A Disc That Starts DISKMON

Introduction

A disc starts whatever its header points at. DISKMON can write sectors. So DISKMON can make a disc start DISKMON.

A Header Just Below 0100h

DISKMON is assembled to run at 0100h. If the MOS loads it there at power-on, nothing in it needs to change. The header has to be loaded too - it is the first seven bytes at the start address - so put the start address seven bytes early, at 00F9h: the header fills 00F9h-00FFh, and DISKMON's first byte lands at 0100h.

Header field	Value
Start	00F9h
End	`LAST`, a label on DISKMON's very last byte
Jump	0100h, where `START` is
Side	0

B writes those seven bytes into memory at 00F9h, then writes memory from 00F9h onwards to drive 1, track 0, sectors 0, 1, 2... - as many as it takes to reach LAST, which the assembler works out: (LAST-00F9H)/512+1.

Write that sum straight into the instructions that need it. If you give it a name with EQU above LAST, z80asm makes it 0 without a word of warning, because it has not seen LAST yet.

Started by the MOS

When the disc boots, the MOS loads DISKMON and jumps to 0100h. There is no DOS: no 0:, nothing at 0005h. START still keeps the stack pointer it was given and switches to its own, the MOS calls DISKMON makes for keys, printing and sectors are all still there - and S, D and L work. Its BDOS commands do not, and S21 deals with that.

The Code

After the line JP Z,CMDU, add:

        CP 'B'
        JP Z,CMDB

After the last line of CMDU (JP CRMAIN), add:

; B: make the disc in drive 1 start DISKMON. A header goes in the 7
; bytes before 0100h - start 00F9h, end LAST, jump 0100h, side 0 - and
; from 00F9h on, as many sectors as that takes are written to track 0.
CMDB:   LD HL,00F9H
        LD (HL),0F9H            ; start: 00F9h
        INC HL
        LD (HL),00H
        INC HL
        LD DE,LAST
        LD (HL),E               ; end: LAST
        INC HL
        LD (HL),D
        INC HL
        LD (HL),00H             ; jump: 0100h
        INC HL
        LD (HL),01H
        INC HL
        LD (HL),0               ; side: 0
        LD A,1
        LD (0FB50H),A
        XOR A
        LD (0FB51H),A
        LD HL,00F9H
        LD B,(LAST-00F9H)/512+1 ; sectors from 00F9h to LAST
        LD C,0
CMDB1:  LD A,C
        LD (0FB52H),A
        LD (0FB53H),HL
        PUSH BC
        PUSH HL
        RST 8
        DEFB 163
        POP HL
        POP BC
        JP NZ,DISCERR
        INC H
        INC H
        INC C
        DJNZ CMDB1
        LD L,(LAST-00F9H)/512+1
        LD H,0
        CALL PDEC
        LD HL,BOOTM
        JP DONE

After the line SIDEM: DEFB ' SIDE ',0, add:

BOOTM:  DEFB ' SECTORS WRITTEN TO TRACK 0',13,10,0

After the line BOOTM: DEFB ' SECTORS WRITTEN TO TRACK 0',13,10,0, add:

LAST:   DEFB 0                  ; the last byte the boot loads

Run it: make work.mfi from PRIMER.DSK with DISKMON on it (S10), run DISKMON from it under XtalDOS, and type:

B
Q

> B
5 SECTORS WRITTEN TO TRACK 0
>

Quit MAME. Now start it with work.mfi in drive 0 and nothing else:

mame -rompath ~/roms einstein -window -nomaximize -ui_active -skip_gameinfo -flop1 work.mfi

What you should see - no XtalDOS sign-on, DISKMON's own prompt. Then D 0:

> D 0
POEM    .TXT  EXTENT 0 RECORDS 3
ROOMS   .DAT  EXTENT 0 RECORDS 128
ROOMS   .DAT  EXTENT 1 RECORDS 72
DISKMON .COM  EXTENT 0 RECORDS 17
>

DISKMON, started by the Einstein itself, reading its own disc's directory. The disc's files are untouched: tracks 0 and 1 are not part of the filesystem.

Change One Thing

U 1 after B. What start, end and jump will it show?
Booted, type T 0:POEM.TXT. What will happen, and why? (Save anything you care about first.)

Summary

A seven-byte header at 00F9h and five sectors of track 0: DISKMON starts straight from the disc, with the MOS and nothing else.

S21: life with no BDOS - what DISKMON must stop doing, and what memory it now has.

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