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assemblyasciidostasmx86-16

TASM program that display letters from a to z (alternate uppercase and lowercase horizontally) using looping

The program must display letters from a to z (alternate uppercase and lowercase horizontally) using looping.

 Sample Output :

AaBb . . . . . . . . . . . . YyZz

This is the code I've used so far but it only outputs the uppercase letters. Please help me how to combine those lower case letters :(

Thank you :)

.model small
.code
.stack 100
org 100h 
start :
       mov  ah, 02h
       mov   cl, 41h 
skip :
       mov  dl, cl
       Int     21h   
       inc    cl
       cmp  cl, 5Ah
       jnz   skip
       Int    27h
end start
Solution

  • If you want them interspersed, the ASCII character set has an offset of 20h between the uppercase and lowercase letters:

    enter image description here

    You can see from that table that moving from A to a requires adding 20h (to go from 41h to 61h) and this is the same for all other letters as well.

    So, instead of simply adding one at the end of the loop, you must first:

    • add 20h.
    • print that character.
    • subtract 1fh (i.e., subtract 20h then add one).

    In other words, change:

    mov  dl, cl
int  21h
inc  cl

    into something like:

    mov  dl, cl     ; load dl with character and print.
int  21h

add  cl, 20h    ; move to lowercase variant and print.
mov  dl, cl
int  21h

sub  cl, 1fh    ; move back to next uppercase variant.

    The code can be made shorter if you know that the interrupt won't clobber the dl register you're using (and the excellent Ralf Brown interrupt list seems to indicate this is so, stating that only al is changed):

    mov  dl, cl     ; load dl with character and print.
int  21h

add  dl, 20h    ; move dl to lowercase variant and print.
int  21h

inc  cl         ; move cl to next uppercase variant.