, which includes a 4-part series on implementing SD card support.

I noticed that all of the command examples were wrapped with code like this:

    // assert chip select
    // ... code to send a command...
    // deassert chip select

Later on, I had a problem where responses were sometimes mis-aligned by one bit. When I also tried sending empty bytes between commands, while the SD card is not selected, these problems disappeared entirely.

Software: Part 3

With the initialisation code complete, I could finally request data from the SD card.

Data on an SD card is arranged in 512-byte blocks by default. I wrote a routine to read one block of data from the SD card into RAM, plus a routine to print sections of RAM.

The result was a program which prints the first kilobyte of the SD card, which is the letter a 512 times, followed by the letter b 512 times.

This screen capture shows the output of a program matching a hexdump of the test file, which is exactly what I was aiming for.

The full code is too long to include in a blog post, but can be found in the repository for this project, mike42/6502-computer on GitHub.

Some lessons

While I was able to read data from an SD card, this took a long time, and there were several points where I nearly hit a dead-end.

I couldn’t see what was happening at a hardware level, and I couldn’t compare my setup with one that worked, because I had never done anything similar before. Spot-the-difference is a powerful debugging tool, and by jumping straight into 6502 assembly, while avoiding existing 6502 implementations, I definitely made things difficult for myself.

With that in mind, there are several ways I could have made this easier:

  • Better equipment. A logic analyser, an SD card writer, and spare SD cards would have been useful for testing.
  • Building somewhere less-constrained first. I could have implemented this first on a Raspberry Pi, then ported the result to 6502 assembly. It would then be possible to verify my hardware setup by running existing code, while still getting the learning experience from writing my implementation from scratch. Note that this would have required a different SD card break-out board.
  • Building something simpler first. For example, Ben Eater released a a YouTube video around the time I was working on this, where he reads from an SPI temperature sensor on a 6502 computer. It would be much easier to get started if I had used any other SPI device on a 6502 first.

The next step in this project is to load programs from an SD card, which would require me to implement a filesystem. This is significantly more complex than what I’ve done here, and I know not to approach it the same way.

More equipment

I picked up a logic analyser, USB SD card reader, alternative SD card module, and a spare SD card.

The logic analyser is compatible with sigrok, which is open source. This is the first time I’ve used this software, and it can decode SD card commands running over SPI, among other things.

The USB SD card reader will allow me to write disk images directly from my development environment, instead of copying them over to a laptop. I do most of my development from a virtual machine, and USB peripherals are easy to pass though.

The alternative SD card module is from SparkFun. Compared with the module I was writing in this blog post, it has a smaller footprint, and includes the pull-up resistors. I’m hoping I can use this to fit an SD card into the 3D-printed enclosure for this project.

Implementing the XMODEM protocol for file transfer

I recently built a 6502-based computer from scratch, and I’m using it as a platform for testing 6502 assembly code. The software on this computer is currently very limited, and re-programming an EEPROM chip to test changes is a slow process.

This blog post covers changes which I made to implement the XMODEM protocol, allowing me to upload programs over a serial connection.

The protocol itself

I chose XMODEM because it’s the simplest protocol which is compatible with modern terminal software. All I need right now is the ability to place some data (machine code) into RAM, so that I can execute it.

An important note that is that modern file transfer protocols would normally run over TCP/IP, while XMODEM runs over a serial connection. This works for me, since it’s the only interface I’ve got.

For what it’s worth, XMODEM is also era-appropriate technology for this retro computer, since it was first implemented in 1977. The 6502 processor came out in 1975.


Implementing XMODEM was the easy part of this process. There are existing 6502 assembly implementations which I could have used, but I decided to work from the description on the Wikipedia article instead. This is partly to make sure I understand it, and partly to keep the software licensing of my ROM as clear as possible.

I have the beginnings of a shell for running commands from ROM, so I implemented two new commands:

  • rx, to receive a file over XMODEM and load it into memory.
  • run to jump to the program

To start the transfer, I found it useful to write a method for reading a character with a time-out. I already have routines for reading and writing characters from serial.

; Like acia_recv_char, but terminates after a short time if nothing is received
  ldy #$ff
  ldx #$ff
  lda ACIA_STATUS              ; check ACIA status in inner loop
  and #$08                     ; mask rx buffer status flag
  bne @rx_got_char
  cpx #0
  bne @x_loop
  cpy #0
  bne @y_loop
  clc                          ; no byte received in time
  lda ACIA_RX                  ; get byte from ACIA data port
  sec                          ; set carry bit

The entire receive process is around 60 lines of assembly code. The process involves ASCII NAK character repeatedly until a SOH character is received, which indicates the start of a 128 byte packet. All padding around each packet is discarded, and the payload is written to memory. This process is repeated until the end of transmission (EOT) is received.

