I was just messing around and wanted to write some code that I could write to a boot sector and boot from.
I proceeded to study assembly(since thats what you have to use when it first boots up), and I have a little program that is supposed to write "Hello World!" to the screen, then wait for a key press, and then print "You may now shutdown you computer...".
It works, but to print the string, you have to give it the length of the string. I wanted to be able to just give it the string and not have to deal with lengths, so I wrote a strlen function. Here it is:
; SHOULD put the length of the string pointed to by bp in cx
push es ; Save the values of modified variables on the stack
mov es,bp ; Set es to the pointer
strlen_loop: ; The loop sub
cmp byte[es:0],0x00 ; If the next character is 0x00, then there are no more characters!
je strlen_end ; Jump to the end
inc cx ; Increment the length
inc bx ; Increment the string pointer
jmp strlen_loop ; Repeat!
strlen_end: ; the end sub
pop es ; Restore modified variables
ret ; return
Like I said, I'm new to assembly, so this seemed like the way to do it, but i'm not sure.
I know my strings are NULL terminated, but it isn't getting the correct string length( it doesn't get stuck forever, but it gets WAY too long)
I think the problem lies in the cmp byte[es:0],0x00 part, but I'm not sure.
Am I doing this correctly?
by the way, I'm using the nasm assembler.