🚀 go-pugleaf

[In the message entitled "Re: SCSI Woes, revisited" on May  9, 18:00, randy hyde writes:]
> MOVD vs. MOVMD (My understanding is that MOVMD is *not* interruptable, that's
> why it was limited to 16 bytes).  Perhaps the execution times of four MOVD
> instrs and one MOVMD instruction are the same.  However, the MOVMD instruction
> is more compact so it impacts the instr cache less.  MOVMD would seem to be
> a better choice in this regard.

Nope. MOVMD is much slower than MOVD for a number of subtle ( and not so
subtle) reasons. MOVSD is also slower than MOVD (modulo the cache effects).
I ran quite a few simulations before the design of the PC532 to figure out
what was best, and why. The effects of the cache, *IOINH, DRAM, and SCSI
are complex (at the best of times, and undocumented in the worst).

Required reading is the NS32532 Data sheet (with instruction timing, or
the NS32GX32 data sheet if you can't find a 532 sheet with timing);
an application note "Using Memory-Mapped I/O with the NS32GX32 or NS32532";
an application note "Series 3200 Instruction Execution Times"; and the
PC532 theory of operation.

This still won't tell you everything (such as *IOINH invalidating the 
instruction cache - nothing is *ever* mentioned about this!). But you will
have better information than a gut feeling... And once you think you know
everything there is to know, hook the whole thing up to a logic analyzer
and find out what it REALLY does (bet you get as many surprises as we
did!).

> 
> Alas, I've been unable to test these two theories out, it seems my drive
> tops out somewhere between 700K-1Mbyte/sec transfer rate (when transferring
> 64K blocks).  Every trick I try to speed it up fails.  The disk transfer
> rate seems to be the limiting factor.  However, I don't want to use this
> as an excuse to stop trying, I may buy a faster SCSI-II drive some day.
> *** Randy Hyde

The current scheme will drive the SCSI bus to 3.8-3.9 megabytes per second
in ASYNC mode (assuming the SCSI device is fast enough - so far only another
PC532 has been fast enough to sustain this rate). In SYNC mode, it should
go to nearly 5 megabytes per second, sustained. That is as fast as the SCSI
chips can support - there is no point trying to go faster.



-- 
Dave Rand
{pyramid|mips|bct|vsi1}!daver!dlr	Internet: dlr@daver.bungi.com