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Thread: Todayís stupid question if Motorola didnít make 5mhz chips why...

  1. #1

    Default Todayís stupid question if Motorola didnít make 5mhz chips why...

    Just curious all cpu photos I can find list the Motorola 68k as coming in 4, 6, and 8 MHz clockings

    Why then was the Lisa 5mhz?

    Video, ram or ???

  2. #2
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    Video probably. DOTCK (the primary clock signal) is 20.375 MHz and used as is for video and divided by 4 to give the CPU clock. Why Apple tried to save a few dollars on a $10,000 workstation by not having video and CPU on unrelated clocks is a mystery to me?

  3. #3
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    Just because a CPU is rated for a given speed, doesn't mean that it must run it at that speed.

    There's definitely a design reason for the CPU running at 5 MHz, but I don't know what that is.

  4. #4

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    Quote Originally Posted by krebizfan View Post
    Video probably. DOTCK (the primary clock signal) is 20.375 MHz and used as is for video and divided by 4 to give the CPU clock. Why Apple tried to save a few dollars on a $10,000 workstation by not having video and CPU on unrelated clocks is a mystery to me?
    Probably this, but RAM also seems like a good guess; if they could get a better deal on slower memory components, that might've seemed like reason enough. But yes, one has to wonder why exactly they would cut corners to such an extent on a machine they were charging so much for, especially given that they were able to bring out the Mac running at a full 8MHz just a year later.
    Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
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    "'Legacy code' often differs from its suggested alternative by actually working and scaling." - Bjarne Stroustrup

  5. #5

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    The micro is specced at 5mhz? Or is it a 6 or 8, just clocked at 5?

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    Quote Originally Posted by 2icebitn View Post
    The micro is specced at 5mhz? Or is it a 6 or 8, just clocked at 5?
    The Lisa CPU boards I have seen pictures of have either a MC68000G8 (the 8 MHz variant) or MC68000L6 (6 MHz). Yes, Apple left a lot of performance on the table.

  7. #7

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    Never knew this. Wasn't the original Mac 7.16? Or was that the Atari, and Mac was 8mhz? Or the Amiga was one or the other. I'm so confused.

  8. #8

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    Quote Originally Posted by 2icebitn View Post
    Never knew this. Wasn't the original Mac 7.16? Or was that the Atari, and Mac was 8mhz? Or the Amiga was one or the other. I'm so confused.
    If Wikipedia is to be believed, the Mac 128k ran at 7.8336 MHz - so not quite 8 MHz, but pretty damn close. The Amiga was 7.16-ish in NTSC territories, IIRC.
    Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
    Synthesizers: Roland JX-10/SH-09/MT-32/D-50, Yamaha DX7-II/V50/TX7/TG33/FB-01, Korg MS-20 Mini/ARP Odyssey/DW-8000/X5DR, Ensoniq SQ-80, E-mu Proteus/2, Moog Satellite, Oberheim SEM
    "'Legacy code' often differs from its suggested alternative by actually working and scaling." - Bjarne Stroustrup

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