; Built-in command: rx
; recives a file over XMODEM protocol, and loads it into memory
USER_PROGRAM_START = $0400   ; Address for start of user programs
USER_PROGRAM_WRITE_PTR = $00 ; ZP address for writing user program

  ; Set pointers
  lda #<USER_PROGRAM_START   ; Low byte first
  lda #>USER_PROGRAM_START   ; High byte next
  ; Delay so that we can set up file send
  lda #1                     ; wait ~1 second
  jsr shell_rx_sleep_seconds
  ; NAK, ACK once
  lda #$15                   ; NAK gets started
  jsr acia_print_char
  lda SPEAKER                ; Click each time we send a NAK or ACK
  jsr shell_rx_receive_with_timeout  ; Check in loop w/ timeout
  bcc @shell_block_nak       ; Not received yet
  cmp #$01                   ; If we do have char, should be SOH
  bne @shell_rx_fail         ; Terminate transfer if we don't get SOH
  ; Receive one block
  jsr acia_recv_char         ; Block number
  jsr acia_recv_char         ; Inverse block number
  ldy #0                     ; Start at char 0
  jsr acia_recv_char
  cpy #128
  bne @shell_rx_char
  jsr acia_recv_char         ; Checksum - TODO verify this and jump to shell_block_nak to repeat if not mathing
  lda #$06                   ; ACK the packet
  jsr acia_print_char
  lda SPEAKER                ; Click each time we send a NAK or ACK
  jsr acia_recv_char
  cmp #$04                   ; EOT char, no more blocks
  beq @shell_rx_done
  cmp #$01                   ; SOH char, next block on the way
  bne @shell_block_nak       ; Anything else fail transfer
  lda USER_PROGRAM_WRITE_PTR ; This next part moves write pointer along by 128 bytes
  cmp #$00
  beq @block_half_advance
  lda #$00                   ; If low byte != 0, set to 0 and inc high byte
  jmp @shell_rx_block
@block_half_advance:         ; If low byte = 0, set it to 128
  lda #$80
  jmp @shell_rx_block
  lda #$6                    ; ACK the EOT as well.
  jsr acia_print_char
  lda SPEAKER                ; Click each time we send a NAK or ACK
  lda #1                     ; wait a moment (printing does not work otherwise..)
  jsr shell_rx_sleep_seconds
  jsr shell_rx_print_user_program
  jsr shell_newline
  lda #0
  jmp sys_exit
  lda #1
  jmp sys_exit

This code is the first time that I’ve used the “Zero Page Indirect Indexed with Y” address mode. It uses a pointer to the program’s location, which is incremented 128 each time a packet is accepted.

The index in the current packet is stored in the Y register, allowing each incoming byte to be written to memory with one operation, which is called in a tight loop.


The other routines mentioned in this source listing are:

  • shell_rx_sleep_seconds – sleeps for the number of seconds in the A register.
  • shell_rx_print_user_program – prints the first 256 bytes of the program for verification.

The checksum is entirely ignored. Everything here is running on my desk, so I’m not too worried about transmission errors.

After a few iterations, I was able to upload placeholder text to RAM.

The screen capture shows 256 bytes of memory (2 packets in XMODEM). The ASCII SUB character (1a in hex) fills out unused bytes at the end, since everything sent over XMODEM needs to be a multiple of 128 bytes.

Assembling some programs

I thought I was almost done at this point, but it turns out that writing a “Hello World” test program to run from RAM is easier said than done.

I started by creating a new ld65 linker configuration which links code to run from address $0400 onwards, which is where programs are loaded into RAM. This system does not have a loader for re-locatable code, and absolute addresses are assigned by the linker.

This is the configuration which I used for these tests. It has some flaws, but it’s good enough to place the CODE segment in the right place.

# Test configuration for user programs
    ZP:     start = $00,    size = $0100, type = rw, file = "";
    RAM:    start = $0100,  size = $0300, type = rw, file = "";
    MAIN:   start = $0400,  size = $1000, type = rw, file = %O;
    ROM:    start = $C000,  size = $4000, type = ro, file = "";

    ZEROPAGE: load = ZP,  type = zp;
    BSS:      load = RAM, type = bss;
    CODE:     load = MAIN, type = ro;
    VECTORS:  load = ROM, type = ro,  start = $FFFA, optional=yes;

If I wanted my test program to terminate, then I needed to find the address of sys_exit in ROM. This routine hands control back to the shell after a command completes.

