Dr. Scott M. Baker Packs 16MB of RAM Into a 1970s Eight-Bit Heathkit H8 Microcomputer
Boasting over four thousand times as much memory as a stock Heathkit H8 build, this open-hardware add-on is a beast by 1970s standards.
Engineer and vintage computing enthusiast Dr. Scott M. Baker has created what may well be the most over-specified Heathkit H8 microcomputer in history, somehow packing 16MB — not kilobytes, but megabytes — of memory in the 70s eight-bit machine.
"I wanted to see how much RAM I could stuff into an old eight-bit [Heathkit] H8," Baker explains. "My original plan was to build four boards, and have it be 32 megabytes, but due to a small error in the board design, I ended up with only two boards, for a total of 16MB."
Originally launched in 1977 as a build-it-yourself kit of parts, a stock Heathkit H8 runs Digital's CP/M operating system on an Intel 8080A processor and required a static RAM (SRAM) card with just 4kB of memory on board in order to actually achieve much. Just one of Baker's new boards, by contrast, offers 2,048 times as much memory — and he's fitted two of them to his personal machine.
The boards themselves are built out of a combination of SRAM and flash memory chips, with roughly equal performance — though the SRAM writes more quickly and with less faffing around in software, but the flash can keep its contents even when the power is switched off. In addition to the four 2MB banks, each comprised of four 512kB SRAM or flash chips, the board includes chip select generators, bus and address programmable logic devices — "I used GAL22V10," Baker notes, "which are period appropriate to the early 1980s," — and 74-series chips for address and data buffers.
"So what is this good for? Do you actually need 16MB of RAM in an H8? Probably not," Baker admits. "But you can easily use it as a series of RAM disks, each one of them 2MB in size. That gives you room for about eight RAM disks. Could you actually use 16MB for a purpose other than a RAM disk? I’m sure you could… The memory is pageable in 16kB pages, so you could easily swap data or code in and out of memory."
This isn't the first memory expansion board Baker has designed for the Heathkit H8, though it's by far the largest. Late last month he showcased the H8 Hellboard, which offered a period-appropriately-generous 64kB of RAM for the system — one-256th of the memory capacity offered by Baker's latest upgrade.
The same Heathkit H8 has received other upgrades in the past, too. Back in March Baker showed off a Raspberry Pi-based add-on which allowed for high-speed file transfer, while in late 2022 he designed and showed off a custom speech synthesis board for the system.
Full details on Baker's build are available on his website, while a schematic, Gerbers, and the PLD source code are available on GitHub under an unspecified open-source license.