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Thread: How to adjust the speed of an 8088-2 CPU?

  1. #11
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    According to my 885 manual, there's a small alternate-action pushbutton switch near the right lower area of the expansion card area (looking at the rear head-on). There's also the CLKCHK command in the Sanyo DOS distribution that indicates the speed of the CPU.

    Sorry for chiming in; I know you wanted to ignore me.

  2. #12
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    Quote Originally Posted by Chuck(G) View Post
    According to my 885 manual, there's a small alternate-action pushbutton switch near the right lower area of the expansion card area (looking at the rear head-on).
    Is it blue? Because the other board I got with the non-working Sanyo has JUST that!

    Edit: AHA! IT IS BLUE! I have a clock reading of 4.655 MHz on the other one! I was suspecting that!

    I got two versions of the same board! One has the switch, the other does not, but has a place FOR the switch!
    Last edited by T-Squared; October 9th, 2020 at 05:05 PM.

  3. #13
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    FWIW, the 885 is the 10MB hard disk version of your system. As far as I'm aware, there's no software way to change the system speed. That button is it.

  4. #14

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    Since you were going to run 8088 MPH: I don't know much about the MBC-775, but it looks like the internal CRT shows RGB video - does this thing have composite video out?

    If so, it'd be nice to know how close the colors are (or aren't) to the expected IBM CGA output.
    int10h.org :: :: :: blog

  5. #15
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    Yes, it does have composite out. And for games like Space Quest I & II, and Burgertime, the composite artifact colors work like a dream. (Space Quest clued me in when I noticed the vertical patterns of lines)

    Check back here later. I'll post some pictures to show the colors.

  6. #16

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    Quote Originally Posted by Eudimorphodon View Post
    Several of the blog posts about it reiterate that claim, but I'm not smart enough to say exactly what they might be referring to. There are several parts of the demo where they "race the beam" and bang on the 6845's registers in various ways that might do "something". My impression was that damaging a monitor by sending it wonky sync signals was more a problem with MDA cards than CGA because the IBM 5151 monitor strictly speaking doesn't "sync", it's more of a direct drive, but maybe it's possible to murder a CGA monitor too, no idea.

    Strictly speaking I don't think it's the tube that would be at risk, but the high voltage power supply that handles the sweep.
    When we wrote 8088 MPH, we weren't sure that wonky sync signals couldn't damage a CGA monitor. I've since learned that (as you said) it's really not an issue for CGA monitors so I think it's perfectly safe for T-Square to go ahead and try running the demo at the higher speed. It'll probably just lose vertical sync in some parts (I don't think it will even lose horizontal sync).

  7. #17
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    Here's how the demo turned out. I wonder if an original 8088 CPU would work properly. The 8088-2 doesn't seem to lock on the proper 4.77MHz signal in slow mode.

    8088 on a Sanyo MBC-775

  8. #18
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    No, it will read the same. The -2 simply means that the CPU has passed the selection criteria for operation at 4.77MHz. It's still the same die. There can be other implementation details that confuse the speed computation. If you really want to know the CPU clock, stick a good 'scope on the clock input to the CPU and measure it directly. If you're relying some software, you're just goofing around.

  9. #19

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    Quote Originally Posted by T-Squared View Post
    Here's how the demo turned out. I wonder if an original 8088 CPU would work properly. The 8088-2 doesn't seem to lock on the proper 4.77MHz signal in slow mode.

    8088 on a Sanyo MBC-775
    Possible clock discrepancies aside: did you try playing with the settings on the initial calibration screen some more? in particular changing "CGA model" to "new"?
    int10h.org :: :: :: blog

  10. #20
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    You know, I may have found my problems. Would bad RAM cause the discrepancies found in the demo?

    In fact, both 8088MPH (Look at the text) and Space Quest had some cut-out portions in the graphics:

    20201010_105626 copy.jpg

    and several programs would either not start, or cause the system to hang, despite their size being able to fit within the 256 KB of memory.

    RAMTEST even found a bad chip in one of the memory banks of the processor board I'm using at the moment.

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