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Terminate but Stay Resident (TSR) program of Real Time Clock (Assembly x86)

April 23, 2016
Published By : Pratik Kataria
Categorised in: ,

code segment para
assume cs:code
org 100h                ;prog seg prefix addrss hex no of 256
jmp initze
savint dd ?             ;for saving address of es:bx.
count dw 0000h          ;count of 17 tics.
hours db ?
mins db ?
sec db ?

testnum:
push ax         ;store all the contents of register
push bx         ;(not to change original values of register)
push cx
push dx
push cs
push es
push si
push di
mov ax,0b800h   ;starting address of display, to access screen memory
mov es,ax    ;es has base and bx has offset by default
mov cx,count
inc cx
mov count,cx
cmp cx,011h    ;hexadecimal value of 17
jne exit
mov cx,0000h
mov count,cx
call time
exit:
pop di
pop si
pop es
pop ds
pop dx
pop cx
pop bx
pop ax
jmp cs:savint   ;jump to normal isr

;——————convert procedure——————–
convert proc
and al,0f0h
ror al,4
add al,30h
call disp
mov al,dh
and al,0fh
add al,30h
call disp
ret
endp

;————————time procedure—————-
time proc
mov ah,02h      ;character outpuT
int 1ah        ;this gives system clk
mov hours,ch
mov mins,cl
mov sec,dh
mov bx,0f90h    ;location for displaying clk
mov al,hours
mov dh,hours
call convert
mov al,’:’
call disp
mov al,mins
mov dh,mins
call convert
mov al,’:’
call disp
mov al,sec
mov dh,sec
call convert
ret
endp

;———————–display procedue—————-
disp proc
mov ah,0ffh     ;for setting attribute. 0ffh -> grey background + blink. 0fh -> black background and no blink
mov es:bx,ax    ;write into video buffer
inc bx
inc bx        ;two times increment bcoz for e.g. 12 is in hours then ‘:’
ret
endp
;——————initialization————————
initze:
push cs        ;code segment
pop ds        ;pop data segment
cli             ;clear int flag so that our program can run
mov ah,35h      ;get original interrupt vector i.e. obtains address of current interrupt handler routine
mov al,08h      ;intrrupt no. 08 -> for timer
int 21h
mov word ptr savint,bx        ;base is saved to savint temporarily
mov word ptr savint+2,es    ;offset is saved. +2 because 2 bytes of base is skipped.
mov ah,25h      ;set int vector to our ISR
mov al,08h
mov dx,offset testnum   ;new add for intrrupt        ;before it was es:bx now it is ds:dx for setting.    ;offset is keyword
int 21h
mov ah,31h              ;make prog resident(request tsr)
mov dx,offset initze    ;size of program
sti            ;set intrrupt flag
int 21h
code ends
end

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Pratik Kataria is a budding programmer, web designer and developer.