To get this, I re-assembled the ROM, but added the flag --debug-info to the ca65 assembler, and --dbgfile boot.dbg to the ld65 linker. This produces a text file, which contains the final address of each symbol.

sym id=45,name="sys_exit",addrsize=absolute,scope=0,def=28,ref=298+192+426+390+157,val=0xC11E,seg=0,type=lab

The simplest test program I could come up with was:

; location in ROM
sys_exit = $c11e

.segment "CODE"
  jmp sys_exit

I was able to assemble this program to 3 bytes of binary and run it. It does absolutely nothing, and simply returns to the shell.

To extend this into a “Hello World” program, I needed to look up the location of two more routines in the ROM.

; locations of some functions in ROM
acia_print_char = $c020
shell_newline   = $c113
sys_exit        = $c11e

.segment "CODE"
  ldx #0
  lda test_string, X
  beq @test_done
  jsr acia_print_char
  jmp @test_char
  jsr shell_newline
  lda #0
  jmp sys_exit

test_string: .asciiz "Test program"

This animation shows the process of uploading the binary and executing it via minicom.

There is nothing special about the invocation for minicom here, it’s just a bit-rate and device name, which in my case is:

minicom -b 19200 -D /dev/ttyUSB0

Automating the boring stuff

This approach is very fragile, because the code is full of hard-coded memory addresses.

Any time I change the ROM, the addresses for these routines could change, and this program will no longer work.

On any real architecture, applications call into the OS via system calls, or maybe a jump table for 8-bit systems. I don’t a stable binary interface, and there is no userspace/kernel distinction, so I decided not to go down that path.

Instead, I wrote a Python script to generate an assembly source file, using the same debug symbols which I was manually reading. This runs after each ROM build.

The format is:

.export VIA_T2C_H                        = $8009
.export VIA_T2C_L                        = $8008
.export acia_delay                       := $c02b
.export acia_print_char                  := $c014

When this file is assembled, it doesn’t generate any code, but it does define all of the constants and labels from the ROM. When applications link against this, they can .import anything which would be available to ROM-based code.

The start of the above script is re-written as:

.import acia_print_char, shell_newline, sys_exit

This means that I have source compatibility, just not binary compatibility.

Bringing everything together

With my initial goal achieved, I was still not happy with the development experience. I started to dig into the murky world of minicom scripting to see if I could automate the process of uploading/running programs, and found that it is indeed possible.

The scripting capabilities of minicom do not appear to be widely used, and I needed to read through the source code to find ways to work around the problems I encountered.

For example, the “send” command was sending characters too quickly for the target machine. There are hard-coded delays between commands in the interpreter, but you can’t use multiple “send” commands to slow things down, because it is also hard-coded to append a line ending each time.

The fix was to use !<, which sends the output of a command verbatim. This script uses echo -n command to send each character individually. This is the exact same procedure as the animation in the previous section. First, the script types “rx”, then sends the file, then types “run”.

# confirm we have a shell prompt
send ""
sleep 0.1
# put into receive mode
expect "# "
sleep 0.1
!< echo -n 'r'
!< echo -n 'x'
send ""
sleep 0.1
# send the program and wait for prompt
! sx -q $TEST_PROGRAM 2> /dev/null
expect "# "
# run program
sleep 0.1
!< echo -n 'r'
!< echo -n 'u'
!< echo -n 'n'
send ""
sleep 0.1
# Terminate minicom when program completes (bit crazy..)
expect "# "
sleep 0.1
! killall -9 minicom

It’s also worth talking about the end of this script. When minicom scripts end, the user is returned to an interactive session. I wanted the minicom process to terminate without clearing the screen, so I run killall -9 minicom as the last line of the script.

This is not the safest thing to include in a script, but it’s certainly effective, and I am quite certain that there is no built-in way to achieve this with the current version of minicom. Realistic alternatives include patching minicom, switching to different terminal software.

The minicom invocation includes the name of the minicom script to execute, and the script will use the TEST_PROGRAM environment variable to name the binary file to run.

TEST_PROGRAM=test.bin minicom -b 19200 -S minicom_autorun.txt -D /dev/ttyUSB0

When running this command, the program executes on the 6502 computer seamlessly, though minicom leaves the terminal in a broken state when it is killed, and bash prints the “Killed” text to the screen.

To work around this, I wrapped the minicom invocation in a shell script which builds the program, suppresses all errors, and always returns 0. This avoids constant reporting that killall -9 is a problem, but it does hide any other problems.

set -eu -o pipefail
make $1.bin
(TEST_PROGRAM=$1.bin minicom -b 19200 -S minicom_autorun.txt -D /dev/ttyUSB0 || true) 2> /dev/null

This script is invoked as:

./ test

A few months back, I wrote an IntelliJ plugin for 6502 assembly, and I’m writing all of my code in PyCharm. The last step was to get this running from my IDE.

