Microcode in Intel's 8087 floating-point chip: the scale instruction
Intel's 1980 8087 floating-point coprocessor established a standard for accurate arithmetic that remains influential in modern computing. The Opcode Collective is reverse-engineering the chip's complex microcode to understand its internal operations. This article specifically analyzes the FSCALE instruction, revealing that it requires over 140 micro-instructions to handle various special cases efficiently.
- ▪The Intel 8087 coprocessor was introduced in 1980 to provide accurate floating-point arithmetic for the IBM PC.
- ▪The FSCALE instruction scales a number by a power of two and is implemented using over 140 micro-instructions.
- ▪The 8087 chip contains a microcode ROM with 1648 micro-instructions that control its operations.
- ▪The chip's datapath is divided into a 16-bit section for exponents and a 64-bit section for significands.
- ▪Researchers from the Opcode Collective are using high-resolution microscopy to reverse-engineer the 8087's microcode.
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| Original publisher | Righto |
| Canonical URL | https://www.righto.com/2026/09/8087-microcode-reverse-engineering-fscale.html |
| Publication time | Sat, 12 Sep 2026 15:49:52 +0000 |
| Retrieval time | 2026-09-12T17:49:59.091Z |
| Last seen | 2026-09-12T17:49:59.091Z |
| Headline source | Publisher (no WeSearch rewrite) |
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| Summary source text | contentText |
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| Cluster | aZUD7fCxeHWF · 1 stories |
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| Commercial reuse | May the content be reused commercially? | Not permitted |
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In the 1970s, floating-point arithmetic was a mess. Computer manufacturers had a dozen incompatible arithmetic standards. Moreover, floating-point systems were designed around hardware simplicity rather than mathematical rigor, leading to problems with numerical stability. This changed when Intel introduced the 8087 floating-point coprocessor chip in 1980, designed to be as accurate as possible, even in the corner cases. The 8087 became popular because it could be installed in the IBM PC, making floating-point operations up to 100 times faster in applications ranging from spreadsheets to CAD. But more importantly, the 8087 became the floating-point standard used by most computers today. The 8087 implemented its instructions in complex low-level code called microcode.
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