A 6502 Home Computer Rebuilt in Logic

Abstract

I have been building an 8-bit home computer around the MOS 6502, and this paper reports where it stands. It is not a clone of any particular machine. The aim is a usable and extensible platform that still runs the existing body of 6502 software. There are two implementations, kept in step: an emulator written in C99, and synthesizable VHDL that runs on real FPGA hardware. They agree on one memory map and use the same ROM images, which supports running EhBASIC and tested 6502 programs on both targets. Full instruction, peripheral, and timing equivalence is not established by a shared ROM alone. On the FPGA the peripheral chips are written in RTL rather than dropped in as opaque IP: a 6502 CPU core (T65), a C64-style video controller with text and bitmap modes, a SID-compatible (MOS 6581) sound chip, 6522/6551 serial and timer I/O, a CIA-1 timer, and a disk controller that reads FAT32 and D64 images in hardware. VIC-II colour and raster registers at their C64 addresses, together with a short build tool, let it play a wide range of original .sid tunes from their own player code, interrupt- and raster-timed players included. The machine boots from SD card and draws coprocessor-accelerated Mandelbrot images over HDMI on a Sipeed Tang Primer 20K; the same core also runs on a Xilinx Spartan-6 board.