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Thread: Pentium 133 board question

  1. #11
    Join Date
    Apr 2015
    Austin, Texas


    Quote Originally Posted by Stone View Post
    Pentium boards require pairs and this is what the OP has.
    Some chipsets can run in "gimped" mode with one stick and half the memory bus width at a significant cost in speed. I've had several 386/486/Pentium boards which could do this.

    The performance impact was pretty noticeable, it's not something I'd recommend today since those old memory modules are generally pretty cheap.

  2. #12


    I miscounted since one was hidden. It does have 4 SIMM slots, though I didn't know that they had to work in pairs since I have used exclusively DIMMS. It is late and I would have to open the machine back up to take pictures of board to attempt to ID it. I'd like to figure out more about the board since it is in a bit of an obscure PC. I will post pictures of it tomorrow afternoon.

  3. #13


    It is quite simple actually.

    The required number of bits depends on the processor chipset:
    - 80286, 80386SX: 16 bit wide memory bus
    - 80386DX, 80486, Pentium Overdrive: 32 bit wide memory bus
    - Pentium, Pentium Pro: 64 bit wide memory bus

    The 486 processor (not the 386!) supports 8/16/32 bit wide busses natively, so chipset support for narrower accesses is trivial. I have never seen a mainboard support this, though.

    The provided number of bits depends on the memory module:
    - 30-pin modules cover 8 bits. SIP or SIMM, FPM or EDO.
    - 72-pin modules cover 32 bits. SIMM, mostly EDO (older ones FPM).
    - 168-pin modules cover 64 bits. DIMM, SD-RAM.

    Easy to distinguish as well:
    - 30-pin modules have no notch
    - 72-pin modules have one notch
    - 168-pin modules have two notches

  4. #14
    Join Date
    Apr 2015
    Austin, Texas


    Don't forget about the weird memory modules that look like or are similar enough to normal memory modules but aren't.

    64/68 pin SIMMs. Former was a special weird memory used on the Macintosh IIfx, latter was used as display memory on several Mac 68k machines. These modules are rare enough today to be hard to stumble upon by accident.

    168 pin EDO DIMMs. Also a weird Apple specific memory, used in the Power Macintosh 6400/6500 and several other models. These are identical to normal 168 pin DIMMs except for the notch in the middle of the stick being in a slightly different position. A dedicated person could force this into a normal memory slot, which would result in fire/smoke. Likewise, installing a normal DIMM in a mac will yield a similar result.

    Also, many 486 chipsets do not like EDO memory and either only work with FPM or have compatibility issues. Some systems may fail to boot with EDO, not be able to count all available memory properly or have a reduced maximum memory capacity. I have one 486 machine which only supports 32 MB of EDO (2 x 16M), but 64M of FPM (2 x 32M.)

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