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Forum Index : Microcontroller and PC projects : 68000 CPU....
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9483 |
This was quite interesting, I thought: 68000 CPU... Smoke makes things work. When the smoke gets out, it stops! |
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| matherp Guru  Joined: 11/12/2012 Location: United KingdomPosts: 10067 |
Basically it was a PDP11 on a chip. I never understood how the awful i86 architecture ended up dominating when the 68000 instruction set was so elegant and better in every way. |
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Martin H. Guru Joined: 04/06/2022 Location: GermanyPosts: 1193 |
I never understood that either. I loved the 68k assembler (but I also came to the 6809 via the Z80 so I had a Motorola background) 'no comment |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7503 |
I read somewhere that IBM originally intended to use the Motorola architecture but there were no support chips available for it at the time. Motorola didn't have the resources or finances to push anything through quickly either. Intel already had CPUs in production together with support chips so IBM changed their minds. As it happens it may have been no bad thing. The indirect addressing system on the 68000 makes it a bit difficult to scale up. IMHO it would have led to more incompatible CPU versions than even Intel have done. We'll never know. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| pwillard Guru Joined: 07/06/2022 Location: United StatesPosts: 307 |
Motorola was not savvy enough to develop an affordable CPU at the time. They also took a design-team hit when a chunk of the team left to form MOSTEK to create the 6502. So technically, the 6502 COULD have been a Motorola product had they not been so narrow-minded. By the time the 68000 became mainstream, DOS, which used the 8086 CPU series, had a significant foothold. It's as if everyone grumbled about the dumb architecture choices of the x86 series, but it was anything but a niche product, thanks to the broad adoption of the PC with MS-DOS. And IBM's criteria for choosing the 8088 for the PC were not its capabilities as much as the cost and availability. Something Motorola couldn't get right. For example, in 1981, when the 6502 was new, it cost $6, compared to the similar offering from Motorola, the 6809, which was priced at $37 (note that it was a better chip). The decision to use the 6502 in a home computer, based on cost, was made clear by its appearance in designs. We can argue that the 68000 is a significantly better chip, but it also took some time to become more affordable. For example, buying an Amiga was always a 'no' for me, as it was beyond my price range for a home computer. |
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| lizby Guru Joined: 17/05/2016 Location: United StatesPosts: 3309 |
Absolutely. When the IBM PC was first rumored, my hope was that it would be 68000-driven. The 8086 was such a disappointment with paged memory access in 64K chunks--compared to an almost unimaginable 1 megabyte of flatly addressable memory in the 68000. The 68000 could be single-stepped in hardware by clocking memory access. I wrote most of a CP/M OS for the 68000, using a Z80 to access all the hardware on an actual CP/M PC. All ready to slot into this imagined 68000 IBM PC. The best-laid plans .... PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed |
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| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 3998 |
I think IBM did make a 68000 PC but inevitably later than the 8088 PC, because as has been posted the 68000 was just not available in quantities with support chips. It was more expensive and not a big seller, I think, and of course by then the 8088-based PC had lots of apps that were what really sold it. Also, then Compaq joined in, with others. The open architecture bus helped, too. John |
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| bfwolf Regular Member  Joined: 03/01/2025 Location: GermanyPosts: 72 |
That's a "slight understatement".  The M6809 is more comparable to the PDP11 (it also supported virtual addressing via an external MMU) – the M68000 has 32-bit registers and an even richer instruction set. The M68K is more of a "VAX light"! But yes: The PDP11 was the template for both (M68K and VAX) architectures.Unfortunately, Motorola had no interest in the "low-cost market" at the beginning. They charged over $100 for an M6800 and were probably able to make a profit in a small segment of industrial control systems. That was the reason why "Chuck" Peddle left Motorla and founded MOS (later acquired by Commodore), and in a very short time designed the 6502 (essentially the first pipelined RISC CPU) and offered it at a very low price. .. I bought an Amiga 2000 in 1986 and have invested a lot of money in it over the years (memory expansion, SCSI controller and HD, 68030 board, CD-ROM drive, deinterlacer card, etc).  I still have it, and it still works! It actually has a beautifully designed multitasking OS!  No: It was really a question of money! Intel made IBM "an irresistibly low offer" for the 8088. They later made huge profits with the 8087 FPU.  The 68000 was simply too expensive! All the support chips for the 68000 actually already existed. UN*X workstations were already being built and sold with it. There was e.g. no need for an external interrupt controller - the logic for soft-vectored IRQs was integrated into the 68K peripheral chips. And besides, it supported all 68xx peripheral chips. The 68451 MMU was actually available pretty early on! But I think the 68851 PMMU came later? One reason for IBM's decision could also have been that the 68000 had a 16-bit memory interface, and IBM only wanted to use 8-bit memory because it was cheaper. The 68008 with its 8-bit memory interface probably came too late? |
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