On 24 Oct 98 at 9:30, Harold E. Schumacher, Jr. wrote:

>         A buddy of mine has an Acer 486SX 25 MHz and we increased the RAM
> from 4MB on board (built into the motherboard) to a total of 8MB; 4
> on board plus another 4MB 72 pin SIMM.  The manual said not to put
> the SIMM in bank 0; for the configuration the SIMM needed to be in
> bank 1.  (I'm telling you this because this may or may not be the
> cause of the problem.)
>         Now, when you turn on the PC, it recognizes all 8MB of memory, but
> then gives the following message:
>
>         RAM Parity Error  Press ESC to turn off NMI, any other key to
> reboot.
>
> We pressed any other key to reboot and get back to the same
> message.  We then pressed ESC, and it went ahead and booted into
> Windows (3.1 BTW), as if there wasn't a problem.  ALl programs
> appear to be working OK.  Two questions---

>         1.  What's causing the error message?

  72-pin SIMMs provide 32 data bits.  72-pin SIMMs with parity
provide 32 data bits and 4 parity bits, which allow a very simple
check for single-bit memory errors.  [Odds are good that a small
defect in a memory chip will affect only a single bit.]

  The CMOS setup may offer an option to disable parity checking of
RAM, and if so then you must disable it.  Or replace this newer SIMM
with one that provides parity bits.

>         2.  What's NMI?

  NMI is the Non-Maskable Interrupt; similar in concept to an IRQ,
but it doesn't go through the Priority Interrupt Controller(s) the
way IRQs do -- it gies directly to the CPU.  PCs have historically
signalled parity failures this way -- up until sometime in the 486
era, all PC memory had to have parity bits.


  Parity checking can be done by a small network of "gate" elements,
in parallel with data access.  Unless it is finding problems, there's
no reason it should slow down other operations on the machine.
  Parity memory should, in theory, require about 12% extra pasrts and
assembly costs versus non-parity.  There was a time (pre-486) when
economies of scale made up for this, but now I think you'll find that
these cost factors translate roughly into a price difference.]

  Parity errors during operation can indicate bad RAM.  However:

1.  I've only ever seen them because a machine was getting too hot.

2.  There is at least one virus that wreaks its damage by making the
CPU think there's a signal on NMI.  A current check for viruses might
not be a bad idea.

David G

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