First post, by aries-mu
Hi all!
I wonder what would have happened in those good old 486 systems with both CPU and FPU sockets if this scenario were to happen:
• 486 SX CPU replaced with a 486 DX one
• 487 FPU installed in its socket
We would have then 1 CPU and 2 FPUs!
1) Would non-multiprocessing operating systems (like DOS) be able to "see" both the FPUs? Indeed, it wouldn't be a multi-CPU system
2) Would FPU-capable applications (like Excel, AutoCAD) be able to "use" both the FPUs under a non-multiCPU operating system like MS-DOS?
3) Would FPU-capable applications (like Excel, AutoCAD) be able to "use" both the FPUs under a multiCPU operating system like Windows NT?
4) Would a multiCPU operating system like Windows NT be able to "see" both the FPUs?
If the answer is NO to all of the above, which of the following would apply?
...a) The system would crash / not boot
...b) The system would work and both OS and apps would ignore the external 487 FPU
...c) The system would work and both OS and apps would ignore the CPU-integrated internal FPU, using only the external 487 FPU
...d) The system would work and the apps would ask which FPU to use (highly unlikely I guess)
In any case, I wonder: by keeping EVERYTHING ELSE identical (even CPU clock, for example a 33 MHz, same internal and external cache, same RAM, etc.), for IDENTICAL tasks which of the following would produce better floating-point performances?
.......i) 486 SX 33 + 487 33 FPU
.......ii) 486 DX 33
My guess would be ii), because for sure an internal-CPU-integrated FPU must have faster CPU==FPU bus/frequency and maybe even bandwidth in terms of bit... I kind of have a semi-subconscious reminiscence as though a CPU==FPU internal bus would be a 128 bit channel??
.......iii) As a variation of points i) and ii), even though the bits of the internal CPU==FPU bus of the DX are the same as the external FPUed bus, what about a 486 SX2 66 + 487 66 FPU (overclocked from a 40 or 50 MHz if not available, and assuming the overclock performance and efficiency would be identical to that of a native 487 66 MHz FPU if it existed officially) vs 486 DX2 66 comparison? (yes, I saw old computer magazines with ads of stores selling 486 SX2 CPUs, so they should be real). In this scenario, the external bus surely has half the speed of the internal CPU clock. So, my guess is that a 486 DX2 66 would perform better, as CPU and FPU would always communicate at 66 MHz, while in the case of 486 SX 66 + 487 66 the data would have to pass through the motherboard, which bus would be clocked at 33 MHz, correct?
.......iv) From which the question: when a 486 CPU is clock-multiplied (let's say DX2s and DX4s), does the internal FPU operate before or after the internal clock multiplier, aka does it operate at the bus or at the CPU frequency?
In my opinion, the answers are:
• From point 1 to point d → Point c) is the one
• Point ii is true
• Point iii → true is that the DX2 66 is faster
• Point iv → FPU operates at the same clock as the CPU
What do you think?
Thanks so much all!
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