Question

I’m writing assembly code that prompts the user for a string of lower-case characters then outputs the same string with all UPPER-CASE characters. My idea is to iterate through the bytes starting at a specific address and subtract 20H (turns a lower case to upper-case) from each one until I reach a byte with a specific value. I’m fairly inexperienced with Assembly so I’m not sure what the syntax for such a loop would look like.

Can anyone provide some sample code or direct me where I can find examples of such syntax?

Any help is greatly appreciated!

Answer 1

Typically, a string is terminated with a null (0x00 in hex). Assuming this is what you choose to do, here’s some sample code. I’m not sure which assembler you’re using, or even which syntax, but this x86 code that should work in MASM:

mov cl, 0             ; cl is the counter register, set it to
                      ; zero (the first character in the string)

start:                ; Beginning of loop
  mov al, bytes[cl]   ; Read the next byte from memory

  cmp al, 0           ; Compare the byte to null (the terminator)
  je end              ; If the byte is null, jump out of the loop

  sub al, 20h         ; Convert to upper case
                      ; A better solution would be: and al, 0DFh

  ; Output the character in al

  add cl, 1           ; Move to the next byte in the string
  jmp start           ; Loop
end: