- 6502 CPU driven by an Arduino Mega
- Introduction
- Pin Mappings
- Disassembling on the fly
- Credits
- Datasheets
- References
This project is about experimenting with a hardcore 6502 processor. Concretely, I'm using a WDC w65c02s8, bought from Ali Express. It was made in 1999... so it's 21 years old, but it seems working.
The processor is driven by an Arduino Mega, which provides:
- a clock signal
- a ROM
- a RAM
The ROM is embedded into the Arduino sketch, as rom.h file. It is made by assembling the source code using CC65 suite and converted to a C array using xxd.
The Arduino Mega also serves as a logic analyser, and dumps the Address bus, Data bus, and some control signals (RWB, SYNC) to its serial port.
In addition, a program is provided to further analyse the serial transmission on the computer attached to the Arduino Mega. At the moment, this disassembles the running instructions.
This diagrams shows the pin mapping between the Arduino Mega pins and the 6502 pins.
The disassemble/serialHost.py Python scripts reads the serial output from the Arduino Mega, and tries to disassemble it on the fly.
It will build the instructions byte per byte, as they are coming.
The program being run in this example is the following loop:
LF000: lda $2000 ; F000 AD 00 20
sta $2000 ; F003 8D 00 20
jmp LF000 ; F006 4C 00 F0
At the time of running this:
- when the 6502 is reading from $2000, the Mega will send the byte value of Pin Port F (Register PINF)
- when writing to $2000, the Mega will write to Pin Port K (Register PORTK)
So the program is continuously copying PINF into PORTK of the Arduino Mega, in other works, it's reading the logic level of the 8 pins of port F, and setting the logic level of the 8 pins of port K to be the same.
Let's see the output of serialHost.py in this case. The first column (before the | is the output from Arduino Mega). After the | is the decoding.
EF85 00 r 1 | ef85 00 brk SNGL
EF85 00 r 0 |
01CA 00 W 0 |
01C9 00 W 0 |
01C8 00 W 0 |
FFFC 00 r 0 |
FFFD F0 r 0 |
F000 AD r 1 | f000 AD .. .. lda $.... ABS
F001 00 r 0 | f000 AD 00 .. lda $..00 ABS
F002 20 r 0 | f000 AD 00 20 lda $2000 ABS
2000 80 r 0 |
F003 8D r 1 | f003 8D .. .. sta $.... ABS
F004 00 r 0 | f003 8D 00 .. sta $..00 ABS
F005 20 r 0 | f003 8D 00 20 sta $2000 ABS
2000 20 W 0 |
F006 4C r 1 | f006 4C .. .. jmp $.... ABS
F007 00 r 0 | f006 4C 00 .. jmp $..00 ABS
F008 F0 r 0 | f006 4C 00 F0 jmp $F000 ABS
F000 AD r 1 | f000 AD .. .. lda $.... ABS
F001 00 r 0 | f000 AD 00 .. lda $..00 ABS
F002 20 r 0 | f000 AD 00 20 lda $2000 ABS
2000 80 r 0 |
F003 8D r 1 | f003 8D .. .. sta $.... ABS
F004 00 r 0 | f003 8D 00 .. sta $..00 ABS
F005 20 r 0 | f003 8D 00 20 sta $2000 ABS
2000 20 W 0 |
F006 4C r 1 | f006 4C .. .. jmp $.... ABS
F007 00 r 0 | f006 4C 00 .. jmp $..00 ABS
F008 F0 r 0 | f006 4C 00 F0 jmp $F000 ABS
Notice how bytes are replaced by ".." when the instruction is incomplete, and they are filling in progressively to decode the complete instruction.
Here are some awesome projects. From some of them I have taken inspiration, from others I have copied lines of code.
- Build a 6502 computer | Ben Eater: This is definitely what started all this.
- Retroshield for Arduino Mega | 8 Bit Force
- My disassembler "logic" started inspired by skilldrick's disassembler in Easy 6502
- I've learned a lot by looking at olivierjan's 6502's routines in his ROM-software-for-6502-SBC