4 ms·

Correct, but you had the option to use fast (external) SRAM for the area which held the registers. Even so, the 64K limit was the bigger problem. Actually 808

Correct, but you had the option to use fast (external) SRAM for the area which held the registers. Even so, the 64K limit was the bigger problem. Actually 8088 really had a huge advantage: easy to port CP/M apps to the IBM-PC. Even if the 68K was ready to go, 8088 was probably the better choice.

Gibbon1 9y ago
One of the things I remember reading in the late 80's was that for IBM the 68000 was a no go because it would have doubled the number of DRAM's needed for a bare bones system. And Motorola didn't have the 68008 ready yet. Important because back then memory was a huge part of the cost of the machine.
- jandrese 9y ago
  By "bare bones" system do you mean 64kb? The original Mac had a 68k and only 128kb of RAM. I guess we are talking about the late 70s here though. The Commodore 64 didn't come out until 82 and it got along just fine with 64kb. IIRC the 68k was an expensive chip period. It was the chip you used if you had money to burn.
  - bluedino 9y ago
    The base IBM PC (in 1981) came with 16KB of memory The Mac came out three years after
    - jandrese 9y ago
      It's hard to imagine getting any actual work done on a machine with 16KB of main memory and no memory manager. A single page of text is 2k, and you can't go too fancy with mappers and memory paging schemes because the code would be too complicated to fit in memory... One can see reasons for C's design tradeoffs if you're worried about machines like that. But even after you abuse every trick in the book it's hard to see how that machine isn't hobbled by its lack of memory.
  - to3m 9y ago
    The 68000 has a 16-bit bus but the 8088 is only 8 bit. I don't know about today, but RAM chips of the time supplied 1 bit per address, with the capacity being a square (since column and row are addressed using the same pins). So if you need twice as many bits per cycle, your options are basically to have twice as many chips (of a quarter the capacity each... probably cost-effective, but now your system has half the RAM), or twice as many chips (of the same capacity each... and now it's twice the cost). Fitting twice as many chips on the board is probably a pain too. (And suppose you go for the 2 x quarter capacity option - now you need 4x as many if you want the same amount of RAM!)
    - kazinator 9y ago
      Obvious reversal: the 8086 has a 16 bit bus, and the 68008 only 8. There is a slower kid in every family. :) Note that both the 8086 (1978) and 68000 (1979) were introduced ahead of, respectively, the 8088 (1979) and 68008 (1982). Basically these 8-bitsters were probably kind of a cost reduction following a familiar pattern in the hardware industry: product catches on, then customers want to put it into more and more things that are cheaper and cheaper, with simpler boards, where big MIPS aren't needed.
      - to3m 9y ago
        And the 1979-1982 period is exactly when the original PC was released, hence the problem!
      - krylon 9y ago
        As the article points out at one point, at least some part of the reason to make the 8088 were existing peripheral devices that were not compatible with the 8086 I/O-wise. (Those reasons are not mutually exclusive, of course.)
        jecel 9y ago
        It was pretty easy to use 8080 peripheral chips with the 8086 and some very few clones did just that. IBM itself had to deal with the problem on the PC AT which had the same i/o chips but a 286 processor. It needed to replicate externally the circuit that the 8088 had internally due to the need to be compatible with the old 8 bit cards as well as the new ISA ones. The 68000 would have been more of a problem since it moved from the matched memory and clock cycle scheme of the 6800 to a four clock cycle scheme with a complicated handshake. A special memory mode and two extra pins made it talk just fine to the 8 bit i/o chips. There was no need to wait for the 68008 for that. One huge mistake that was made in the 8088 and 68008 (and I will suppose the TMS9980 as well, though I haven't checked) was that they didn't have a simple way to take advantage of page mode access in DRAMs like the original ARM did. If they had, the gap in performance compared to the 16 bit bus models would have been smaller.
  - krylon 9y ago
    > only 128kb of RAM I remember reading the first IBM PC came with 16k. A former colleague of mine once reminisced about programming on a PC in the late 1980s that only had 256k of RAM (although I think that must have been an old or low-end machine).
  - Gibbon1 9y ago
    > 128kb of RAM. Probably 16 64kX1 DRAMS. > IIRC the 68k was an expensive chip period. Fuzzy memory but the 68000 was a 64 pin ceramic package. I remember comments that the IC testers of the day didn't have enough IO to test them. That upped the cost as well.

