Day 15

Comparing Strings

	Yesterday, we got a string of text from the user. Today, we'll learn how to do a case
insensitive comparison of 2 strings. However, there's a catch, these strings must be NULL
terminated (end with a 0 byte instead of a $). So instead of putting a $ on the end like
last time, we'll put a 0 at the end.

Service 2Fh
	Service 2Fh is the Multiplex Service. It allows communication between Windows and DOS.
We'll be using Function 121Eh to compare strings. That's right, a 16bit number. For 16bit function
numbers, all of AX is the function number.

How to use Function 121Eh
	Here's our input again and move down a lines:

		mov ah,0Ah
		mov ds,cs
		mov dx,offset len
		int 21h
		mov ah,2
		mov dl,10
		int 21h
		mov dl,13
		int 21h		

	This puts a 0 at the end making a NULL-terminated string.

		mov bx,offset act
		mov dx,offset buffer
		add dl,byte [bx]
		mov bx,dx
		mov byte [bx],0  ; note that the zero was $ last time

	Here we do the comparison:

		mov ax,offset msg  ; compare input with msg variable
		mov si,ax          ; SI is the first string
		mov ax,offset buffer
		mov di,ax          ; DI is the second string, it shouldn't matter what order.
		mov ax,121Eh    ; function number
		int 2Fh         ; call Multiplex Service
		je Yep         ; if strings equal goto Yep
		jmp Nope       ; goto Nope
	Yep:
		mov dx,offset good
		mov ah,9
		int 21h    ; tell the user it's good
		mov ah,4Ch
		int 21h   ; exit
	Nope:
		mov dx,offset bad
		mov ah,9
		int 21h   ; tell user no match
		mov ah,4Ch
		int 21h  ; exit

		len db 255
		act db 0
		buffer db 256 DUP 0

		msg db "Elmo",0
		bad db "How can you not say 'Elmo'!$"
		good db "I like Elmo too!$"
	

And they say ASM is hard!? If you don't get it, ask me!

This Day In Review

	Everyone always wants to compare strings, so here you go!

	Have some fun with this!,
		-Mike


Intro - Day 16