🚀 go-pugleaf

In article <PRENER.92Dec12010123@prener.watson.ibm.com> prener@watson.ibm.com (Dan Prener) writes:
>In article <1992Dec11.060038.27494@netcom.com> norm@netcom.com (Norman Hardy) writes:
>
>> How about sharing expensive functional units (multiply, divide, 
>> DES) between processors on the same chip. This gives the economic 
>> advantages of heterogeneous processors (division of labor) without 
>> the software pain. The processors would appear to the operating 
>> system as CPUs sharing memory. Indeed this approach makes special 
>> purpose hardware more attractive as functional units.
>
>Doesn't that make saving the state of a processor somewhat complicated?
>--
>                                   Dan Prener (prener@watson.ibm.com)

I see no need to change the programmer's model in order to share 
functional units. Functional units have no state if you interrupt 
between instructions as in most architectures. On processors such 
as the 88K the floating point units are frozen upon interrupt so 
that interrupt latency may be less than the divide latency. At 
worst this can be handled by providing a register per processor 
per functional unit to capture the results of operations issued 
by the interrupted processor. The interrupt hardware might even 
be cagey and favor for interruption a processor with no 
outstanding instructions in shared units.

The 88K has a systolic multiplier and I imagine that few programs 
issue multiples more than 10% of the cycles. I imagine that 
sharing such a multiplier between four instruction streams would 
produce little interference. Perhaps the 88K has too much 
multiply hardware. I don't know the tradeoffs.