[HN Gopher] The Pentium FDIV bug, reverse-engineered
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The Pentium FDIV bug, reverse-engineered
Author : croes
Score : 84 points
Date : 2024-12-11 18:33 UTC (2 days ago)
(HTM) web link (oldbytes.space)
(TXT) w3m dump (oldbytes.space)
| perdomon wrote:
| This dude pulled out a microscope and said "there's your
| problem." Super impressive work. Really great micro-read.
| layer8 wrote:
| To be fair, he knew what the problem was (errors in the lookup
| table) beforehand.
| Cumpiler69 wrote:
| This is probably one of the reasons Intel went to a microcode
| architecture after.
|
| I wonder how many yet to be discover silicone bugs are out there
| on modern chips?
| KingLancelot wrote:
| Silicone is plastic, Silicon is the element.
| wmf wrote:
| They always used microcode:
| https://www.righto.com/2022/11/how-8086-processors-microcode...
|
| I'm not sure when Intel started supporting microcode updates
| but I think it was much later.
| Lammy wrote:
| Older Intel CPUs were already using microcode. Intel went after
| NEC with a copyright case over 8086 microcode, and after AMD
| with a copyright case over 287/386/486 microcode:
|
| - https://thechipletter.substack.com/p/intel-vs-nec-the-
| case-o...
|
| - https://www.upi.com/Archives/1994/03/10/Jury-backs-AMD-in-
| di...
|
| I would totally believe the FDIV bug is why Intel went to a
| _patchable_ microcode architecture however. See "Intel P6
| Microcode Can Be Patched -- Intel Discloses Details of Download
| Mechanism for Fixing CPU Bugs (1997)"
| https://news.ycombinator.com/item?id=35934367
| kens wrote:
| Intel used microcode starting with the 8086. However, patchable
| microcode wasn't introduced until the Pentium Pro. The original
| purpose was for testing, being able to run special test
| microcode routines. But after the Pentium, Intel realized that
| being able to patch microcode was also good for fixing bugs in
| the field.
| molticrystal wrote:
| The story was posted a couple days ago and ken left a couple
| comments there: https://news.ycombinator.com/item?id=42388455
|
| I look forward to the promised proper write up that should be out
| soon.
| Thaxll wrote:
| So nowdays this table could have been fixed with a microcode
| update right?
| jeffbee wrote:
| With a microcode update that ruins FDIV performance, sure. Even
| at that time there were CPUs still using microcoded division,
| like the AMD K5.
| mega_dingus wrote:
| Oh to remember mid-90s humor
|
| How many Intel engineers does it take to change a light bulb?
| 0.99999999
| jghn wrote:
| An anecdote regarding this bug that always cracks me up. My
| college roommate showed up with a shiny new pentium machine that
| year, and kept bragging about how awesome it was. We used some
| math software called Maple that was pretty intensive for PCs at
| the time, and he thought he was cool because he could do his
| homework on his PC instead of on one of the unix machines in the
| lab.
|
| Except that he kept getting wrong answers on his homework.
|
| And then he realized that when he did it on one of the unix
| machines, he got correct answers.
|
| And then a few months later he realized why ....
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(page generated 2024-12-13 23:00 UTC)