USER subroutine

The USER subroutine (not called, always jumped to by the BIOS) is required 68k code which needs to perform actions according to the command value set in BIOS_USER_REQUEST ($10FDAE ) .

At the end of the routine, a jump to BIOSF_SYSTEM_RETURN is needed to give back control to the BIOS.

Handling code

JT_User:
    dc.l   StartupInit
    dc.l   EyeCatcher
    dc.l   Game
    dc.l   Title

User:
    move.b  d0,REG_DIPSW
    clr.l   d0
    move.b  BIOS_USER_REQUEST,d0
    lsl.b   #2,d0
    lea     JT_User,a0
    movea.l (a0,d0),a0
    jsr     (a0)
    jmp     BIOSF_SYSTEM_RETURN

USER_REQUEST values

Command 0

Initilization of the soft/debug DIPs in the 68k user RAM. Used only once, when the game is started for the first time on a MVS system (probably when the BIOS doesn't find the game's NGH number in the backup RAM).

Set BIOS_USER_MODE ($10FDAF ) to 0 during operations, and don't forget to kick the watchdog as interrupts are disabled.

Initilization of the rest of the user RAM, the display, and I/O.

Viewpoint, which has its backup RAM save pointer set to $100000 (header $10E) and game save size set to $0200 (header $112) does this:

	move.b	#$00,BIOS_USER_MODE
	lea	$100000,a0		;Start of game save block
	move	#$001F,d1		;32*4 longword writes = 512 bytes
	moveq	#0,d0
.cl:
	move.l	d0,(a0)+
	move.l	d0,(a0)+
	move.l	d0,(a0)+
	move.l	d0,(a0)+
	dbf	d1,.cl
	move.b	d0,REG_WATCHDOG
...
        jmp     BIOSF_SYSTEM_RETURN

Typical use is load default data (default rankings, etc) into work-backup area, then call SYSTEM_RETURN.

Command 1

Show the game's own boot logo animation (when $114 = 1).

More information needed (end flag, BIOS_USER_MODE value ?).

Command 2

Show the game's demo (the recommended duration is ?).

Command 3

Show the game's title screen. This is called when a coin is inserted on MVS systems. BIOS_COMPULSION_TIMER ($10FDDA ) is used as a count-down timer for auto-start.