Ted Fried's MCL86jr Is a Speedy FPGA Upgrade for Your Slowpoke IBM PCjr

Embedded hardware engineer Ted Fried has built a field-programmable gate array (FPGA) accelerator board for IBM's classic PCjr — offering a fourfold or higher performance boost over the stock hardware while topping its memory up to a generous 640kB.

"The MCL86jr is a 8088 drop-in replacement for the IBM PCjr which expands the system's memory to the full 640 KB and can run either cycle accurate or accelerated," Fried explains of the board, which slots into place where the original processor would normally live. "It uses the MCL86 EU [soft CPU] core combined with a minimum-mode BIU [Bus Interface Unit] and access to a 512kB static RAM to bring the PCjr to the full 640kB of RAM."

IBM launched the PCjr — "Personal Computer Junior" — in 1984 as a follow-up to the IBM PC, released three years earlier, promising the same Intel 8088 CPU and IBM Basic Input/Output System (BIOS) but in a lower-cost device with gaming-friendly features. Its relatively high launch price, partial IBM PC compatibility unpleasant "chiclet" keyboard, and limited 64kB or 128kB of RAM meant it wasn't a great success — particularly when up against pocket-friendly devices like the Commodore 64 in the casual use and gaming markets.

Fried's accelerator board addresses at least two of these problems. Replacing the original Intel 8088 chip, the board is built around an AMD Xilinx Spartan-6 running a soft-core 8088 originally designed for use with the MiSTer vintage computing emulation project. In cycle-accurate mode, this acts close enough to the original hardware for software to run as-normal — while a 512kB SRAM chip breaks through the 128kB limit and brings any PCjr up to the maximum possible 640kB base memory.

It's the board's accelerated mode, which is likely to be of most interest to PCjr owners, though: while potentially less accurate, it offers a major performance boost of at least fourfold and potentially up to sixfold depending on workload — "about as fast as an IBM PC/AT," Fried notes, referring to IBM's Personal Computer/AT, launched later in 1984 and offering an Intel 80286 running at up to 8MHz for around five times the cost of the PCjr.

Design files and sources for the board MCL86jr board are available on Fried's GitHub repository, under an unspecified open-source license.