Calling the Main ROM from Option ROM

STDCALL:
       DI
       SHLD    HOLDH           ;Caller's HL
       XCHG
       SHLD    HOLDD           ;Caller's DE
       POP     H               ;(HL)=Routine to Call
       MOV     E,M
       INX     H
       MOV     D,M             ;DE=Routine to Call
       INX     H               ;HL=Return Address
       PUSH    H
       LXI     H,OPON          ;return through OPON
       PUSH    H
       PUSH    D               ;Return Address
       LHLD    HOLDD           ;Caller's DE
       XCHG
       LHLD    HOLDH           ;Caller's HL

       PUSH    H               ; 0363, FB C9  EI, RET
       lxi     h,0363h         ; return here ---> EI; RET
       XTHL                    ; works in T200 and M100

       jmp     STD_ON



INTCALL:
       SHLD    INTH            ;Caller's HL
       XCHG
       SHLD    INTD            ;Caller's DE
       POP     H               ;(HL)=Routine to Call
       MOV     E,M
       INX     H
       MOV     D,M             ;DE=Routine to Call
       INX     H               ;HL=Return Address
       PUSH    H
       LXI     H,OPON          ;return through OPON
       PUSH    H
       PUSH    D               ;Return Address
       LHLD    INTD            ;Caller's DE
       XCHG
       LHLD    INTH            ;Caller's HL

       JMP     STD_ON                          ;make sure STD_ON does not enable interrupts!



;---------------------------------------------------------
;STD_ON: Turns off the Option ROM (or turns on Standard ROM)
;dedicated to rom calls and interrupts
;---------------------------------------------------------


                                            ; modification to STDCALL to delay the
EI till the end.
       .ORG    007EH
STD_ON:
       PUSH    PSW
       PUSH    H               ; 26C8, F1 C9  POP PSW, RET
       LXI     H,26C8H         ; it returns to this location --> pop psw; ret
       XTHL
                               ; return to location pushed on stack
       lda     0FF45h
       ani     0FEh            ; manage FF45 properly
       sta     0FF45h
       OUT     0E8H
       RET                     ; RET can be found at 008EH in stdrom
                               ; in both M100 and T200



OPON:          ; this code in upper ram somewhere safe, like ALTLCD
                    ; this routine critical for proper return from
STDCALL and INTCALL

       di
       PUSH    PSW             ; Model 100 logic.
       lda     0FF45h
       ori     001h            ; manage FF45 properly
       sta     0FF45h          ; sensitive - cannot take an interrupt here
                               ; either the rom switch is screwed up, or
                               ; PSW gets trashed, or FF45 gets trashed

                               ; if the interrupt occurs after sta 0FF45 but before
                               ; out E8, then we might not return to REXROM (and the
                               ; rest of port E8 gets screwed up)
                               ; we would return to somewhere in main rom.
       OUT     0E8H

       POP     PSW
       ei
       RET