How the Intel 8087 FPU Knows Which Instructions to Execute
#Intel8087
![<div><img alt="" class="attachment-large size-large wp-post-image" height="596" src="https://hackaday.com/wp-content/uploads/2026/02/8087_decoding-labeled_ken_shirriff.jpg?w=800" style="margin: 0 auto; margin-bottom: 15px;" width="800" /></div><p>An interesting detail about the Intel 8087 floating point processor (FPU) is that it’s a co-processor that shares a bus with the 8086 or 8088 CPU and system memory, which means that somehow both the CPU and FPU need to know which instructions are intended for the FPU. Key to this are eight so-called <code>ESCAPE</code> opcodes that are assigned to the co-processor, as explained <a href="https://www.righto.com/2026/02/8087-instruction-decoding.html" target="_blank">in a recent article</a> by [Ken Shirriff].</p>
<p>The 8087 thus waits to see whether it sees these opcodes, but since it doesn’t have access to the CPU’s registers, sharing data has to occur via system memory. The address for this is calculated by the CPU and read from by the CPU, with this address registered by the FPU and stores for later use in its BIU register. From there the instruction can be fully decoded and executed.</p>
<p>This decoding is mostly done by the microcode engine, with <a href="https://hackaday.com/2026/01/11/the-intel-8087-and-conditional-microcode-tests/">conditional instructions</a> like <code>cos</code> featuring circuitry that sprawls all over the IC. Explained in the article is how the microcode engine even knows how to begin th](https://files.mastodon.social/cache/media_attachments/files/116/111/545/493/411/586/original/40a5af0322ba796a.jpg)
How the Intel 8087 FPU Knows Which Instructions to Execute https://hackaday.com/2026/02/21/how-the-intel-8087-fpu-knows-which-instructions-to-execute/
#Retrocomputing #ReverseEngineering #Intel8087
Conditions in the Intel 8087 floating-point chip's microcode
https://www.righto.com/2025/12/8087-microcode-conditions.html
#HackerNews #Intel8087 #FloatingPoint #Microcode #Technology #History #ComputerArchitecture
Extracting ROM constants from the 8087 math coprocessor's die
http://www.righto.com/2020/05/extracting-rom-constants-from-8087-math.html
#Intel8087 #FloatingPointArithmetic #ComputerHistory #Math #Intel
Client Info
Server: https://mastodon.social
Version: 2025.07
Repository: https://github.com/cyevgeniy/lmst