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Comment by burnt-resistor | Hacker News Reader

Very cool.

It's all about that 80-bit/82-bit floating point format with the explicit mantissa bit just to be extra different. ;) Not only is it a 1:15:1:63, it's (2(tag)):1:15:1:63, whereas binary64 is 1:11:0:52. (sign:exponent [biased]:explicit leading mantissa bit stored?:manitissa remaining)

Other pre-P5 ISA idiosyncrasies: Only the 8087 has FDISI/FNDISI, FENI/FNENI. Only the plain 287 has a functional FSETPM. Most everything else looks like a 387 ISA-wise, more or less until MMX arrived. That's all I know.

I'm curious what the CX-83D87 and Weiteks look like.

Keep up the good work!

PS: Perhaps sometime in the (near) future we might get almost 1:1 silicon "OCR" transcription of die scans to FPGA RTL with bugs and all?

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> I'm curious what the CX-83D87 and Weiteks look like.

The Weitek's were memory mapped. (At least those built for x86 machines.).

This essentially increased bandwidth by using the address bus as a source for floating point instructions. Was really a very cool idea, although I don't know what the performance realities were when using one.

http://www.bitsavers.org/components/weitek/dataSheets/WTL-31...

This is nuts, in the best way.

The operand fields of a WTL 3167 address have been specifically designed so that a WTL 3167 address can be given as either the source or the destination to a REP MOVSD instruction. [

Single-precision vector arithmetic is accomplished by applying the 80386 block move instruction REP MOVSD to a WTL 3167 address involving arithmetic instead of loading or storing.

haha - took me a while to figure out that's Mauro Bonomi's signature

iirc the 3167 was a single clocked, full barrel shift mac pipeline with a bunch (64?) of registers, so the FPU could be driven with a RISC-style opcode on every address bus clock (given the right driver on the CPU) ... the core registers were enough to run inner loops (think LINPACK) very fast with some housekeeping on context switch of course

this window sat between full PCB minicomputer FPUs made from TTL and the decoupling of microcomputer internal clocks & cache from address bus rates ...

Weitek tried to convert their FPU base into an integrated FPU/CPU play during the RISC wars, but lost

A significant difference in 80387 versus 80287 & 8087 was that in 387 you could no longer select the "projective" behavior for infinities (where positive and negative infinities are identical).

This feature had not been included in the IEEE standard, so it was no longer implemented.

Testing whether this feature works or not was used in the programs running on an 80386 CPU to detect whether the attached FP coprocessor was a 287 or a 387 (because the hardware allowed both; 387 was launched later than 386, so initially a 386 had to be coupled with a 287, if a hardware FPU was needed).

That 80/82 bit format turned out to be problematic, as it was only used internally, and stored as 64 bits in memory. This meant that the same calculation could give different results based on whether an intermediate value had spilled to memory or not.

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