Check first character in String in assembly

Question

Im using NASM

I'd like to compare the first character of a string in assembly to see if its blank.

            mov byte [NAME], 40
            lea dx, [NAME]
            mov ah, 0ah
            int 21h

            cmp [NAME],""
            je somewhere

This doesnt work.. any advice?

Jester · Accepted Answer · 2015-10-04 12:56:20Z

2

For starters, the int21/0ah puts the actual contents at NAME+2 since at NAME you specify the buffer length and at NAME+1 the actual length is returned. All of this is of course documented.

The cmp should use a size specification and also simple quotes. As such, cmp byte [NAME+2], ' ' could work.

answered Oct 4, 2015 at 12:56

Jester

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3 Comments

user3079563 Over a year ago

i used NAME times 43 db "$", so will it be cmp byte [NAME +2], '$' ?

Michael Over a year ago

@user3079563: What are you trying to achieve with cmp byte [NAME +2], '$'? If you wanto to check if the character is a space, compare against ' ', not '$'. If you want to check if nothing was written to the buffer, check the byte at [NAME+1] (number of characters actually read, excluding CR)

Peter Cordes Over a year ago

if [NAME] holds the buffer len, and the system call doesn't clobber that, @user3079563 should just have the length byte in the data section, and leave out the mov byte [NAME], 40 instruction, right?

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