identifying PC Simms

Chuck Guzis cclist at
Fri Sep 30 16:57:51 CDT 2005

On 9/30/2005 at 3:24 PM Jules Richardson wrote:

>> I beliee that most PCs could make use of the added parity bit, while 
> > Macs didn't care if it was present.
>(ignoring attributions here I know)
>Isn't that the other way around? Nearly all PCs I've come across don't 
>care about parity, but the rest of the world always seemed to make use 
>of it.

Nope, PC's right up through the PS/2 used NMI (int 2) to diagnose memory parity and bus errors, as well as 80x87 exceptions.  PS/2's also had a watchdog timer that could be activated (int 15h, ah=C3) to trip if the timer interrupt went un-serviced.  The NMI could be selectively masked, of course (port 61h).

There were a fiew BIOSes out there ("were" is operative, isn't it?  No one actulaly makes new PC's using 30-pin SIMMs.) that would note the lack of the parity bit and simply mask it at POST time.

I can recall that 30 pin non-parity SIMMs were specifically called out by some retailers as being for "Macintosh only).


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