ISA bus throughput

Jim Leonard trixter at
Tue Jan 31 17:30:50 CST 2006

mbbrutman-cctalk at wrote:
> 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.)  

I've successfully used my Sound Blaster Pro + 1X CDROM drive to play 
Sherlock Holmes (a video title, no less) on a 12MHz 286, so it is 
possible.  1X is, as I'm sure you remember, 150KB/s.

As for giggles, it's really useful.  Most 286s don't have 650MB of 
storage :-)

> Well, let's go throught some quick math.  6 million clock cycles per second is a
> 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.

Don't forget memory timing.  Different boards/manufacturers did 
different things.

> 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?

I believe the AT and later used tight processor loops as they beat the 
speed of DMA but I could wrong.
Jim Leonard (trixter at  
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