Updates:

• 2025-11-20 – Thanks to a comment from Jerry Stratton, I have the start of a disassembly for the “DOS” code which would load at $2600. Article updated with what I have, so far.

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To this date I still think one of the most useful products Sub-Etha Software ever created was MultiBoot. For CoCo hard drive users of the era, most of us had to boot from a floppy disk. That boot disk would contain the necessary drivers to access the hard drive. With boot files, “one size does not fit all” so most of us had a stack of them — one for running a BBS with serial drivers installed, one with the Sierra Online drivers for playing those games, one with all the graphics drivers for that stuff, etc.

MultiBoot allowed putting multiple boot files on one floppy disk. Type “DOS” and a menu is displayed. Choose the one you want, then that boot file is made active and the process continues.

MultiBoot was written by myself and Sub-Etha co-founder, Terry S. Todd. I wrote the OS-9 frontend in C, and Terry wrote the actual MultiBoot code in RS-DOS assembly. He would then provide that code to me as data, then my C front end program would “install” it by copying it out to the boot sector of the boot disk.

That code looks like this in the MultiBoot source code:

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
};

char mb_sec33_15[] = { /* Terry's RS-DOS MultiBoot code V1.12... 512 Bytes */
   142,56,0,16,142,38,0,236,129,237,161,140,57,0,37,247,23,1,152,23,1,171,
   32,32,32,32,32,32,32,32,77,117,108,116,105,66,111,111,116,32,86,49,46,49,
   50,13,32,98,121,32,84,101,114,114,121,32,84,111,100,100,32,38,32,65,108,
   108,101,110,32,72,117,102,102,109,97,110,13,32,32,32,32,32,67,111,112,121,
   114,105,103,104,116,32,40,67,41,32,49,57,57,51,32,98,121,13,32,32,32,32,
   32,32,32,83,117,98,45,69,116,104,97,32,83,111,102,116,119,97,114,101,13,
   0,16,142,5,192,16,159,136,23,1,53,32,32,32,32,32,85,115,101,32,85,80,47,
   68,79,87,78,32,116,111,32,115,99,114,111,108,108,13,91,69,78,84,69,82,93,
   32,83,101,108,101,99,116,115,32,32,32,91,66,82,69,65,75,93,32,81,117,105,
   116,115,0,246,58,235,23,1,16,206,4,164,246,58,255,16,39,0,209,240,58,235,
   193,8,37,2,198,8,52,20,223,136,189,58,208,53,20,48,136,32,51,200,32,90,
   38,238,182,58,234,176,58,235,198,32,61,195,4,163,31,1,134,106,167,132,16,
   190,58,253,49,63,38,5,50,98,22,0,151,173,159,160,0,39,241,198,96,231,132,
   198,255,129,94,39,17,129,10,39,36,129,13,39,62,129,3,38,197,15,113,126,
   140,27,182,58,234,39,187,247,1,85,74,183,58,234,177,58,235,36,3,122,58,
   235,22,255,126,182,58,234,76,177,58,255,36,160,247,1,86,183,58,234,182,
   58,235,139,7,177,58,234,36,228,124,58,235,32,223,246,58,234,92,141,112,
   48,27,52,16,142,0,234,204,2,0,237,132,204,0,1,237,2,204,1,218,237,4,173,
   159,192,4,109,6,38,79,16,174,4,49,168,21,53,64,198,5,166,192,167,160,90,
   38,249,134,3,167,132,173,159,192,4,109,6,38,50,126,38,2,134,87,183,149,
   201,134,16,183,255,34,204,63,0,253,255,188,189,246,82,126,169,40,174,228,
   166,128,39,6,173,159,160,2,32,246,175,228,57,166,128,39,251,173,159,160,
   2,32,246,126,215,9,142,59,0,93,39,236,48,136,32,90,38,250,57,0,0,0,0,0,
   0,0,0,0,0,0,0,0,0,0,0,0,0,0,66,66,0
};

Are there any 6809 disassembler gurus out there that might take this data and reverse it back into 6809 assembly source code?

Sadly, the only source code from Terry that I have found is that for his MultiBasic product. I cannot even find the master disks for his InfoPatch, and I do not think I ever had sources to ShadowBBS, OS9Term or anything else he wrote.

Thank you for your attention to this matter.

Disassemblies

The “DOS” code would load into memory at &H2600. The first two bytes must be “OS”, and indeed that matches the first two bytes in the data. Per a suggestion from Jerry Stratton, I wrote a program to POKE this data into memory, then used EDTASM Z-BUG to disassemble and do a bit of cleanup.

Where the “??” comment is was something Z-BUG could not decode. There is a value of 2 there. No cluck. BUT, I see a reference to DSKCON (C004) so I am betting if I look up how that works some of this code might make sense.

Help is appreciated!

NOTE: I kept making adjustments to the code until it would compile with lwasm… It produces the same bytes, but there is this weird “2” in it, which I recreated using “FCB 2”. What is that all about??

; /* Terry’s RS-DOS DOS startup code… 50 bytes */

; lwasm dos.asm -fbasic -odos.bas --map --list

DSKCON  EQU     C004

; 7E - CBUFAD RMB 2 - CASSETTE LOAD BUFFER POINTER


        ORG     $25EF

L25EF   FCB     0
L25F0   FCB     0

        ORG     $2600

OS      FCB     'O
        FCB     'S

        LDA     #$D
        STA     $D3         ; CBUFAD
        LDD     #$3800
        STD     $D1         ; VD1 - scratch pad vars
        INC     $D3         ; VD3 - scratch pad vars
        LDX     #$EA        ; DCOPC - DSKCON op code 0-3
                            ; DCDRV - DSKCON drive number 0-3
        LDD     #$200       ; 512? or...
        STD     ,X          ; DCOPC=02, DCDRV=00
        NEG     $86         ; GRBLOK - block for set/reset/point
        BRN     L25EF
        ADDD    $ED         ; DSEC - DSKCON sector number 1-18
L261B   FCB     2           ; ??
        LDD     $D1         ; VD1
        STD     4,X         ; DCBPT - DSKCON data pointer
        JSR     [$C004]     ; DSKCON
        TST     6,X         ; DCSTA - DSKCON status byte
        BNE     L2630
        INCA
        CMPA    #$3D        ; 61
        BLO     L262D
L262D   STD     $7E         ; CBUFAD - cassete load buffer pointer
        RTS
L2630   NEG     $7E         ; CBUFAD - cassete load buffer pointer
        STB     $9          ; INPFLG - input flag (read=0, input<>0)
        NEG     $0          ; ENDFLG - stop/end flag (+ stop, - end)

        END

To be updated as more information comes in…