ISA bus throughput

Tony Duell ard at
Tue Jan 31 17:58:32 CST 2006

> I revived an old 6Mhz PC AT today by removing the original Western Digital based
> hard disk controller and replacing it with a newer, generic IDE controller. 
> The large boat-anchor class 5.25" full height hard disk had died, and without a
> suitable replacement a small IDE drive was chosen.  The rest of the machine is
> original, including the EGA card with the extra memory daughterboard and the
> full slot monochrome card.
> While discussing this beast, the topic of putting a CD-ROM on it came up.  (I
> would never do such a thing except for giggles.)  The topic of the CD-ROM led

Why not? It works.

> to a throughput question - could this old monster even do it?  As in, is the
> ISA bus up to reading from the CD-ROM at a reasonable speed?

Depends on what you mean by 'reasonable speed'. AFAIK most CD-ROMs have 
internal buffers, and will read quite well even on a slow machine (you 
are limited by the bus speed, so don't try anything that _needs_ high 
throughput). I wouldn't want to try a CD burner though.

You don't want ot know about the CD-ROM drive I have on this machine (a 
much-hacked PC/AT with most of the original bits). It's an external 
Philips unit, and it looks like a converted audio CD player. Darn it, it 
_is_ a converted audio CD player. Most of the bits, including the 
mechanism (CDM-4), decoder chips, case, etc are the same as those of a 
contemporary Philips CD-plauer. They added an ASIC to grab the data 
stream after the decoder and send it to the PC, and re-programmed the 
microcontroller to accept comamnds from the PC.

It's just as well I realised it was a modified CD player. The mains 
switch failed mechanically (wouldn't latch on), and the service mnaual 
for the CD-ROM drive (yes, I have it) didn't list it. Fortunately the 
manual for the audio player did, I ordered that, and fortunately again it 
was the right part.

> When writing to a hard disk, would this machine have used a tight processor
> loop, or would it have used DMA?  Under what circumstances would it use DMA to
> transfer data to a hard disk?

The PC/AT hard disk contoroller and the mainboard BIOS do not use DMA. 

> And one last question ..  Unlike the PC and XT, the AT BIOS handles hard drives.
>  It didn't blink when I removed the crusty WD based controller and replaced it
> with a no-name WinBond based controller.  Does the new IDE controller really
> look that much like the old controller that the BIOS can't tell?

YEs, that was the idea behind IDE. It's upward compatible with the ST506 
controller (WD2001 or something like that). I have an IDE card in this 
PC/AT. It's just a few TTL buffers and a PAL as the address deocder. It 
uses the mainboard BIOS, there is no extension ROM on that card.


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