Lowercase / Uppercase in Assembly Language

In this blog post I want to show how to lowercase letters by using Assembly Language.


The JNZ instruction tests ZF each time through, and knows to stop the loop when the counter goes to 0.

The operands of the ADD instruction are worth a closer look. Putting EBX inside square brackets references the contents of what is found in the address, rather than the address itself. But more important, the BYTE size specifier tells NASM that we’re only writing a single byte to the memory address in EBX. NASM has no way to know otherwise. It’s possible to write one byte, two bytes, or four bytes to memory at once, depending on what you need to do.

And here is the output: