Retro-emulation

Hats off to friend Andy for pointing to a special issue on programmable logic in the December 2009 Circuit Cellar (online articles are kept a month behind, so you’ll have to wait for the goods). The most intriguing article, ‘Retrocomputing on an FPGA’ by Stephen A. Edwards, follows a pattern we discussed in tMarch 2009 in this blog – the emulation of obsolete electronics in modern FPGAs (the earlier case involved emulation of a General Instruments controller for playing Pong).

Just as nostalgia for 50s rock and roll gives way to nostalgia for 80s synth-pop, the obsession with imagined 19^th-century “steam-punk” history has given way to a desire to bring back the archaic video games and desktop platforms of the 1970s. Douglas has reached for a simple but compelling architecture of the time, the Apple II+ based on a Mostek 6502 (Having problems remembering a pre-Mac Apple history? Go talk to grandpa, young’un).

In theory, the Apple II+’s 48 Kbytes of system memory could almost fit within the confines of the Altera Cyclone II chosen to emulate the architecture, but Douglas elected instead to use off-chip SRAM for memory. The SRAM was chosen for ease of interface compared to a DRAM, though this design had to use multiple data streams to emulate the Apple tri-state bus.

The Apple II+ used a technique for video display almost forgotten in the 21^st century: in the era before dedicated computer monitors, a composite-color NTSC signal was generated for display on a standard TV set. Douglas had to create a VGA line-doubling circuit to allow the emulated Apple II+ to be used with standard monitors. Similarly, Douglas had to emulate the Disk II controller, used with some of the earliest floppy disks for consumers, and allow disk images to be used with SD cards.

This project represents but the first step in an Apple II+ development which could include joystick and peripheral cards, neither of which Douglas emulated. The emulated system consumes just a fraction of the power of the 1970s computer – 5W for an FPGA vs. 22W for the Apple II+ itself.

Why bother emulating such archaic systems of yore? You might as well ask a hobbyist why one would want to study games and fashions of the medieval era. The retro-emulation movement is bound to blossom to bring back the applications from PC AT, Nintendo, Sega, and early Mac environments, simply for the fun and challenge — and to show those young whippersnappers what Pong and Commander Keen were all about.