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

  1. #1
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    Default How to adjust the speed of an 8088-2 CPU?

    Since I have gotten my Sanyo MBC-775 working, I had considered running 8088MPH to see if it would work, despite the CPU in the system not being an 8088, but an 8088-2.

    (I'm putting the IRCjr attempt on hold, for now, if you read my previous topic about the Sanyo.)

    Indeed, the pre-demo test says that the measurement is 40% above the normal 4.77mhz, and I didn't want to risk burning out the internal picture tube from overdrive, given the warning in the pre-test.

    Is there a way, either through hardware jumpers or software configuration, to change the speed from 8MHz/7.16MHz to 4.77?

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    Pardon my ignorance, but how can one damage the CRT by running the CPU too fast?

    On the old 8088 systems, the old Landmark Speed test is a decent "quick and dirty" check on CPU speed.

    Most 8088 "turbo" systems had some sort of multi-keystroke capability to reduce the speed to 4.77MHz for compatibility reasons.

  3. #3
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    I think it's more the timing of the video than anything else. Here's what the disclaimer states in the pre-test:
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    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.
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    The PC Magazine review indicates that a compatibility slow down was supposed to be added to the MBC-775. Are there any readmes on the disks that came with the system? I would suspect if Sanyo provided the ability, it would be using software so checking the disks and seeing if any unusual programs are provided would be a necessary step. While using names like SLOW or TURBO was common, Sanyo did have a tendency to do things their own way.

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    I tried Wikipedia's suggestion to use Ctrl-Shift-(+) or Ctrl-Shift-(-) or Ctrl-Shift-(?), but they didn't seem to work.

    Furthermore, the Landmark benchmark shows a CPU speed of 7.995MHz.

    Quote Originally Posted by krebizfan
    Are there any readmes on the disks that came with the system?
    These systems were picked up from a lot sale, and the casings were in pretty rough shape. They didn't come with any disks, unfortunately. I've been looking for Sanyo MBC manuals, but I haven't found any yet. (Plus I'm using a different 8088 BIOS, a Phoenix Technologies BIOS, for compatibility's sake.)

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    Swapping to a third party BIOS would almost certainly neuter any kind of key combination speed switching unless it’d been specifically built for the machine. Presumably speed is controlled by twiddling a bit on an I/O port, but without the original software or a tech manual...
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    Here's some teledisk-format archives of some Sanyo disks:

    http://www.retroarchive.org/maslin/disks/sanyo/

    Maybe there's some kind of speed setting program on them.

    Out of curiosity, how compatible does the machine seem to be with other graphics software? There's an old review of the system out there that says it wouldn't run MS Flight Simulator (a common target for PC compatibility) and mentions vaguely there may have been other cases where software could be heard running in the background but with no video. If that indicates that the CGA implementation isn't register compatible then 8088MPH will almost certainly faceplant right out of the gate.
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    I've got the MBC885 manual, which system also uses a 8088-2 documented as 4.77/8 MHz. Should I paw through it for an answer?

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    Quote Originally Posted by Eudimorphodon View Post
    Swapping to a third party BIOS would almost certainly neuter any kind of key combination speed switching unless it’d been specifically built for the machine.
    I had considered that too. It's a bit difficult to use Sanyo's v2.33 BIOS, though, with how buggy Sanyo's attempt at the IBM BIOS was; at least how I've heard it.

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