[HN Gopher] Two interesting XOR circuits inside the Intel 386 pr...
___________________________________________________________________
Two interesting XOR circuits inside the Intel 386 processor
Author : _Microft
Score : 77 points
Date : 2023-12-16 17:26 UTC (1 days ago)
(HTM) web link (www.righto.com)
(TXT) w3m dump (www.righto.com)
| kens wrote:
| Author here for any questions. Hopefully you're not tired of the
| 386 yet!
| h2odragon wrote:
| Not at all tired of the 386; an unrelated question comes up
| tho.
|
| Have you done much on "high power" semiconductors yet?
|
| It occurs to me that we're throwing around amps on ICs now that
| would've been well past "magic smoke" territory not all that
| long ago; perhaps you might find some ESC chips or something
| that demonstrate how this progress is being made?
| kens wrote:
| I've looked at a few higher-power chips, such as the
| venerable 7805 voltage regulator, but I don't know if that's
| high-power enough for you :-) There's no high-power secret
| inside that chip except it uses large transistors.
|
| https://www.righto.com/2014/09/reverse-engineering-
| counterfe...
| rzzzt wrote:
| Parent might be thinking of those Gallium-Nitride FETs that
| enable manufacturers to build suspiciously small power
| bricks.
| ace2358 wrote:
| I thought that was due to the higher switching frequency
| of the power transistors, perhaps unrelated to higher
| current draw in the transistor?
| ksaj wrote:
| If you do a writeup on the prefetch queue and want some
| original code for tampering with it, let me know. I did some
| deep experimentation with it in the 90's, and still have the
| code archived.
|
| The prefetch queue could be (ab)used in some very interesting
| and sometimes baffling ways. One of the creations causes a
| divide by zero if you try to debug it, even if viewing the code
| doesn't look like it should.
|
| Also, I did some experimenting with shrinking code by replacing
| INT3h with INT21h (one byte, versus two), which similarly lent
| itself to anti-debugger effects.
| kens wrote:
| It sounds like you did some very interesting experimentation
| with the prefetch queue. Realistically, I'm unlikely to
| investigate the 386 prefetch queue to that level of detail,
| but if I do, your tests would be useful.
| mmastrac wrote:
| Is the self-test exposed in any interesting ways? Any
| interesting JTAG-like interfaces that could be explored?
| kens wrote:
| I was thinking of writing about the self-test circuitry. It
| doesn't expose things in a JTAG-like way, which came later.
| The main self-test is that if you boot the chip with the BUSY
| pin set, it goes through a self-test of all the PLA and
| microcode entries, accumulating a signature using linear-
| feedback shift registers. It then XORs with the correct value
| and writes the result to a register. So the visible result is
| you see 0 in a register, not too exciting.
|
| There are also self-test instructions to do things such as
| write entries to the TLB and read entries, to make sure it is
| operating correctly.
|
| For details, there's a paper on "Design and Test of the
| 80386" by Pat Gelsinger, who is now CEO of Intel.
| https://ieeexplore.ieee.org/document/4069991 (paywalled).
| freshbob wrote:
| I'm always amazed at how people can make out anything without
| having the individual metal layers present as well to show
| vertical and horizontal connections in these die photographs.
| kens wrote:
| The 386 has two layers of metal, which makes it an order of
| magnitude more difficult to reverse engineer. I don't have a
| good process for removing one layer of metal at a time, so I
| end up having to puzzle over faint patterns in the metal to
| determine what is going on.
|
| The two layers of metal also make it much harder to make
| diagrams that show what is going on in the circuit without
| turning into a tangle of lines. In this article, I decided not
| to try, and went straight to schematics.
| boringuser2 wrote:
| The sisyphean efforts of chip designers kind of amaze me in the
| sense that all of this is lurking just beneath the surface of the
| computing revolution, and will one day be outdated or simply lost
| for good.
___________________________________________________________________
(page generated 2023-12-17 23:00 UTC)