ISA bus throughput

Chuck Guzis cclist at
Tue Jan 31 20:11:04 CST 2006

On 1/31/2006 at 7:46 PM Michael B. Brutman wrote:

>Chuck - you mentioned that the superio controller card would have a BIOS 
>extension.  Are you certain?  One card that I tried definitely did, as 
>it had the intelligence to handle larger drives.  

Some controller cards have BIOS extensions, some don't.  Most IDE ones that
were built to handle larger IDE drives do; garden-variety ones don't.
You may get in a bind with a larger IDE drive as the original AT was pretty
limited in its support for large drives.

>I was thinking that if each instruction takes a few cycles that even the 
>tightest of loops would waste a lot of cycles, but I found a gem in the 
>286 user's guide - REP.  Apparently you can use REP INx and REP OUTx 
>instructions to generate a tight loop that doesn't require subsequent 
>instruction fetches until the loop ends.  So that would allow a 286 to 
>push the bus much harder than an 8088/8086 class machine would.  (The 
>8088/8086 would have to keep fetching instructions, which would suck.)

As I mentioned, the operative instructions are INS and OUTS--work just like
STOS and LODS, except the input is through port (DX).   Even on a
Pentium-class machine, the original AT 16-bit bus timings are observed for
ISA devices, so at some point, processor speed just doesn't matter any


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