I added a build configuration which invokes the script. The script is completely generic, and I can use it to run other programs by changing the argument.

I can now write a program, click run, and watch it execute on real hardware, all without leaving my IDE. It took a lot of effort, but this is a huge improvement from where I started.


A fast compile/test/run cycle is a huge gain for developer productivity on any system. I can now get fast, accurate feedback on whether my 6502 assembly programs work on real hardware. The first program I’m writing involves interfacing with an SD card, which will be a lot more approachable with this improvement.

I was surprised at how tricky it was to assemble and link a small program to run in RAM instead of ROM, and I can see that my linker configuration will need to be improved as I write programs which use more data. My basic plan is to test features by uploading them over XMODEM, then add them to the ROM when they are working.

The full source code for this project, which includes hardware and software, can be found on GitHub at mike42/6502-computer.

Designing a 3D printed enclosure for my KiCad project in Blender

I recently built a small 6502-based computer on a custom PCB, which I designed in KiCad. This blog post is about the process of building a 3D-printed case to house this project, using Blender.

This is my first ever 3D print, so I had a few things to learn.

Quick background

My effort on this project has been focused on making a design which works, then transferring that design to a PCB. Since this is my first electronics project, I’m working sequentially, and have completely ignored the need for a case until now.

I could have saved some time if I positioned the ports and mounting holes to match pre-made electronics enclosures, but I’ll take that as a lesson for my next project.

The requirements for the enclosure are simple:

  • Ability to mount the PCB inside the case
  • Cut-out for power input
  • Cut-outs for cable routing to expansion ports
  • Cut-outs for power and reset buttons

My PCB design files are in KiCad, and I decided to design the case in Blender, since I already know how to use it. Blender is not a CAD tool, but it is excellent for general-purpose 3D work, and will do just fine for this task.

Getting scale

My first challenge was to get the PCB into Blender at the correct scale, so that I could build a model around it. This is mostly just juggling file formats, but also note that Blender works in metres by default. I’ve set it to millimetres, with a scale of 0.001. There are guides on the web about how to set this.

I tried two different methods.

Method 1: Image import

In KiCad, I added two “dimensions” to show the distance between the centre of the mounting holes, then plotted the front copper layer as an SVG.

I used Inkscape to convert from SVG to PNG. I then used GIMP to add cross-hairs to the centre of the mounting holes.

In Blender, I added a single vertex at the origin, then extruded it on the X axis to match the measured dimension. I then added a reference image (the PNG file exported above), and scaled/moved the reference image until the cross-hairs lined up with the vertex.

Method 2: Model import

KiCad can export its 3D model to a STEP file. I imported this file into FreeCAD, then exported it to STL.

Lastly, I imported the STL into Blender.

The scale is correct with both of these methods, because I can align the image and model.

The second method produces much larger .blend files, but has less room to make undetected errors, so I’ll be using it in future.

Building a box

I modeled out the case as a lid and base with 3mm walls, and used boolean modifiers to cut out spaces for buttons and cables.

One challenge was rounding the corners without the two pieces intersecting. I built the box as a square, then beveled it in wire-frame view. The spaces are larger than necessary, and I will try using modifiers slot the pieces together with a fixed tolerance if I need to do this again.

The result was two pieces which sit together but do not attach, with rounded corners and no sharp overhangs, for ease of manufacturing.

Manufacturing and test

I exported the STL files, and sent them to a local manufacturer, since I don’t have my own 3D printer. The print is black PLA, printed with fused deposition modeling, with an 0.2mm layer height and 30% infill.

When I received the parts, I first checked that the two pieces fitted together, then placed a blank PCB over the base to check that the holes lined up. PLA shrinks as it cools, but this aligned perfectly, which is either good luck or correct calibration.

The computer’s board will be installed on standoffs, which are secured by a nut on the other side. I like the idea of using heat-set inserts instead, but I don’t want to try too many new things at once, so I’ve skipped the idea for now.

Next steps

At the time of writing, I’m still waiting for some parts to assemble this computer with a power/reset button, which will be the end of the hardware side of this project.

I’m quite happy with how this turned out as my first ever 3D print. If I need to make enclosures for future projects, I’ll take another look at open source parametric CAD tools such as OpenSCAD and FreeCAD, which are more targeted to this kind of work.

The STL files for this case are available on GitHub at mike42/6502-computer, critique is welcome.