PC6502

My 6502 project in a PC104 like formfactor.
This is VERY VERY unstable, I do not know what works and what doesn't, use at your own risk of sanity and wasted money/chips

Status

2025-09-08 - PCBs ordered, Parts ordered.
2025-09-11 - Edited the firmware based on Grant Searl's code. Created this Repository

CPU Board

This board has a 6502, RAM and ROM on board, along with an ATF1502 for address decoding. There is also a 60-fake-50 (instaled of 6850) which is not my design but emulates the 6850 with USB. The CPLD code and .JED files are in /Software/CPLD

VIA Board

This board has two 6522 VIAs and an ATF1502 for address decoding, The VIA outputs are brought out to PMOD style headers (though are 5V) The CPLD code and .JED files are in /Software/CPLD

Memory Map

The memory map is subject to change in some parts, though I expect the RAM, ROM and Peripheral blocks to stay the same.

High Level

Start	End	Size (Dec)	Size (Hex)	What is it	Notes
0x0000	0x7FFF	32768	0x8000	RAM	This is devided into sub parts below
0x8000	0xBFFF	16384	0x4000	Peripherals	See table below for more info
0xC000	0xFFFF	16384	0x4000	ROM	More info below

RAM

Start	End	Size (Dec)	Size (Hex)	What is it	Notes
0x0000	0x00FF	256	0x0100	Zero Page	Used by the 6502
0x0100	0x02FF	256	0x0100	Stack	Used by 6502
0x0200	0x7FFF	32256	0x7E00	User RAM	un-allocated RAM

ROM

Start	End	Size (Dec)	Size (Hex)	What is it	Notes
0xC000	0xEFFF	12288	0x3000	MS Basic	Basic Interpreter
0xF000	0xFDFF	3584	0x0E00	BIOS	This is where common things are held, like getting input
0xFE00	0xFFF9	506	0x1FA	Monitor	Monitor program
0xFFFA	0xFFFF	6	0x0006	Jump vetors	Vectors for power on, NMI, etc

The OS is based on Grant Searle's 6502 computer work - http://searle.x10host.com/6502/Simple6502.html I've modified this to fit with my memory map, but it's not yet tested. I do plan to add bits and bobs to it as I need (e.g. LOAD, SAVE, DIR).

Peripherals

I expect frequent changes as I add/remove things;

Address	subAddr	RW	What is it	Notes
0xBFFF	n/a	RW	Console Control/Status	60-fake-50, mostly the same as 6850, but no config
0xBFFE	n/a	RW	Console Control/Status	60-fake-50, mostly the same as 6850, but no config
0xBFE-	0 - F	RW	6522 VIA A	Datasheet - https://eater.net/datasheets/w65c22.pdf
0xBFD-	0 - F	RW	6522 VIA A	Datasheet - https://eater.net/datasheets/w65c22.pdf

The Future

I have a bunch of things I want to try adding, some ideas I have are below, some may happen, some may not and they're not in any particular order;

• Power Board: LAmost certainly will be USB-C PD to 5V, I'm aware that some of the things I want to add can be quite heavy on power consumption, so this seems like a sensible way to future proof it to an extent.
• RA8875 based display & keyboard interface: something that can display text and simple graphics and let my type without needing my laptop. I'll probably buy an off the shelf display with this on and memory map it as a peripheral somewhere and it will almost certainly be PS/2 keyboard.
• RTC & ASCII Display: Maybe an RTC and an ascii display (I have a couple of QDSP2118 displays somewhere that I'd like to use)
• A Sound Engine: I'm thinking of my own simple Wavetable based sound engine, possibly with analogue VCF/VCAs. Some kind of tracker software or even MIDI interface to go with it?
• Compact Flash / SD Storage: I'm going to want something in the long run as retyping code is going to get tedious quickly. I have found a 6502 SPI interface here, so that may speed things up - https://sbc.rictor.org/65spi2.html
• Some kind of HDMI grpahics adapter: My current thinking is the videobeast (https://feertech.com/microbeast/videobeast.html) but that needs a big hole in RAM.
• Transputer Link Adapter/Card: because I still want to use these, but I think having them "speed up" computation on a 6502 is going to have a bigger impact than a 1GHz PC.