2025-05-02


Horseshit


Info Rental / ShowBiz / Advertising

TechSuck / Geek Bait

  • HN Jobs:

  • The absurdly complicated circuitry for the 386 processor's registers

    the circuitry that implements the 386's registers is much more complicated than one would expect. For the 30 registers that I examine, instead of using a standard circuit, the 386 uses six different circuits, each one optimized for the particular characteristics of the register. For some registers, Intel squeezes register cells together to double the storage capacity. Other registers support accesses of 8, 16, or 32 bits at a time. Much of the register file is "triple-ported", allowing two registers to be read simultaneously while a value is written to a third register. Finally, I was surprised to find that registers don't store bits in order: the lower 16 bits of each register are interleaved, while the upper 16 bits are stored linearly.

    The 386's register circuitry also shows the curse of backward compatibility. The x86 architecture supports 8-bit register accesses for compatibility with processors dating back to 1971. This compatibility requires additional circuitry such as the shuffle network and interleaved registers. Looking at the circuitry of x86 processors makes me appreciate some of the advantages of RISC processors, which avoid much of the ad hoc circuitry of x86 processors.

Trump

Left Angst