Awesome

======================================================== A Verilog HDL version of the old MOS 6502 and 65C02 CPUs

Original 6502 core by Arlet Ottens

65C02 extensions by David Banks and Ed Spittles

========== 6502 Core

Arlet's original 6502 core (cpu.v) is unchanged.

Note: the 6502/65C02 cores assumes a synchronous memory. This means that valid data (DI) is expected on the cycle after valid address. This allows direct connection to (Xilinx) block RAMs. When using asynchronous memory, I suggest registering the address/control lines for glitchless output signals.

Also check out my new 65C02 project

Have fun.

========== 65C02 Core

A second core (cpu_65c02.v) has been added, based on Arlet's 6502 core, with additional 65C02 instructions and addressing modes:

• PHX, PHY, PLX, PLY
• BRA
• INC A, DEC A
• (zp) addressing mode
• STZ
• BIT zpx, absx, imm
• TSB/TRB
• JMP (,X)
• NOPs (optional)
• 65C02 BCD N/Z flags (optional, disabled)

The Rockwell/WDC specific instructions (RMB/SMB/BBR/BBS/WAI/STP) are not currently implemented

The 65C02 core passes the Dormann 6502 test suite, and also passes the Dormann 65C02 test suite if the optional support for NOPs and 65C02 BCD flags is enabled.

It has been tested as a BBC Micro "Matchbox" 65C02 Co Processor, in a XC6SLX9-2 FPGA, running at 80MHz using 64KB of internel block RAM. It just meets timing at 80MHz in this environment. It successfully runs BBC Basic IV and Tube Elite.

============ Known Issues

The Matchbox Co Processor needed one wait state (via RDY) to be added to each ROM access (only needed early in the boot process, as eventually everything runs from RAM). The DIHOLD logic did not work correctly with a single wait state, and so has been commented out.

I now believe the correct fix is actually just:

always @(posedge clk ) if( RDY ) DIHOLD <= DI;

assign DIMUX = ~RDY ? DIHOLD : DI;