Changing the FPU in a Sun 3 System:

Fortunately the 68882 FPU and the 68881 FPU are functionally and pin-out compatible.

The standard FPU for the Sun3-Series is the MC68881. This chip can be replaced by the Pin and Command compatible MC68882 FPU. Just take the 68881 out of it's socket and put a 68882 in. make sure you have the chip pointing in the same direction as the one you pulled out.

The FPU I removed was rated for 16Mhz. The FPU I replaced was rated for 25Mhz. This will become important when I move into my next phase of hardware hacking - the CPU replacement.

The change-over to a 68882 gains up to 60% Float-Performance or more because the 68882 has a better cycle-time for several FPU-Commands. However, most usage performance is negliable at this time since very little on the Sun 3/160 is using the FPU.

The Operation

The removal of the CPU board (501-1208 4MB Sun 3004 CPU, 16.67MHz 68881) went smoothly, sorry, I didn't take any pictures (I'll have to do so next time). There are a number of jumpers on the board - if anyone knows what these jumpers do, contact me!

I viewed 2 different crystals on the CPU half of the main board (the lower half is consumed by RAM), one is 16Mhz exactly and the other is 33.333Mhz. One of these oscilators controls the CPU rate, and I suspect it's the 33.333Mhz, since oscilator rate is usually halved for CPU clock. For example, on the SUN 3/60, the 20Mhz chip is run by a 40Mhz crystal. The other factor that allowed me to make this determination is that 33.333 halved is EXACTLY 16.67 (the actual rate of the SUN's CPU). Hey, it ain't rocket science!

This gives us an intermediary potential for overclocking the original 16Mhz 68020 simply by replacing this 33Mhz crystal with one of a higher rate. It also gives us a fallback option should my 33Mhz 68030 modification fail.

I'll have to start looking for higher clocked crystals and I'll also keep an eye out for a higher clock rated 68020 since overclocking puts severe strain on lower rated CPU's.