;FILENAME:	UNIT3.TXT
;
; ASSEMBLY LANGUAGE COURSE, UNIT 3
; By Don Stoner Revision: 2008.06.17
;
;
; WARNING: Assembly language programs,
; like the ones you are going to be
; experimenting with here, can do
; absolutely anything! Eventually, as
; you progress through these units,
; YOU *WILL* CRASH the HARD DRIVE OF
; THE COMPUTER YOU ARE USING! Make sure
; that there is ABSOLUTELY NOTHING on
; that computer that you can't afford
; to loose! In fact, having a dedicated
; computer would be a very good idea.
;
; To assemble, type:
;
; MAKE UNIT3
;
; To run, type:
;
; UNIT3
;
; Tell NASM where the code goes:

	ORG 100h	;code at 100h

; Next, the real code:

	; This time we'll output four
	; letters - and make it look
	; easier:

	; Output an "A" (same as before)
	;
	MOV AL,41h	;ASCII "A"
	MOV AH,0Eh	;select "output"
	MOV BL,0	;select page and
	MOV BH,0	;graphics color
	INT 10h		;BIOS routines

	; Output a "B", but this time
	; we'll let the assembler figure
	; out the ASCII codes for us:
	;
	MOV AL,"B"	;ASCII "B"
	MOV AH,0Eh	;select "output"
	MOV BL,0	;select page and
	MOV BH,0	;graphics color
	INT 10h		;BIOS routines

	; Instead of writing all of the
	; details required by the BIOS
	; routine over again each time,
	; well just move them to a
	; subroutine at the bottom of
	; this file, which we will name
	; "ASCII_OUT"  The name is
	; arbitrary, we choose it, but
	; we do have to spell it the
	; same way every time we use
	; it; with NASM, we even have
	; to make sure we match all
	; upper and lower case letters.
	;
	; But here all we need is the
	; command to "call" that
	; subroutine:

	MOV AL,"C"     ;Choose a "C"
	CALL ASCII_OUT ;call subroutine

	; Same thing for last letter:

	MOV AL,"d"	;lowercase "d"
	CALL ASCII_OUT

	INT 20h		;back to DOS

; end of main program
;--------------------------------------
; but ...

; Down here, past the end of the main
; program, where it won't get executed
; at the wrong time, is where we put
; the subroutine:

; This is the label we have chosen. The
; colon at the end helps identify it as
; a label so NASM won't get confused.

ASCII_OUT:

; The rest of this is just the routine
; we had copied three times before. We
; don't load the AL register here
; because it contains whatever letter
; we decide to print - and that is
; selected in the main code above

	MOV AH,0Eh  ;select "output"
	MOV BL,0    ;select page and
	MOV BH,0    ;graphics color
	INT 10h	    ;BIOS routines

; Finally, instead of just falling off
; the end of the world, we have to tell
; the computer to return to just after
; whichever "CALL" instruction sent us
; here:
	RET

; And this is the end of the code