;FILENAME:	UNIT2.TXT
;
; ASSEMBLY LANGUAGE COURSE, UNIT 2
; By Don Stoner Revision: 2008.06.17
;
; WARNING: Assembly language programs,
; can do absolutely anything! As you
; progress through these units, YOU
; *WILL* CRASH YOUR HARD DRIVE.
; Make sure that there is ABSOLUTELY
; NOTHING on your computer that you
; can't afford to loose!
;
; To save time, you should create a
; "batch file" titled MAKE.BAT:
; (ted make.bat) which first tells
; the computer to edit a file, then
; to assemble it:
;
;    ted %1.txt
;    nasn -o%1.com %1.txt
;
; To use it, just type:
;
; MAKE UNIT2
;
; at the FreeDOS command prompt. This
; will first put you in the editor to
; make any changes you want, then it
; will assemble your file. If there
; were any errors which need fixing,
; just type "make unit2" again. If
; there weren't any errors, type:
;
; UNIT2
;
; at the command prompt to run your
; program. Like before, we start with
; the ORG "pseudo op":
			;tell NASM to
	ORG 100h	;put the code
			;at 100h (256)

; Next, the first line of real code:

	MOV AL,41h	;ASCII "A"

; This is a "move" instruction. It
; tells the computer to put a 41h (65
; decimal) into one of the registers in
; the computer. That register is the
; one named "AL" (the "low" byte of the
; two-byte "AX" register). There are
; several different registers in the
; computer which all have different
; purposes. The number 65 (41h), in the
; operand, happens to be the ASCII code
; for the capital letter "A". ("ASCII"
; stands for American Stardard Code for
; Information Interchange).
;
; The next line of code:

	MOV AH,0Eh    ;select "output"

; is very similar to the previous one.
; It loads a 14 (0Eh) into the register
; named AH ("high" byte of the two-byte
; "AX" register). The "14" is not an
; ASCII code for a letter, instead it
; will be used, below, to tell the
; computer's BIOS code which one of a
; family of different BIOS routines we
; are going to want to use. ("BIOS"
; stands for Basic Input/Output System)
; 14 (0Eh) happens to be the code to
; output a letter (the ASCII "A" in the
; AL register) to the monitor.
;
; Next:
	MOV BL,0	;select page and
	MOV BH,0	;graphics color

; these two instructions are sometimes
; necessary to make the "A" display
; correctly. We won't be worrying about
; what they mean until later on.
;
; And:
	INT 10h

; Like the "INT 20h" (exit to FreeDOS)
; command, INT 10h executes one of the
; routines identified by the table of
; addresses at the bottom of memory.
; "Interrupt" 16 (10h) is the 16th
; entry in the table; it was set up by
; the BIOS ROM which is part of the
; computer's "motherboard" instead of
; by the operating system (FreeDOS).
;
; Interrupt 16 (10h), function 14 (0Eh)
; tells the BIOS routine to write the
; contents of register AL (which we
; loaded with an ASCII "A") onto the
; computer's monitor. This time, when
; you run the program, it should put
; an "A" on the monitor before it
; returns to the operating system.
;
; Can you guess how to display a "B"?
;
; This may seem like a whole lot of
; work for not getting very much done,
; but, as you will see, it gets much
; easier. We only have to do each
; thing the hard way once; after that
; we can just reuse it.
;
; And finally, like before, we exit:

	INT 20h	  ;back to FreeDOS

; See how much simpler this exit
; command looks this time than it did
; last time?