inter.asm

;******************************************************************************* 
; TIMER INTERRUPTION
; AUTHOR: MANUEL PABLO GONZALEZ CALZON 
; AH=01 FOR ENABLE TIMER, ANYTHING ELSE FOR DISABLE
;*******************************************************************************

CODE SEGMENT
ASSUME CS:CODE
ORG 256

START: JMP CHECK
TIME DB 33H,30H ;TIME IN TENS AND THEN UNITS IN ASCII CODE
SECOND DB 0; HAS A SECOND PASSED
ENABLED DB 0; IS THE TIMER ENABLED
DIR_ANT_INT DW ?,? ; OLD 1CH INTERRUPTION DIRECTION TO REPLACE IT WHEN UNINSTALLING

ISR PROC FAR
STI ;ALLOW STACKING OF OTHER ROUTINES
PUSH  DX DS
PUSH ES BX

;CHECK AX FOR ENABLE OR DISABLE
CMP AH,01H
JE ENABLE

;DISABLE
MOV [ENABLED],0
JMP ENDRUTINE


ENABLE:
;GET MAX TIME
MOV BH,[TIME]
MOV BL,[TIME+1]
;STORE IT ON MEMORY
MOV DS:[SI+1],BL
MOV DS:[SI],BH
;ENABLE
MOV [ENABLED],1

;CHANGE VIEW FOR THE FIRST TIME
MOV DI,DS:[SI]
CALL CHANGEVIEW

;END RUTINE
ENDRUTINE:
POP BX ES
POP DS DX 
IRET
ISR ENDP


TICKS DB 0 ; TICKS FROM THE LAST SECOND

TIMER PROC FAR
PUSH AX DX DS
PUSH ES BX
STI
CMP [ENABLED],0 ;CHECK IF TIMER IS ENABLED
JE NOSEG
INC [TICKS];A NEW TICK HAS PASSED
NORES:
CMP [TICKS],18 ; TICKS == 18 ? A SECOND HAS PASSED (APROX, IN REAL IT WOULD BE 18,2)
JNZ NOSEG
;SET SECONDS AND RESTART TICKS
INC [SECOND] ; SECOND = 1
AND [TICKS],0 ; TICKS = 0

;CHANGE VALUES IN MEMORY AND ON SCREEN
MOV BX,DS:[SI]
CMP BH,30H
JE DECDEC ;IF TENS DIGIT IS EQUAL TO 0 DECREASE THE TENS AND SET UNITS TO 0

;IF NOT JUST DECREASE THE UNITS
DEC BH
MOV DS:[SI],BX
JMP NXT

DECDEC:
MOV BH,39H
DEC BL
MOV DS:[SI],BX

;RESTART SECONDS AND PRINT THE NEW VALUE ON SCREEN
NXT:
MOV [SECOND],0
MOV DI,DS:[SI]
CALL CHANGEVIEW 
;IF THE TIMER REACHES 0 DISABLE IT
CMP BX,3030H
JNE NOSEG
MOV [ENABLED],0

;END RUTINE
NOSEG:
POP BX ES
POP DS DX AX
IRET
TIMER ENDP

CHANGEVIEW PROC NEAR 
PUSH AX DX SI BX CX

;GET CURSOR
MOV BH,0
MOV AH,03H
INT 10H

;STORE PREVIOUS CURSOR IN SI
MOV SI,DX

;SET CURSOR
MOV DL,41
MOV AH,02H
INT 10H

;PRINT FIRST VALUE
MOV CX,DI
MOV BX,DI
MOV AL, CL
MOV CX,1
MOV BH,0
MOV BL,4 ;SET COLOR TO RED
MOV AH,09H
INT 10H

;INCREASE CURSOR VALUE
MOV DL,42
MOV AH,02H
INT 10H

;PRINT SECOND VALUE
MOV CX,DI
MOV BX,DI
MOV AL, CH
MOV CX,1
MOV BH,0
MOV BL,4 ;SET COLOR TO RED
MOV AH,09H
INT 10H

;REESTORE CURSOR PREVIOUS POSITION
MOV DX,SI
MOV AH,02H
INT 10H

POP CX BX SI DX AX
RET
CHANGEVIEW ENDP

CHECK:
MOV AH, DS:[80H]
CMP AH, 0
JE END1

;IF ARGUMENT IS /I INSTALL
MOV AX, DS:[82H]
CMP AX, "I/"
JNZ UNINST
CALL INSTALL

;IF ARGUMENT IS /I UNINSTALL
UNINST:
CMP AX, "U/"
JNZ END1
CALL UNINSTALL

END1:
MOV AX, 4C00H
INT 21H



INSTALL PROC
MOV AX, 0
MOV DS, AX
MOV AX, OFFSET ISR
MOV CX, CS

CLI
;SET INTERRUPTION 30H TO ISR RUTINE
MOV DS:[30H*4], AX
MOV DS:[30H*4 + 2], CX

MOV AX, DS:[1CH*4] ; GET 1CH ORIGINAL INTERRUPTION 
MOV DX, DS:[1CH*4 + 2] ; GET 1CH ORIGINAL INTERRUPTION OFFSET

;STORE INFO
MOV [DIR_ANT_INT],AX
MOV [DIR_ANT_INT+2],DX

;SET 1CH TO TIMER INTERRUPTION
MOV AX, OFFSET TIMER 
MOV DS:[1CH*4],AX
MOV DS:[1CH*4 + 2],CX

STI
MOV DX, OFFSET CHECK
INT 27H
INSTALL ENDP

UNINSTALL PROC
PUSH AX BX CX DX DS ES
MOV CX, 0
MOV DS, CX
MOV ES, DS:[30H*4 +2]
MOV BX, DS:[2CH]

MOV AX, OFFSET ISR
MOV CX, CS

CLI
;UNINSTALL 30H RUTINE
MOV WORD PTR DS:[30H*4], CX
MOV WORD PTR DS:[30H*4 + 2], CX

; RESTORE PREVIOUS INTERRUPTION VECTOR
MOV AX, ES:[DIR_ANT_INT]
MOV CX, ES:[DIR_ANT_INT+2] 
MOV WORD PTR DS:[1CH*4],AX
MOV WORD PTR DS:[1CH*4 + 2],CX
STI

;RELEASE MEMORY
MOV AH, 49H
INT 21H
MOV ES, BX
INT 21H


POP ES DS DX CX BX AX
RET
UNINSTALL ENDP

CODE ENDS
END START