GoKawiilNot too long ago I had a need and an opportunity to re-acquaint myself with the mechanism used for software emulation of the 8087 FPU on 8086/8088 machines.
As mentioned elsewhere, the 8086 CPU (1978) had a generic co-processor interface first utilized by the Intel 8089 I/O processor (1979) and later the Intel 8087 FPU (1980), initially called the Numeric Processor Extension or NPX.
The 8087 was a somewhat expensive add-on, assuming that a given system actually had a socket to plug the 8087 into (IBM PCs did, but other 8086/8088 systems did not necessarily have one). There was a largish class of software which could significantly benefit from the 8087 (e.g. spreadsheets), but in the era of shrink-wrapped software, there was a significant incentive to ship software which could use an 8087 when present, yet would still run on a bare 8086/8088 machine with no FPU.
There was also a desire to develop and test floating-point software without having to install an 8087 into every system. Given the initial limited availability of 8087 chips, it was in Intel’s best interest to give developers a way to write 8087 software without requiring 8087 hardware.
Intel released the E8087 software emulation package together with the 8087 chip. This is evidenced by the original Numerics Supplement to The 8086 Family User’s Manual from July 1980, Intel order no. 121586-001. The Numerics Supplement outlines how the E8087 package works. Actually there were two packages — the full E8087 library, and also a “partial” PE8087 library which implemented just enough functionality for Intel’s PL/M language tools. Intel’s PL/M compiler was the first high-level language translator capable of utilizing the 8087.
Because the 8086 had no facility for emulating an FPU (unlike the 80286 and later processors), the emulation mechanism was somewhat complex and required tight cooperation of assemblers/compilers, linkers, and run-time libraries.
Assembler/Compiler – Intel Original
The assembler or compiler generated “emulatable” 8087 code. The translator in fact produced normal 8086/8087 code, but the object modules included special fix-ups for every 8087 ESC instruction and for (F)WAIT.
Early on, Intel established the convention that the WAIT mnemonic was translated directly to the WAIT opcode, while the FWAIT mnemonic could be emulated.
A key fact is that the language translator did not directly produce 8087 emulation code. It only prepared object modules for emulation while emitting regular 8087 instructions, and the actual decision whether to emulate or not was made at link time.
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