; LABELS.ASM
;
; Purpose:
;  - Show the difference in syntax between two types of labels:
;      Labels on CODE require a colon after the label (code=colon).
;      Labels on DATA must not be followed by a colon (data=don't).
;  - Demonstrate a common error in coding loop algorithms.
;
; Algorithm PseudoCode:
;
;    initialize counter from MAXL
;    do {
;       output an asterisk
;       decrease counter by value in DOWN
;    } while ( counter is not zero )
;    exit with zero return code
;
; Note1: This code has a bug - the loop never terminates.  Why?
;
; Note2: 
; Many comments in this file are intended to teach students about the
; meaning of various assembler instructions.  They are NOT suitable
; for students documenting their own code, since they do not refer to
; any process or problem being solved.
;
; Students: Do NOT copy the teacher-style comments into your own code.
;
; -Ian! D. Allen - idallen@idallen.ca - www.idallen.com
;  July 2000, November 2009
;
;----------------------------------------------------------------------
; Both the Code Segment (CS) and Data Segment (DS) are the same.
Labels  segment
        assume  CS:Labels,DS:Labels ; .COM format uses only one segment
        org     100h            ; .COM format executable starts at 100h
;----------------------------------------------------------------------
START:  MOV     BX,MAXL ; initialize BX to start the loop

DOLOOP: MOV     DL,CHAR ; output this CHAR on the screen...
        MOV     AH,02h  ;   using DOS service 02h and...
        INT     21h     ;   interrupt number 21h (remember the 'h'!)

        SUB     BX,DOWN ; decrease BX and loop back up if not zero
        JNZ     DOLOOP  ; this is a conditional "jump if not zero"

        MOV     AH,4Ch  ; code to terminate this LABELS program
        MOV     AL,0h   ; set a zero return status from LABELS
        INT     21h     ; call the DOS service routine to do the job

MAXL    DW      0FFh    ; max loop start value
DOWN    DW      2h      ; go down by this much each time
CHAR    DB      '*'     ; output this character
;----------------------------------------------------------------------
Labels  ends
        end start       ; "start" is the label on the first instruction