Gibbon1 9y ago

One of the things I remember reading in the late 80's was that for IBM the 68000 was a no go because it would have doubled the number of DRAM's needed for a bare bones system. And Motorola didn't have the 68008 ready yet. Important because back then memory was a huge part of the cost of the machine.

jandrese 9y ago
By "bare bones" system do you mean 64kb? The original Mac had a 68k and only 128kb of RAM. I guess we are talking about the late 70s here though. The Commodore 64 didn't come out until 82 and it got along just fine with 64kb. IIRC the 68k was an expensive chip period. It was the chip you used if you had money to burn.
- bluedino 9y ago
  The base IBM PC (in 1981) came with 16KB of memory The Mac came out three years after
  - jandrese 9y ago
    It's hard to imagine getting any actual work done on a machine with 16KB of main memory and no memory manager. A single page of text is 2k, and you can't go too fancy with mappers and memory paging schemes because the code would be too complicated to fit in memory... One can see reasons for C's design tradeoffs if you're worried about machines like that. But even after you abuse every trick in the book it's hard to see how that machine isn't hobbled by its lack of memory.
- to3m 9y ago
  The 68000 has a 16-bit bus but the 8088 is only 8 bit. I don't know about today, but RAM chips of the time supplied 1 bit per address, with the capacity being a square (since column and row are addressed using the same pins). So if you need twice as many bits per cycle, your options are basically to have twice as many chips (of a quarter the capacity each... probably cost-effective, but now your system has half the RAM), or twice as many chips (of the same capacity each... and now it's twice the cost). Fitting twice as many chips on the board is probably a pain too. (And suppose you go for the 2 x quarter capacity option - now you need 4x as many if you want the same amount of RAM!)
  - kazinator 9y ago
    Obvious reversal: the 8086 has a 16 bit bus, and the 68008 only 8. There is a slower kid in every family. :) Note that both the 8086 (1978) and 68000 (1979) were introduced ahead of, respectively, the 8088 (1979) and 68008 (1982). Basically these 8-bitsters were probably kind of a cost reduction following a familiar pattern in the hardware industry: product catches on, then customers want to put it into more and more things that are cheaper and cheaper, with simpler boards, where big MIPS aren't needed.
    - to3m 9y ago
      And the 1979-1982 period is exactly when the original PC was released, hence the problem!
    - krylon 9y ago
      As the article points out at one point, at least some part of the reason to make the 8088 were existing peripheral devices that were not compatible with the 8086 I/O-wise. (Those reasons are not mutually exclusive, of course.)
      - jecel 9y ago
        It was pretty easy to use 8080 peripheral chips with the 8086 and some very few clones did just that. IBM itself had to deal with the problem on the PC AT which had the same i/o chips but a 286 processor. It needed to replicate externally the circuit that the 8088 had internally due to the need to be compatible with the old 8 bit cards as well as the new ISA ones. The 68000 would have been more of a problem since it moved from the matched memory and clock cycle scheme of the 6800 to a four clock cycle scheme with a complicated handshake. A special memory mode and two extra pins made it talk just fine to the 8 bit i/o chips. There was no need to wait for the 68008 for that. One huge mistake that was made in the 8088 and 68008 (and I will suppose the TMS9980 as well, though I haven't checked) was that they didn't have a simple way to take advantage of page mode access in DRAMs like the original ARM did. If they had, the gap in performance compared to the 16 bit bus models would have been smaller.
- krylon 9y ago
  > only 128kb of RAM I remember reading the first IBM PC came with 16k. A former colleague of mine once reminisced about programming on a PC in the late 1980s that only had 256k of RAM (although I think that must have been an old or low-end machine).
- Gibbon1 9y ago
  > 128kb of RAM. Probably 16 64kX1 DRAMS. > IIRC the 68k was an expensive chip period. Fuzzy memory but the 68000 was a 64 pin ceramic package. I remember comments that the IC testers of the day didn't have enough IO to test them. That upped the cost as well.