Question

I am writing the code for the following program,

Objective:

Count the number of spaces pressed by the user. Break if enter is pressed.

Code:

.MODEL SMALL
.STACK 100H
.DATA
    TEXT DB "Enter Character"
.CODE
    MOV AX,9
    MOV DX,TEXT
    INT 21H
    BACK:
        INT21H
        CMP AL, " "
        JE INCREMENT
        CMP AL, "\n"
        JE OUT
        JMP BACK
INCREMENT:
        INC ; which register do I use here? I feel that I am out of registers
        LOOP BACK
OUT:
;display the count

In the above code, I have marked the area where I am feeling that I don’t have anymore registers left to use. I have to use AX,AL,DX,DL registers in the code. Which other register can I use to store the count? Please help, thanks.

Answer 1

You have the following general-purpose registers that you can use:

AX (AH is 8 most significant bits of AX, AL is 8 least significant bits of AX)
BX (BH is 8 MSBs of BX, BLis 8 LSBs of BX)
CX (CH is 8 MSBs of CX, CL is 8 LSBs of CX)
DX (DH is 8 MSBs of DX, DL is 8 LSBs of DX)
BP (individual halves aren’t accessible directly)
SI (individual halves aren’t accessible directly)
DI (individual halves aren’t accessible directly)

Btw, you have to set AH to 1 before int 21H that reads a character into AL from the keyboard.

And your text string must end with a dollar sign if you want to print it using int 21H function 9.

Also, Enter’s code is 13 (‘\r’), not 10 (‘\n’).

A few more issues:

• Your stack is too small for interrupts. Bump it up to 4 KB. If you don’t, they’ll overwrite some code or data in your program or in some other place. You don’t want that.
• Since this is an .EXE program (and not .COM) you must set DS to point to the data segment. See your book or notes or assembler documentation on how to do that. ES should probably be set to the same value.