Ever wonder what it would have been like if IBM has chosen the Motorola 68000 instead of the Intel 8088?

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I was wondering what the IBM Personal Computer would have been like if they had chosen the Motorola 68000 instead of the Intel 8088, so I used my MCL86+ to emulate the 68000 and find out!

The MCL86+ is a board which uses a Teensy 4.1 to emulate a microprocessor in C code as well as use its GPIOs to emulate the local bus of the Intel 8088. It can be used as a drop-in replacement for the Intel 8088 and can be cycle accurate as well as run in accelerated modes.

For this project I swapped the 8088 emulation code with my MCL68 code which emulates the Motorola 68000. The 8088 local bus emulation was kept so that all of the 68000’s memory reads and writes could pass through to the IBM motherboard.

Emulating the 68000 is fine, but not very useful without an operating system or some other application to run on it. I chose to use Gordon Brandley’s 68K BASIC which was published in Dr. Dobbs Journal back in 1985.

I should note that just the 68000 emulation is running inside of the MCL86+. All of the IBM motherboard peripherals, video, and memory are used for this project. Video is drawn to the IBM MDA card’s video memory over the ISA bus. BASIC is run out of 64 KB of the motherboard DRAM which is refreshed by the 8327 DMA controller by an interval set by the 8253. The keyboard receives the serial data and generates an interrupt through the 8259 and is cleared by accesses to the 8255. Basically all of the major components and glue logic of the IBM motherboard are used!

Even now, our giant supercomputer desktop machines boot as though they were a vintage 1981 PC for a few minutes on each startup. But the PC wasn’t the first personal machine from IBM and, in fact, the IBM 5100 was not only personal, but it was also portable. Well, portable by 1970s standards that also had very heavy video cameras and luggable computers like the Osborne 1.

The IBM 5100 had a brief three-year life from 1975 to 1978. A blistering 1.9 MHz 16-bit CPU drove a 5-inch CRT monitor and you could have between 16K and 64K of RAM along with a fair amount of ROM. In fact, the ROMs were the key feature and a giant switch on the front let you pick between an APL ROM and a BASIC ROM (assuming you had bought both).

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let me know how you feel.

But yeah, thanks for spending some time to talk about this subject here
on your blog.