Keyboard shift status

[phreaky]

Hi all, im new to assembly and tasm and all that, taught myself from a book. Just wondering if anyone could tell me whats wrong with this code?
Thanks

;
.model small
.stack 100h
.data

msg7 db 'Insert on $'
msg6 db 'Caps lock on $'
msg5 db 'Num lock on $'
msg4 db 'Scroll lock on $'
msg3 db 'Alt pressed $'
msg2 db 'Ctrl pressed $'
msg1 db 'Left shift pressed $'
msg0 db 'right shift pressed $'

.code

Start:

mov ah, 02h ;keyboard passes bit
int 16h
mov cx, 0 ;Initialize cx to 0

ShrLoop: shr al, 1 ;Shift al one bit to the right

jc numberCheck ; jump to cl number when carry = 1
inc cl ;increment cl counter
jnc shrloop ;restart loop when carry = 0


cmp cl, 8 ; compare cl with 8 if true
je finish ; then finish

numberCheck: cmp cl, 0
je no0
cmp cl, 1
je no1
cmp cl, 2
je no2
cmp cl, 3
je no3
cmp cl, 4
je no4
cmp cl, 5
je no5
cmp cl, 6
je no6
cmp cl, 7
je no7

mov bx,0 ;Set bx to 0 so
shr bx,1 ;the carry can be reset
ret


no7: mov dx, offset msg7
call puts ; Calls procedure to display string
ret
no6: mov dx, offset msg6
call puts
ret
no5: mov dx, offset msg5
call puts
ret
no4: mov dx, offset msg4
call puts
ret
no3: mov dx, offset msg3
call puts
ret
no2: mov dx, offset msg2
call puts
ret
no1: mov dx, offset msg1
call puts
ret
no0: mov dx, offset msg0
call puts
ret


finish: mov ah, 4Ch ;End Program
int 21h




puts: ;display a string

mov ah, 9h
int 21h

ret


End start

 

[TessaBonting]

Dear phreaky,

You just made one mistake:

shr al,1 shifts right al 1 bit and place the tightmost
bit in the carry.
The leftmost bit however is shifted in by a 0.

So: if al = 0FFh, afther 1 shift al = 07Fh
and so on.

If the strating value of al = 000h, your check will never
stop, becose the carry is clear, inc cl will not change
to carry and the jnc shrloop will make your program
"hang"


ShrLoop: shr al, 1 ;Shift al one bit to the right

jc numberCheck ; jump to cl number when carry = 1
inc cl ;increment cl counter
jnc shrloop ;restart loop when carry = 0


cmp cl, 8 ; compare cl with 8 if true
je finish ; then finish


I hope i helped you a bit,

Tessa