Path: utzoo!mnetor!uunet!husc6!think!ames!pasteur!ucbvax!MITRE-BEDFORD.ARPA!jhs
From: jhs@MITRE-BEDFORD.ARPA
Newsgroups: comp.sys.atari.8bit
Subject: boot disks
Message-ID: <8803290341.AA03091@mitre-bedford.ARPA>
Date: 29 Mar 88 03:41:48 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 58

For Steven Klassen and anyone else who cares, here is a brief description of
the boot process.  When you power up with a disk drive attached and turned on,
and with a disk in it, etc., the O/S will read the first sector of the disk
into memory starting at address $0700 and then will look at what it got to
determine how to proceed.  The first few bytes of data loaded have the
following meanings:

	$0700   BFLAG   Flag (normally 0)
	$0701   BRCNT   Boot Record Count (nr of consec sectors to read into
			  memory for boot record (typically 3).
	$0702   BLDADR  Boot Load Address, i.e. where to put the stuff.
	$0704   BIWTARR Initialization address
	$0706   JMP XBCONT  This is an actual jump instruction followed by
				a 2-byte address indicating where to go.
	$0709   SABYTE  Number of sector buffers to allocate
	$070A   DRVBYT  Bit map of the disk drive unit numbers to plan for.
	$070B           Unused
	$070C   SASA    Buffer Start Address
	$070E   DFSFLG  DOS Flag
	$070E   DFLINK  Sector at which DOS.SYS starts
	$070F   BLDISP  Displacement in sector to sector link (differs for
				DD versus SD disk format).
	$0712   DFLADR  Address for loading DOS.SYS file.

This nomenclature assumes you will want to load DOS from the disk, as you
would if your program planned to do disk I/O in the normal way, i.e. using
DOS services.  If not, some of the DOS-related names should still give you
a clue as to what the loader will try to do with these locations.

First the O/S loads BRCNT consecutive sectors into memory starting with the
first sector (sector 1) starting at BLDADR.  Then it will do a JSR into the
7th byte into the area loaded, i.e. BLDADR+$6, or the JMP instruction at
$0706 in the above listing.  If what you are booting is a DOS disk, then the
address XBCONT will lie in the block of sectors initially loaded and the code
there will already be in place and will continue the load in whatever way
strikes its fancy.

If you are booting your own thing without DOS, then evidently you need to
start sector 1 with a 0 byte, then a byte containing the number of sectors
you want loaded in the first gulp, then 2 bytes saying where to load these
sectors, then 2 bytes saying where to JMP to initialize the program, and
finally -- 6 bytes into the load area -- a JMP to the code you have provided
to continue the load process.  Everything after $0706 is apparently up to you.
In particular, the initialization address called BIWTARR at $0704 will only
get used if your code references it, I believe.  By the way, if BRCNT is set
to zero it is interpreted as 256 sectors, so you can let the bootstrap loader
load a whole program if it is anywhere up to 256 sectors long, so long as the
sectors are contiguous.  If it is longer or non-contiguous, you will have to
provide code to read the sector links and figure out where to go for the next
sector.

I hope this is enough to get you started.  I also hope it is true.  I am
playing back what they say in "Inside Atari DOS", which pertains to DOS 2.0S.
Your DOS may differ, depending on road conditions and your style of driving.
I'm sure there are other readers of this august journal who can give you
the benefit of their greater experience with how it all REALLY works.

-John Sangster / jhs@mitre-bedford.apra