[Coco] MultiBoot - multiple boot files on one disk (can someone dis-asm this for me?)

[Don Barber]

The first two bytes are 'OS' in ascii which the DOS command looks for to 
ensure its loaded a valid boot track.

Thanks,

Don

On 3/3/25 20:36, Sean Conner via Coco wrote:
> It was thus said that the Great Allen Huffman via Coco once stated:
>
>> One of out utilities I was most proud of was MultiBoot. “Back in the day”
>> when you had different boot floppies (for the BBS with those drives, for
>> the hard drive, for the Sierra games, etc.) Terry and I came up with an
>> idea of swapping the os9boot file out dynamically. MultiBoot was the
>> result. Type “DOS” and a menu comes up showing the boot files on the disk.
>> Select one and go. One disk, multiple boot files (up to 16, space
>> permitting, if I recall).
>>
>> Terry wrote the RS-DOS portion that lives on the “DOS” track, and the code
>> it ran from there, and I wrote the OS-9 frontend. You’d point it to a disk
>> with multiple os9boot files and add the ones you wanted.
>>
>> Unfortunately, I do not have Terry’s assembly source — but I do have the
>> data that made it up:
>>
>> char mb_sec34_1[] = { /* Terry's RS-DOS DOS startup code... 50 bytes */
>>     79,83,134,13,151,211,204,56,0,221,209,12,211,142,0,234,204,2,0,237,132,
>>     134,33,214,211,237,2,220,209,237,4,173,159,192,4,109,6,38,8,76,129,61,37,
>>     221,126,57,0,126,215,9
>> };
>>
>> Is there anyone out there who might do me a favor of turning this back
>> into the assembly? I’d like to include it with the GitHub repo I am
>> putting together of all the Sub-Etha Software stuff.
>    Here's the assembly for the above data:
>
> inbuf	equ	$D1
> sector	equ	$D3
>
> DCOPCv	equ	$EA
> DCOPC	equ	0
> DCDRV	equ	1
> DCTRK	equ	2
> DSEC	equ	3
> DCPBT	equ	4
> DCSTA	equ	6
>
> startup	clra			; ???
> 	comb			; ???
> 	lda	#13		; starting sector - 1
> 	sta	sector
> 	ldd	#$3800		; buffer address
>
> again	std	inbuf		; save updated buffer ptr
> 	inc	sector
> 	ldx	#DCOPCv		; smaller than LDX [$C006]
> 	ldd	#READ*256+0	; read op, drive 0
> 	std	DCOPC,x		; tell RS-DOS
> 	lda	#33		; track 33
> 	ldb	sector		; get sector number
> 	std	DCTRK,x		; tell RS-DOS
> 	ldd	inbuf		; get buffer
> 	std	DCBPT,x		; tell RS-DOS
> 	jsr	[DSKCON]
> 	tst	DCSTA,x		; check status
> 	bne	done
> 	inca			; adjust buffer
> 	cmpa	#$3D		; are we done? (last address $3CFF)
> 	blo	again		; if not, continue
> 	jmp	$3900		; jump to starting code
> done	jmp	$D709		; report I/O error to BASIC
>
>    I'm puzzled by the first two instructions, which do nothing.  I'm also
> curious that it loads data off the last five sectors of track 33.  The DOS
> track is track 34, not 33, and it's an entire track, and a granual in RS-DOS
> is 9 sectors, not 5.
>
>    I haven't fully disassembled the larger blob as it looks like it'll take
> more work as there appear to be gaps in the instruction stream.  Do you
> recall which address the other blob was assembled to?
>
>    -spc
>