🚀 go-pugleaf

In article <22011@cup.portal.com> cliffhanger@cup.portal.com (Cliff C Heyer) writes:
>>> 4. Are any board makers making (or have made) motherboards with ESDI and/or
>>> SCSI interfaces ON BOARD to bypass the 8MHz AT bus? 
>>Even that's not enough.  SCSI, for example, is still only 8 bits wide, and it's
>>maximum transfer rate of 4 megs/second is in the same ballpark as the fastest
>>AT buses.  This is what Amiga hard disk controllers do (DMA), and we get up to 900k/sec
>>through the filesystem using asynchronous SCSI (1.5 megs/sec). 
>
>Sounds like Commodore is really *trying* to put out a "best" product...I wonder
>about many others. For example, IBM PS/2s always score the lowest in disk I/O. I
>assume this is because they want to *encourage* customers to shift the needed
>I/O BW to IBM big iron.

	I've never quite understood why msdos computers are so slow at FS
access and I/O in general.  Note that that 900K/sec is through the filesystem,
not just raw I/O speed (with a good drive I've seen 1.3-1.4 meg/sec through
the I/O system, maybe 1.0-1.1 through the filesystem, using asynch scsi (1.5
meg/sec max)).

	We try real hard to provide excellent HD/filesystem speeds compared
to any other micro (and often end up with 4-10x better on the same HD).

>What about SYNCHRONOUS SCSI in the Amiga? (4.0MB/sec) Or DMA & memory 
>can't handle it at this speed(?)
>
>Do sync & async are about the same cost? Why use synchronous SCSI vs. 
>async? 

	Sure, synchronous scsi con be done, though the controller we're
discussing didn't have a fast enough clock on-board to use the 4 meg/sec
rate (7 Mhz clock available, need a 10(?) Mhz clock to do 4 meg/sec - note
I'm just talking clocks, not bus speeds).  The current amiga bus bandwidth
is about 3.5 meg/sec, if I remember correctly.  If we had the clock we
could approach that.

>Actually I must admit I don't know that much about the AT bus. 
>What is the AT bus rated at in aggregate and effective MB/sec throughput?

	I suspect it's based on the speed of the processor (causes amusing
problems for boards designed for lower speeds, I suspect).

> (For example, I assume it is
>16 bits wide? Does this mean at 8 MHz it's aggregate throughput is 16MB/sec?
>But I suppose because of DOS I/O is done via 8 bit bytes...which would give
>8MB/sec. Still this sounds awfully high - A 15MHz ESDI drive transfers at
>15Mbits/sec which is 1.5MB/sec, but this is said to be too fast for 
>the AT bus.....and so the 10MHz ESDI  is used.)

	8Mhz doesn't mean 8 million xfers of 16 bits per second.  For example,
the amiga bus clock is 7.16 Mhz, 16 bits wide.  However, bus transfers require
a number of cycles (on almost any bus/processor I've seen).

>Cliff


-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
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Common phrase heard at Amiga Devcon '89: "It's in there!"