kens

51 ms·

1. 811.▲
   by kens 3y ago

   The Intel 8087 math coprocessor (1980) had too much microcode to fit into a standard ROM. It was implemented with a special ROM that stored two bits per transistor by using four different transistor sizes, almost doubling the density. Each
2. 812.▲
   by kens 3y ago

   Thanks!
3. 813.▲
   by kens 3y ago

   Yes, I cut-and-pasted that paragraph from the 286 manual and clearly I shouldn't have trusted the OCR :-)
4. 814.▲
   by kens 3y ago

   Thanks, I've fixed the typo.
5. 815.▲
   by kens 3y ago

   Since the decoder is implemented in a PLA-style matrix, you don't save any space by omitting transistors. So that's not a motivation. Random thing: chip transistor counts usually count "transistor sites" rather than phys
6. 816.▲
   by kens 3y ago

   I hadn't noticed that characteristic of 8F. Looking at the microcode, the subopcode bits are ignored, so I think your analysis is correct. I agree that it's very weird that PUSH rm and POP rm are arranged completely differently. T
7. 817.▲
   by kens 3y ago

   Thanks! It's fixed now.
8. 818.▲
   by kens 3y ago

   My transistor-level simulator shows that the 8086 will generate a divide by 0 interrupt if you give the AAM instruction a divisor of 0. But I haven't tried this on a real chip.
9. 819.▲
   by kens 3y ago

   You can click on the three dots (at least in desktop YouTube) and then "Show Transcript", and then you have a transcript that you can skim.
10. 820.▲
    by kens 3y ago

    A computer architecture book such as Hennessy and Patterson will describe microcode in detail. It's a bit confusing because microcode has changed meaning a bit over time. "Classical" microcode, such as the 8086, replaces hard
11. 821.▲
    by kens 3y ago

    When I'm done with the 8086, I might look at the 68000 but I don't have any specific plans.
12. 822.▲
    by kens 3y ago

    There's a reason why every 8086 opcode matches something. If an instruction didn't match anything, I think the microcode engine would spin idle and the instruction would never terminate. (You need a RNI micro-instruction to end mi
13. 823.▲
    Undocumented 8086 instructions, explained by the microcode(righto.com)

    242 points by kens 3y ago|99 comments
14. 824.▲
    by kens 3y ago

    But it's a programming challenge :-) My quick Python program gets 4 solutions out of 1296 possibilities: ['RGBY', 'GRYB', 'BYRG', 'YBGR'] So the answer is that each position has 25% chance of the
15. 825.▲
    by kens 3y ago

    I'd use brute force, since there aren't that many possibilities.
16. 826.▲
    by kens 3y ago

    There's a lot of discussion here about getting tested for the gene, but the gene isn't as important as you might think from the title. The article states that the gene is associated with "1.6-fold higher odds of developing lo
17. 827.▲
    by kens 3y ago

    I haven't looked at the signals from that perspective, but I'd say that both sets of signals are derived from various internal signals that are more fundamental.
18. 828.▲
    by kens 3y ago

    I haven't really timed it let alone optimized it, but about 50 clock cycles per second. If I had a nice graphical display like Visual 6502, it would be way slower.
19. 829.▲
    by kens 3y ago

    No worries! I gave up on stressing about the spelling years ago :-)
20. 830.▲
    by kens 3y ago

    There's not really much to the minimum and maximum modes. You can think of it as multiplexers on the relevant pins, selecting the signal for the appropriate mode. (Of course, the logic isn't quite that clean but that's the ba
21. 831.▲
    by kens 3y ago

    Thanks, I've fixed that section since I kind of mangled it.
22. 832.▲
    by kens 3y ago

    Yes, the simulator is extremely helpful for analysis. I started doing the analysis on paper, but it's very easy to make a mistake and end up confused. The simulator is also very helpful when trying to understand complicated state machi
23. 833.▲
    by kens 3y ago

    Thanks! I'm trying to do what I can to preserve this history and knowledge.
24. 834.▲
    by kens 3y ago

    I have a transistor-level 8086 simulator that mostly works but needs some cleanup and bug fixes. For now, I'm concentrating on analysis of the 8086 rather than finishing the simulator.
25. 835.▲
    by kens 3y ago

    Author here if anyone has questions.
26. 836.▲
    by kens 3y ago

    I don't understand your math. 0.01% of 60,000 is 6.
27. 837.▲
    by kens 3y ago

    The die is 620x680 mil. https://archive.org/details/guidetoriscmicro0000unse/page/26...
28. 838.▲
    by kens 3y ago

    My officemate Paul Heckbert wrote a ray tracer in PostScript after we realized that the laser printer's processor was much more powerful than our "real" computer. As you might expect, the interpreted PostScript ray tracer was
29. 839.▲
    by kens 3y ago

    Of course I have a source :-) https://www.google.com/books/edition/Avionic_Systems_Design/... It was upgraded from the HAC-32 (Hughes system with the i960 MX) to PowerPC around 2002. See page I-12 of https:&
30. 840.▲
    by kens 3y ago

    That's a nice collection of documents on the i960 RM and RN chips.