;FILENAME:	UNIT1.TXT
;
; ASSEMBLY LANGUAGE COURSE, UNIT 1
; By Don Stoner Revision: 2008.06.21
;
; 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.
;
; First thing, all lines that start
; with a simicolon (like these here)
; are merely comments. Since assembly
; language is somewhat cryptic, it is
; very important to explain what the
; code you write does and how it works.
; Otherwise, it will be very difficult
; to figure it all out again if you
; have to use the same code later.
;
; It is assumed that you will be using
; the Tiny Editor (TED) to write your
; code, the Netwide Assembler (NASM) to
; assemble it (convert it from text
; files like this one into executable
; machine language files), and that you
; will be running FreeDOS (a free REAL-
; MODE opreating system). All of these
; are free and should be installed on
; your computer.
;
; To edit this file, type:
;
; TED UNIT1.TXT
;
; at the FreeDOS command prompt. You
; will learn best by experimenting,
; so please try changing anything
; you are curious about. Make mistakes!
; To see what your changes have done,
; you need to assemble and run your
; program. To do this, first type:
;
; NASM -ounit.com unit1.txt
;
; at the command prompt. If there are
; errors, the assembler will tell you
; which line numbers contain mistakes
; and also give some rather cryptic
; clues regarding what is wrong. If
; there were no errors, then type:
;
; UNIT1
;
; to run your program. Now we are
; ready for some code:
			;(You can put
	ORG 100h	;comments over
			;here too.)

; This first command tells NASM (the
; assembler) at which address to start
; the code it generates. "ORG" means
; point of origin. 100h is the point
; of origin NASM will use. The "h"
; in 100h means the number is written
; in hexadecimal (base 16, hereafter
; "hex") instead of in normal necimal
; numbers. The hex digits are: 0, 1,
; 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D,
; E, and F (for 0-15 respectively).
; 10H means 16 and 100h means 16 x 16
; or 256. Memory offset 256 (100h) is
; where FreeDOS is going to load this
; program (it always loads it in the
; same place) and NASM will sometimes
; need to know this to translate your
; assembly code into machine language.
;
; The ORG command is calles a pseudo-
; op (as opposed to a real operation)
; It doesn't generate any code; it
; just tells the assembler where to
; put any code that it does generate.
;
; Next:
	INT 20h

; This command really does generate
; some code; specifically, it makes
; two bytes of code. The first byte
; (it will be 0CDh, the leading zero
; is to identify it as a number instead
; of a mispelled word) is called the
; "operation code" (hereafter "op
;  code"). It tells the computer what
; to do. In this case, the op code
; tells the computer to perform an
; "interrupt" - more specifically,
; interrupt number 32 (20h - which is
; the second byte of the instruction,
; called the operand). Interupt 20h has
; already been set up for us by the
; operating system (FreeDOS); all we
; have to do is use it.
;
; The INT 20h instruction causes the
; computer to jump to the 32nd (20h)
; entry in a table of 256 addresses
; which is stored at the bottom of the
; computer's memory. The 32nd address
; sends the computer to a routine in
; the FreeDOS operating system which
; will transfer control back to
; FreeDOS from our program.
;
; That's right; this two-byte program
; doesn't do anything except end
; itself!