Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!att!lzaz!hcj
From: hcj@lzaz.ATT.COM (HC Johnson)
Newsgroups: comp.sys.atari.st
Subject: Re: AHDI (Was: GDOS, AHDI, FONTWID?)
Summary: ahdi is just a TSR program
Keywords: ahdi
Message-ID: <627@lzaz.ATT.COM>
Date: 13 Jun 89 14:52:08 GMT
References: <18600014@uxh.cso.uiuc.edu> <1543@atari.UUCP> <1257@pkmab.se>
Organization: AT&T ISL Lincroft NJ USA
Lines: 43

In article <1257@pkmab.se>, daniel@pkmab.se (Daniel Deimert) writes:
> 
> Talking about AHDI - what does it do exactly? It installs some kind of
> driver, but what does that driver consist of. Is it possible for me to
> write my own driver?

AHDI (generic) is not a mystical program.
It is a Terminate-Stay-Resident program that when loaded does
these things:
a.	Attempt to read from each disk on the system.
b. 	For each partition labeled GEM, store its starting sector (and size).
c.	TOS does all disk (floppy and hard) io thru low memory vectors 
	(address) called hdv_rw, hdv_media, hdv_bpb. At boot time, these
	addresses are in the kernel, floppy disk handler.
d.	AHDI substitutes its own address for the kernel ones at these location
	(and remembers the old ones).
e. 	TOS references a drive as 0-15.  0 and 1 are floppies, so AHDI passes
	these calls to the saved vector addresses.
	2-15 may be hard drives, and if a starting sector, and drive address
	are saved for the number referenced, then it will do the disk operation.

f. 	note that if a ram disk is added, it does the same thing in substituting
	its addresses, so first its called (last one first), then passes
	control to (presumedly) AHDI, which will do the work, or pass control
	to the floppy driver in the kernel.

When AHDI scans for active disk drives it can do several things.
Atari Brand:
	The Atari host adapter is a little sick and will not indicate
whether drive 1 is present.  It does OK on drive 0.  SO, Atari's AHDI program
only tests for drive 0 on each SCSI address from 0-7.  As soon as it finds
no drive, it stops looking, and the only drives letters that can be accessed
are those partitions it found.

BMS/ICD/SUPRA:
	These allow 2 or more drives on each controller.
The Adaptec 400x series only support 2 drives.  Each drive is tested on each
controller until a drive 0 is not found.  

Howard C. Johnson
ATT Bell Labs
att!lzaz!hcj
hcj@lzaz.att.com