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Fake65c02, an enhanced fork of fake6502

Ben Eater's videos on using the 6502 to build a breadboard computer left me aching to play with a 6502 of my own. This project is being developed so I can work on 6502 software without building my own breadboard computer, although I'm still planning on doing that down the line.

This project includes an application to run 6502 code with a simple IO framework for writing to serial, halting the CPU, and (todo) handle interrupts. See the roms subdirectory for various small assembly files. Of particular note is roms/tests/test65c02.s, which tests each added CMOS instruction and prints pass/fail messages to the designated serial port memory address in roms/lib.s. Note that you must have an up-to-date version of vasm6502_oldstyle, as older versions do not support include ../ directives. I had to compile it from source for my roms to build on FreeBSD.

Licensing

fake65c02.c and fake65c02.h are released and maintained in the public domain to respect Mike Chambers' work and contribution to the public domain.

Everything else is licensed under the MIT license.

Notes on usage

In order to provide a method of pairing a fake65c02_t instance with your own data structure, a void* m field is included in the struct. It is unused by fake65c02.c and is reserved solely for use as a pointer to your data structure. An example of its usage can be found in main.c as a method of determining how to access each fake65c02_t's ram/rom banks from a static and shared memory access functions. Feel free to give it a NULL value if you will not be using it in your read and write functions.

Differences from fake6502.c

  • Added CMOS instructions (see below)
  • Refactored API to use a fake65c02_t struct as context, ala Lua
  • Added a header file for ease of use, although the API is not very stable at the moment.

Attribution

Mike Chamber's fake6502 provides everything I needed to get up and running with 6502 emulation. His work allowed me to get this project (and dependent projects) up and running far faster than if I had had to write my own emulator from scratch.

Differences between NMOS 6502 and CMOS 65c02 provided a nice list of changes for me to implement. The instructions are listed in a table below for ease of access when working on my fake65c02.c

Instructions added in the CMOS 65c02

Instruction OP Description
PHX $DA push X onto the hardware stack, without disturbing A.
PLX $FA pull X off the hardware stack, without disturbing A.
PHY $5A push Y onto the hardware stack, without disturbing A.
PLY $7A pull Y off the hardware stack, without disturbing A.
STZ abs $9C At the 16 bit addr indicated by the operand, store zero.
STZ abs,X $9E At the 16 bit addr indicated by the operand plus X, store zero.
STZ ZP $64 At the ZP addr indicated by the operand, store zero.
STZ ZP,X $74 At the ZP addr indicated by the operand plus X, store zero.
BIT ZP,X $34 (new addressing mode for the BIT instruction)
BIT abs,X $3C (new addressing mode for the BIT instruction)
JMP (abs,X) $7C (new addressing mode for the JMP instruction)
BRA rel $80 Branch Relative Always (unconditionally), range -128 to +127
TRB addr $1C Test & Reset memory Bits with A.
TRB ZP $14 Test & Reset memory Bits with A.
TSB addr $0C Test & Set memory Bits with A.
TSB ZP $04 Test & Set memory Bits with A.

Instructions added in the WDC and Rockwell variants of the 65c02

Instruction OP Description
BBR ZP $0F-$7F Branch if specified Bit is Reset.
BBS ZP $8F-$FF Branch if specified Bit is Set.
RMB ZP $07-$77 Reset specified Memory Bit.
SMB ZP $87-$F7 Set specified Memory Bit.

Instructions added in the WDC variant of the 65c02

Instruction OP Description
STP $DB SToP the processor until the next RST.
WAI $CB WAIt. It's like STP, but any interrupt will resume execution.

Instructions Implemented

CMOS

  • PHX
  • PLX
  • PHY
  • PLY
  • STZ abs
  • STZ ZP
  • STZ abs,X
  • STZ ZP,X
  • BIT ZP,X
  • BIT abs,X
  • JMP (abs,X)
  • BRA
  • TRB addr
  • TRB ZP
  • TSB addr
  • TSB ZP

WDC and Rockwell

  • BBR ZP
  • BBS ZP
  • RMB ZP
  • SMB ZP

WDC

  • STP
  • WAI