Question

I got stuck at this thing,I want to see if right shift button has been pressed,so I have this assambler code:

mov ah,2
int 16h           ;calling INT 16,2 - Read Keyboard Flags interrupt      
mov ah,10000000b
shl al,7 
and al,10000000b
cmp al,ah         ;check if first bit is 1
je rshift 
jmp final


rshift:
mov ah,9
lea dx,rsh  ;rsh is a string that says that "right shift button has been pressed"
int 21h
jmp final  

final:             ; quit program
mov ax,4c00h
int 21h

why is it not working ?,I think the problem is that int 16,2 is not working properly,if so why is that? here is what INT 16,2 is supposed to do:

AH = 02
on return:
AL = BIOS keyboard flags (located in BIOS Data Area 40:17)

|7|6|5|4|3|2|1|0|  AL or BIOS Data Area 40:17
| | | | | | | `---- right shift key depressed
| | | | | | `----- left shift key depressed
| | | | | `------ CTRL key depressed
| | | | `------- ALT key depressed
| | | `-------- scroll-lock is active
| | `--------- num-lock is active
| `---------- caps-lock is active
`----------- insert is active

I never see the message,I ve looked at the AL register in debug and it doesn t seem to change after I call INT 16,2.I m running Win 7 on a x86 arhitecture,and I m working with tasm

Answer 1

How are you testing this?

Out of curiosity, i made a simple program (code below). When run under Windows console, it detects left shift (Result: 2) but never detects right shift (expected Result: 1, but only got Result: 0).

When run under pure DOS (in VMWare), it correctly displays all combinations (0 to 3).

So it seems to be an artifact of NTVDM (Windows DOS emulation), though I don t have any sources to cite.

My code:

.model small

.code

start:
    mov ax, seg msg
    mov ds, ax

    mov ah, 2
    int 16h

    and al,3        ; get two lower bits - both SHIFTs
    add digit, al   ; convert to decimal

    lea dx, msg
    mov ah, 9
    int 21h

    mov ax, 4c00h
    int 21h

.data

msg     db   Result:  
digit   db   0 
        db  13,10, $ ,0

.stack
        db  16384 dup(?)

end start

Answer 2

you should be able to clean up your check by doing :

test al,1
jnz rshift

Answer 3

I cannot explain why your code isn t working, but you can replace

mov ah,10000000b
shl al,7 
and al,10000000b
cmp al,ah         ;check if first bit is 1

by

test al, 1

which does the same thing and is more idiomatic, if there is such a thing as an idiom in assembly.

EDIT: As Michael points out in the comment below, you need to reverse the conditional jump if you use the test instruction. test al, C sets ZF if and only the bitwise and of al AND C is zero.

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