Analyzing the FScale Instruction in Intel’s 8087 FPU
One of these, the FSCALE instruction turned out to be far more complicated than assumed, with one might assume to be a straightforward powers-of-two scaling turning out to entail over 140 micro-instructions and three levels of sub-routine calls just to handle all cases. The annotated die shot in the heading image shows the functional blocks that are used by this one x87 instruction, to give some kind of idea of what amount of hardware even ‘just’ scaling a floating point number involves. Much like with the x86’s CISC-style ISA, these 8087 instructions break down into individual steps that involve everything from loading values into registers, performing operations, checking for and handling error conditions as well as stack management.
- ▪One of these, the FSCALE instruction turned out to be far more complicated than assumed, with one might assume to be a straightforward powers-of-two scaling turning out to entail over 140 micro-instructions and three levels of sub-routine c
- ▪The annotated die shot in the heading image shows the functional blocks that are used by this one x87 instruction, to give some kind of idea of what amount of hardware even ‘just’ scaling a floating point number involves.
- ▪Much like with the x86’s CISC-style ISA, these 8087 instructions break down into individual steps that involve everything from loading values into registers, performing operations, checking for and handling error conditions as well as stack
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| Original publisher | Hackaday |
| Canonical URL | https://hackaday.com/2026/09/13/analyzing-the-fscale-instruction-in-intels-8087-fpu/ |
| Publication time | Sun, 13 Sep 2026 14:00:33 +0000 |
| Retrieval time | 2026-09-13T14:01:49.992Z |
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Opening excerpt (first ~120 words) tap to expand
Analyzing The FScale Instruction In Intel’s 8087 FPU No comments by: Maya Posch September 13, 2026 Title: Copy Short Link: Copy During his continuing analysis of the architecture and microcode of Intel’s highly influential 8087 floating point unit (FPU) co-processor, [Ken Shirriff] has now arrived at the point where he can put together how the 8087’s microcode implements various x87 instructions. One of these, the FSCALE instruction turned out to be far more complicated than assumed, with one might assume to be a straightforward powers-of-two scaling turning out to entail over 140 micro-instructions and three levels of sub-routine calls just to handle all cases.
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