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| Authored by: Anonymous on Sunday, March 31 2013 @ 12:33 PM EDT |
There *was* a 80487SX 'FPU'. I remember it well, because
it is the only chip 'missing' in my collection of the Intel x86-
ish chips :-/
For the 8088 and 8086 'the difference' was in the bus-
width: 8-bits external databus for 8088, 16-bits for 8086.
The FPU for both was the 8087. The 8087 was quite
expensive; About -estimated- 10 times the price of the
CPU.
Let's 'forget' about the 80188/80186 (and variations...) for
a moment.. I think they also used the 8087 FPU.
For the 80286 there was only the 80287 FPU, no buswith
tricks or SX/DX versions (yet).
For the 80386, the SX/DX naming came into 'full play':
80386SX was externally 16-bits data bus, and address bus
limited to 16 MBytes address space. The 80386DX has a
full 32-bits wide external data bus and 32-bits address
space. Both had a companion FPU, the 80387SX and
80387DX respectively.
With the '486, the SX/DX was used to indicate if the chip
has an internal FPU. the 80486DX does, the 80486SX does
not have a built-in FPU.
The 80487SX *does* exist, but: it was extremly rare and
an 'oddball': On a likewise 'rare' special 80486SX/80487SX
supporting motherboard that supports such (a
motherboard with sockets for two of those 'big' chips..) you
could 'add' the 80487SX getting a 80486SX/80487SX
combination that does have a hardware FPU.
There is however a snag: The 80486-s, both SX and DX,
also have an on-chip cache. That -for the time- was very
tricky to integrate with an external FPU (and FPU-only..).
The creative solution was that the 80487SX was 'a bit'
more than a FPU: it was basically a complete 80486DX.
On a 80486SX/80487SX combi-motherboard, the 80486SX
was completely de-activated/halted, and the 'FPU' was
doing *all* the work (instruction processing, integer and
floating-point).
The 80487SX was very hard to spot in the wild, and -in
Intel's good tradition of expensive FPU's- much more
expensive than the 80486DX. I did 'spot' a 80487SX on a
computer 'dump' *once*, long after 'the time' of the
80486, but even then, the thing was still too expensive for
me...
From 80586 on, the FPU was always integrated into the
processor chip.[ Reply to This | Parent | # ]
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- a minor nit - Authored by: Anonymous on Tuesday, April 02 2013 @ 03:24 AM EDT
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