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Reverse Engineering the Intel 386 Processor's Register Cell

posted by hubie on Monday November 13 2023, @05:29AM   Printer-friendly

owl writes:

https://www.righto.com/2023/11/reverse-engineering-intel-386.html

The groundbreaking Intel 386 processor (1985) was the first 32-bit processor in the x86 line. It has numerous internal registers: general-purpose registers, index registers, segment selectors, and more specialized registers. In this blog post, I look at the silicon die of the 386 and explain how some of these registers are implemented at the transistor level. The registers that I examined are implemented as static RAM, with each bit stored in a common 8-transistor circuit, known as "8T". Studying this circuit shows the interesting layout techniques that Intel used to squeeze two storage cells together to minimize the space they require.

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  • (Score: 4, Informative) by maxwell demon on Monday November 13 2023, @08:17PM

    by maxwell demon (1608) on Monday November 13 2023, @08:17PM (#1332792) Journal

    When I first understood how those machines worked, how they overlapped the addressing so that 32 bits could only address a 20 bit space, I thought there must be a good reason for the waste of having so very many different ways to address the same byte of RAM, but I couldn't think of any.

    The reason for the 8086 segmented architecture was backwards compatibility. While the 8086 was not machine code compatible to the 8085, it was still possible to automatically translate most machine code. This was in part thanks to the segmented architecture. Basically a program could ignore that there were more than 16 bits of address space.

    Also note that back in that time, 1 MB was a huge amount of memory.

    --
    The Tao of math: The numbers you can count are not the real numbers.
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