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owl [soylentnews.org] writes:

In 1985, Intel introduced the groundbreaking 386 processor, the first 32-bit processor in the x86 architecture. To improve performance, the 386 has a 16-byte instruction prefetch queue. The purpose of the prefetch queue is to fetch instructions from memory before they are needed, so the processor usually doesn't need to wait on memory while executing instructions. Instruction prefetching takes advantage of times when the processor is "thinking" and the memory bus would otherwise be unused.

In this article, I look at the 386's prefetch queue circuitry in detail. One interesting circuit is the incrementer, which adds 1 to a pointer to step through memory. This sounds easy enough, but the incrementer uses complicated circuitry for high performance. The prefetch queue uses a large network to shift bytes around so they are properly aligned. It also has a compact circuit to extend signed 8-bit and 16-bit numbers to 32 bits. There aren't any major discoveries in this post, but if you're interested in low-level circuits and dynamic logic, keep reading.

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