ESDI vs SCSI

[James Van Artsdalen]

In <98835 at sun.Eng.Sun.COM>, williamt at sun.UUCP (William A. Turnbow) wrote:

> 3) Last time I looked at the AT bios it didn't use DMA in the disk
> routines.  So it really isn't a factor.

DMA isn't really an option.  Many (most?) hard disk controllers don't
even have the "fingers" on the card for the DMA lines.

> 5) As for SCSI controllers supporting variable number of sectors per
> track, that is only in the interface presented to the user (computer).

I had expected that someone would build a controller to present a
WD1010 interface to the CPU, but no one has.  All of the IDE drives
have user-definable geometry, so the technique is well known.  What
happens is that when the drive is initialized, the CPU sends the
geometry the CPU wants to the WD1010 registers.  The IDE drives looks
at the resultant capacity and sees if it fits.  If so, the IDE drive
just translates each incoming request into the underlying geometry.
The same could be done with SCSI except that the underlying geometry
would be in linear blocks.

This IDE remapping capability is why one needn't find exact drive-type
entries in the BIOS for IDE drives (though you loose some space if you
don't).  All you do is find an entry with the same (or less) net
capacity and just use it, without regard to the number of heads or
sectors.  The IDE drive will remap your choice into the real geometry.
-- 
James R. Van Artsdalen          james at bigtex.cactus.org   "Live Free or Die"
DCC Corporation     9505 Arboretum Blvd Austin TX 78759         512-338-8789