OPTROM Switching

Some time was spent in developing an improved method to call main ROM from option ROM, and to handle interrupts from within the optrom.

The goal was to

• use only the stack
• minimize code size
• reduce execution time

OPON: .EQU 0FAA4H

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ROM start

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.org 0000h

RST0: di JMP program ; standard entry .DB 0,0,0,0 RST1: RET .DB 0,0,0,0,0,0,0 RST2: RET ;Not used .DB 0,0,0,0,0,0,0 RST3: RET ;Not Used .DB 0,0,0,0,0,0,0 RST4: RET ;Not Used .DB 0,0,0 TRAP: DI CALL INTCALL RST5: RET ;Not Used .DB 0,0,0 RST55: DI CALL INTCALL RST6: JMP STDCALL ;RST 6 used as short call to a Standard ROM routine. .DB 0 RST65: DI ;Replaces the 6.5 interrupt and sets up a call to 6.5 in Standard ROM CALL INTCALL RST7: RET ;Not Used .DB 0,0,0 RST75: DI ;Replaces the 7.5 interrupt and sets up a call to 7.5 in Standard ROM CALL INTCALL

.org 0040h

OPON_img: ; auto copied on power up ; 8 bytes PUSH PSW ; temp store psw mvi a,01h OUT 0E8H pop psw ret

.db 00h

.org 0048h

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The STDCALL routine allows a program running in the

the Option ROM to call and return to an address in the

Option ROM.

Syntax is to use a RST 6 plus the address to be called.

RST 6

rvect--> DW 04B44H (cvect)

pvect-->

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STDCALL: ; 23 bytes ; stack = rvect xthl inx h inx h xthl ; fix up first stack entry ; stack = pvect ; hl=hl'

push h lxi h, OPON xthl ; stack = pvect, OPON ; hl=hl'

push h push d ; stack = pvect, OPON, hl', de'

ldsi 06h ; point de to stack location of return vector

lhlx ; hl=pvect dcx h dcx h ; point hl=pvect-2 xchg ; de=pvect-2 lhlx ; hl=cvect

pop d ; de=de' xthl ; hl=hl', stack= pvect, OPON, cvect

jmp STDON

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Routine for the hardware traps.

return address on entry is related to which interrupt occured.

after interrupt occurs

ivect DI

call INTCALL

rvect -->

so stack = pvect, rvect

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INTCALL: ; final stack must be opon, introutine ; interrupts are off ; enter with return location on stack, indicating int routine ; stack = pvect, rvect ; 20 bytes push h push d ; stack = pvect, rvect, hl', de'

ldsi 04h

lhlx ; hl= rvect

dcx h dcx h dcx h dcx h ; hl = ivect

push h ; stack = pvect, rvect, hl', de', ivect

lxi h,OPON shlx ; stack = pvect, OPON, hl', de', ivect

pop h ; hl=ivect pop d ; de=de'

xthl ; stack = pvect, OPON, ivect ; hl=hl', de=de'

jmp STDON

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OPEXIT/STDON

Turns off the Option ROM (or turns on Standard ROM)

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.org 0085h

STDON: PUSH PSW PUSH H ; 26C8, F1 C9 POP PSW, RET LXI H,26C8H ; it returns to this location --> pop psw; ret XTHL

OPEXIT: ; return to location pushed on stack xra a OUT 0E8H RET ; RET can be found at 008EH in stdrom ; in both M100 and T200

EXIT: .org 00094H pop h lxi h,0000h xthl ; restart

jmp OPEXIT