There's memory bandwidth and there's memory transaction thruput

sqrmax at sqrmax at
Sat Feb 13 19:27:56 UTC 1999


>I've long wondered why Squeak runs better on some processors
>than others. Some of the surprises have been:
>  4. it runs really slowly on 386/486 and 68K machines
>     (almost certainly due to insufficient registers)

The 68K has much more registers than the intel processors. Intel ones (386 
and up) have 4 general use registers named A, B, C and D (with suffix X 
meaning 16 bits, with extra E prefix meaning 32 bits, or with H and L suffixes to 
mean lower 8 bits and higher 8 bits of the first 16 bits), then segment and 
offset registers named (add an E prefix as needed for their 32 bit sibling in 
protected or real mode) CS, DS, SS, ES, DI, SI, SP, BP, FS and GS. These last 
registers have limited arithmetic capabilities, especially the ones ending 
in S (for segment). The 68K has 8 32bit data registers (d0 to d7) and 8 32bit 
index registers (a0 to a7) fully arithmetic capable (to my knowing). I think 
the 68K with its variants lacks the raw speed of the Intels, although it's 
much more maneuverable. I'd like to see intel processors with some of the 68K 
characteristics (like asynchronous IO for instance, why do you think intel 
machines need FIFOs everywhere?).

Intel processor based motherboards for the old processors lack a lot of 
bandwidth, esp at the data bus. I'd also like some stuff taken right off Amiga 
500 motherboards... *25* DMA channels for instance A bus master synchronizing 
chip in charge of the DMAs, with a memory bus running at 4x the clock of the 
cpu. Custom chips to leave the cpu number crunching alone. No wonder it's 
such a great piece of hardware.

I feel that Intel machines are like a non-turn-off-able rocket in a twisted 
road, they keep crashing at the curves but go incredibly fast fast in the 
few feet the road is straight. 68Ks are like a Ferrari, not the most powerful 
engine, and so much slower than a rocket,  but excellent grip and finesse. The 
rocket is so fast... and crashes so bad all the time...


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