View Full Version : Interrupt bugged 8088 c79 only, how late were they used?

January 28th, 2016, 05:07 PM
Years ago i bought an 5150 at a local goodwil store,my original intent was to dump the guts and use only the case to make a "retroized" custom PC ( I know, I should be stoned or burnt at stake for this sacrilege).
Fortunately nothing happened and the machine sits safely in a crawlspace.
I do remember openning it though, and what I noticed at the time was that it had an Intel 8088 copyrighted 79 only, and as far I know that CPU had a serious bug that was quickly rectified by Intel .
They released a new version copyrighted 79 and 81 (or 82 I don't remember exactly) that was bug free, and I think they were sending it as a replacement together with a new BIOS rom ( if I'm not mistaken).
Wonder if that (having such an early CPU) was an indication of the PC being a early one or it really didn't matter that much?
I remember checking the serial number at the time, and it did't seem like anything unusual, also the power supply was as far I remember just plain vanilla siver not black.
Guess a real question is, how far into the production of original PC's, did they use the early 8088 c79 only ?
Anyone knows?

January 29th, 2016, 07:43 AM
I don't have the answer to your question specifically but I wouldn't worry about it.

From Ralph Brown's Interrupt List:

Some _very early_ 88 and 86 processors do not disable interrupts following
a MOV sreg,reg. This causes them to crash when an interrupt uses the stack
between MOV SS,reg and MOV SP,op. These versions carry a copyright message
for 1978 on the package. Later, corrected revisions, carry both 1978 and
1981 as the copyright year.
Normally interrupts would be disabled between the move to SS and execution
of the instruction following it on 88 and 86es. A workaround is to manually
disable the interrupts when reloading SS. The 286 and higher processors only
disable interrupts after a MOV SS, in contrast to earlier CPUs, including
the NECs, who do this with all MOV sreg,op instructions.

An unsolvable problem occurs when an unmaskable interrupt or exception
takes place while executing the instruction pair on an old 88 or 86.
There are conflicting messages though about this type of interrupts having
no effect on the bug.

This bug is so well known that I doubt you'll be able to find software that don't have the workaround for it (including software never intended for the 808x class of PCs).