8-bit IDE hard drives, controllers

[Eric J Korpela]

On Tue, 22 Feb 2005 15:37:12 -0600, Jim Leonard <trixter at oldskool.org> wrote: 
> My apologies; I was indeed referring to an 8-bit ISA bus interface card that
> claims to work with "modern" (ie 16-bit) IDE drives up to a size of 540MB.
> 16-bit ISA bus cards are quite plentiful, but 8-bit ISA bus cards of this
> nature (that interface to modern drives) are fairly rare.  If the card works as
> advertised, I will report back here.

Is this the Acculogic sIDE-1/16?  I've used a couple, and they typically work.
The logic on the card is fairly trivial.  Anyone could design and
build one in short
order. (In fact if you look at most IDE interfaces for 8 bit machines
they often match
the sIDE-1/16 down to the chips used.) The hardest part is writing the BIOS.  

The 504MB limit could probably be surpassed with a change to the BIOS
on the sIDE
card or with a memory resident  BIOS.  The IDE spec gives 64k
cylinders, 16 heads,  256 sectors. The BIOS int 13h interface allows
1k cylinders, 256 heads, and 63 sectors.
If the drive reports more than 1k cylinders, the BIOS can play around
with it's heads and
cylinder counts (i.e. make a 16k cylinder, 16 head drive look to the
caller like a 1k cylinder 256 head drive)  Messing with the sector
count isn't usually necessary because most IDE
drives report 63 or fewer sectors.   That'll at least get you up to
the 8.4GB BIOS limit.  To
get past that you'll need to use a different OS than DOS or a hacked
DOS that doesn't use
int 13h.

Eric