Day 20

Writing Files

	See Day 19 for the note that I want to make, the same thing applies here.

Opening and Writing a File
	Before we can read anything, we need to open it first:

		jmp start
		filename db 'elmo.txt',0	
		handle dw 0   ; will be the file handle. the number that DOS assigns to the open file.
		len db 255   ; we need an input buffer this time.	
		act db 0
		buffer db 255 DUP 0
	start:
		mov ah,3Dh   ; 3Dh of DOS Services opens a file.
		mov al,1   ; 0 - for reading. 1 - for writing. 2 - both
		mov dx,offset filename  ; make a pointer to the filename
		int 21h   ; call DOS
		mov handle,ax   ; Function 3Dh returns the file handle in AX, here we save it for later use.

	Get input:

		mov ah,0Ah
		mov dx,offset len
		int 21h

	DOS Service Function number 40h writes to a file.

		mov ah,40h
		mov bx,offset act ; pointer to number of bytes read from user.
		mov cl,byte [bx]   ; get the contents of the byte at the pointer.
		; note that even though CX takes the length, CL physically IS the low byte of CX.
		mov dx,offset buffer  ; pointer to the actual data in DX.
		mov bx,handle    ; BX needs the file handle.
		int 21h   ; call DOS

	Now we can just exit, because we've accomplished our mission:

		mov ah,4Ch
		int 21h      ; Function 4Ch (Exit Program)
	
It looks like a lot like that, but don't forget that you can make your own subroutines and CALL them.

This Day In Review

	You can probably tell I used the previous Day's file as a template for this, but oh well!

	Happy files!,
		- GbaGuy


Intro - Day 21