;FILENAME:	UNIT7.TXT
;
; ASSEMBLY LANGUAGE COURSE, UNIT 7
; By Don Stoner Revision: 2008.06.25
;
; WARNING: Assembly language programs,
; can do absolutely anything!
; Eventually, 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!
;
; To edit, assemblem and test, type:
;
; MAKE UNIT7
; UNIT7
;
; Code Starts at 100h
;
	ORG 100h
	;
	; This time we echo the typed
	; keys in both ASCII and "hex."
	; To make things easier, we'll
	; make subroutines for "HEX"
	; output, for "Space," and for
	; "Carriage return."
	;
	; Tell NASM where to loop back:
	;
LOOP_BACK_TO_HERE:
	;
	; First, we read a key into AL
	;
	CALL ASCII_IN
	;
	; We won't bother saving AL;
	; we'll have the subroutines
	; take care of that.
	;
	; Next, we echo it in ASCII:
	;
	CALL ASCII_OUT	;AL out
	;
	CALL SPACE_OUT	;Output a space
	;
	CALL HEX_OUT	;in Hexadecimal
	;
	CALL CRLF_OUT  ;And output a
	;	       ;Carriage Return
	;	       ;and a Line Feed
	;
	; Finally, we either loop back,
	; or exit.
	;
	CMP AL,"x"	;Loop if "x"
	JNE LOOP_BACK_TO_HERE
	INT 20h	    ;Else, back to DOS

; End of Main code
;--------------------------------------
; Subroutines:

;
; Output a Space
;
; (PUSH and POP commands "save data on"
; and "restore data from" the "stack."
;
SPACE_OUT:
	PUSH AX	   ;save AL (and AH)
	MOV AL," "	;output an " "
	CALL ASCII_OUT	;ASCII space
	POP AX	   ;restore AL (and AH)
	RET
;
; Output a "Carriage Return" and a
; "Line Feed"
;
CRLF_OUT:
	PUSH AX	   ;save AL (and AH)
	MOV AL,13  ;output a C.R.
	CALL ASCII_OUT
	MOV AL,10  ;output a L.F.
	CALL ASCII_OUT
	POP AX	   ;restore AL (and AH)
	RET
;
; Output AL in Hexadecimal
;
HEX_OUT:
	PUSH AX	   ;save AL (and AH)
	SHR AL,4   ;step 1: MSN -> LSN
	CALL LSN_OUT	;LSN (MSN) out
	POP AX	   ;restore AL (and AH)
LSN_OUT:
	PUSH AX	   ;save AL (and AH)
	AND AL,0FH	;step 2
	OR AL,"0"	;step 3
	CMP AL,":"	;step 4
	JC HEXOK2	;step 4
	ADD AL,"A"-":"	;step 4
HEXOK2:	CALL ASCII_OUT	;step 5
	POP AX	   ;restore AL (and AH)
	RET
;
; Output AL in ASCII
;
ASCII_OUT:
	MOV AH,0Eh ;select "output"
	MOV BL,0   ;select page and
	MOV BH,0   ;graphics color
	INT 10h	   ;BIOS routines
	RET	;return to main code
;
; Input AL in ASCII
;
ASCII_IN:
	MOV AH,0    ;normal keyboard
	INT 16H	    ;key into AL
	RET	;return to main code