**************************************************************
;
; THIS PROGRAM TOGGLES THE LST: PORT OF A CP/M OPERATING SYSTEM
;
;EQUATES LIST
;
     RESET     EQU       0H        ; CP/M SYSTEM RESET
     IOBYTE    EQU       3H        ; CP/M IOBYTE ADDRESS
     LOUT      EQU       5H        ; CP/M LISTOUT
     POUT      EQU       9H        ; CP/M PRINT STRING
     CR        EQU       0DH       ; ASCII CR
     LF        EQU       0AH       ; ASCII LF
;
     ORG       100H                ; STANDARD CP/M STARTING POINT
     LXI       SP,STACK            ; START A STACK
;
     LDA       IOBYTE              ; LOAD IOBYTE TO ACC
     CPI       0                   ; IS IT PARALLEL ?
     JZ        SER                 ; IF NOT CHANGE
;
; CHANGE TO PARALLEL PORT SUBROUTINE
;
     LXI       D,SMSG              ; POINT TO SERIAL MSG
     MVI       C,POUT              ; PRINT IT
     CALL      LOUT
     MVI       A,0                 ; PARALLEL VALUE OF IOBYTE
     STA       IOBYTE              ; INSTALL
     JMP       FINISH
;
;CHANGE TO SERIAL PORT SUBROUTINE
;
SER: LXI       D,PMSG              ; POINT TO PARALLEL MSG
     MVI       C,POUT              ; PRINT IT
     CALL      LOUT
     MVI       A,80H               ; SERIAL VALUE FOR IOBYTE
     STA       IOBYTE              ; INSTALL
     JMP       FINISH
;
FINISH: CALL   RESET               ; SYSTEM RESET
;
;STORAGE AREAS
;
SMSG:   DB     CR,LF,'CHANGING FROM SERIAL TO PARALLEL.',CR,LF,'$'   


                           4



PMSG:   DB     CR,LF,'CHANGING FROM PARALLEL TO SERIAL.`,CR,LF,`4`
        DB     40
STACK;  DB     0                   ; START STACK
;
END