The IBM PC 5150: What if?
Posted by Jim Leonard on December 27, 2012
Did you know that the IBM PC was not originally designed around the i8088? Various sources conflict somewhat (including offline sources such as “Blue Magic: The People, Power and Politics Behind the IBM Personal Computer“), but the general consensus is that early 5150 designs considered CPUs that were both less powerful (such as the MOS 6502 and Zilog Z80) and also more powerful (such as IBM’s POWER predecessor 801, or the Motorola 68000 CPU).
I find this fascinating to daydream about sometimes. What if the IBM PC had not been built around the 8088? How would have the personal computer industry progressed in the 1980s? Who would the leader(s) be today?
The CPU possibility I keep drifting to is the 68000. The IBM PC+8088 design had limitations that the industry spent nearly 15 years working around, the most infamous being limited to 1MB RAM (640KB typically available) and, adding insult to injury, only being able to access it one 64K segment at a time. But while an address translation chip could have provided a flat 16:16 address space, there were some annoying limitations in the 8088 itself, such as only having four general-purpose 16-bit registers (and of those four, each had a specialization married to some sets of instructions, so you couldn’t use them as flexibly as you would have liked). The 68000 by comparison was immensely more powerful and had none of these limitations: It had eight 32-bit general-purpose registers, and another eight 32-bit addressing registers. On top of that, the 68000 had a flat 16MB address space (no segments!) The 68000 had some drawbacks too (big-endian architecture, misaligned code would crash the CPU) but the core architecture was so much more powerful that it would have drastically changed how the IBM PC was perceived and used.
However, it would have also drastically changed how the IBM PC was marketed. The original price of the PC was already expensive at launch ($1565 without any monitor or disk drives, about $3800 today) but designing the machine around the 68000 would have required more expensive components. This, along with IBM’s existing familiarity with the i8086 family which would shave months off the time to market, was ultimately behind IBM’s decision to abandon all others and go with the i8088.
The 68000 was used in more powerful home computers that came after the IBM PC, such as the Apple Macintosh, Atari ST, and Amiga 1000. On launch day for each of those computers, they greatly outshone the original IBM PC. (BTW, the Amiga launch is particularly impressive and fascinating to watch although I can only find a small snippet of the full launch video online.) If the 68000 had been used in the PC, would those later machines have ever existed? While it’s interesting to fantasize that a 68000-based PC would have prevented the Atari ST and Amiga from ever existing, I think a more realistic outcome would be that the PC would have been treated like the Apple Lisa: Powerful, but way too expensive, and perceived as a high-end business workstation that would find its way into some niche business markets but never considered a computer for the home. (As to what computer(s) would have triumphed in the PC’s absence, that’s a What If? for another day.)
What about other potential CPUs? What do you think might have happened to the IBM PC if it had been based on the same CPU as found in the Commodore 64 or the ZX Spectrum?