From: mbbrutman-cctalk at brutman.com
I revived an old 6Mhz PC AT today by removing the
original Western Digital
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
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?
Well, let's go throught some quick math. 6 million clock cycles per second is
lot, but when you consider that each transation on the
bus takes 4 to 5 cycles,
and instructions take 4 to 5 cycles (average) on a 80285, the throughput might
not be so hot.
Forget the CD-ROM and let's just pretend it's an I/O device on the bus, like a
fast hard disk. Given a PC AT running at 6Mhz, what can reasonably be expected
for bus throughput? (Obviously this depends on the loop and the instructions
used.) I would have guess 500K or so per second, but now I'm second guessing
myself and I'll actually have to write a small benchmark to try it.
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?
Hard disk controller have at lest one sector RAM buffer and most
newer ones have at least a track worth of RAM. The controller doesn't
write the sector until the buffer has the new data. Reads are buffered
the same so that bus speeds are not issues, like for floppies.
And one last question .. Unlike the PC and XT, the AT BIOS handles hard
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?