title SpaceInvaders										(spacegame.asm)
 
; Dani Horowitz
; CSC111 x86 Assembly Programming
 
INCLUDE Irvine32.inc
 
.data			; begin data segment
	use_default		byte	0		; don't use user variables?	
	; ##### USER VARIABLES #####
	; ## used if use_default=1 #
	; ##########################
	dodge_bullets	byte	1		; boolean for whether invaders can dodge bullets
	invader_count	dword	20		; number of invaders wanted (min 14)
	bottom_row		byte	20		; screen height (console vs fullscreen)
	; ##########################
	; ##########################
	invader_char	=	2		; character symbol to represent
	player_char		=	1		; character symbol to represent
	bullet_char		=	30		; character symbol to represent
	invader_color	=	red
	player_color	=	lightCyan
	bullet_color	=	yellow
	default_color	=	white
	; ##########################
	invader_steps	byte	1	; number of rows and columns invaders move at a time
	game_running	byte	0	; boolean value for whether game is running or not
	; ##########################
	bullet_row		byte	0	; current location of bullet (dh)
	bullet_col		byte	0	; current location of bullet (dl)
	invader_col		byte	5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70
	invader_row		byte	2, 5,  8,  5,  2,  5,  8,  5,  2,  5,  8,  5,  2,  5
	current_loc		byte	?	; player's current column (dl) location
	direction		byte	?	; player's left/right direction
	inv_direction	byte	0	; current direction of invaders
	; ##########################
	time			dword	0	; iterations of the game loop (loop counter)
	delay_time		dword	70	; length of pause time
	points			dword	0	; invaders killed
	counter			dword	0	; invaders that escaped
	number_invaders	dword	14	; current number of invaders
	icounter		dword	0	; invader loop counter
	score			sdword	0
	; ##########################
	msg_bottom		byte	21	; bottom location for message (dh)
	msg_left		=		2	; left location for message (dl)
	msg_right		=		65	; right location for score (dl)
	; ##########################
	stdInHandle		dword	?			; keyboard input handler
	buffer			byte	80 DUP(?)	; keyboard input buffer
	bug_fix_counter	byte	0
; ##################################################
.code			; begin code segment
; ##################################################
 
; ##################################################
INIT_KEYBOARD PROC
; get handle to standard keyboard input
; populate the stdInHandle variable
; ##################################################
	; abstraction is a wonderful, glorious thing!
	INVOKE		GetStdHandle,		STD_INPUT_HANDLE
	mov			stdInHandle, eax
	ret
; ##################################################
INIT_KEYBOARD ENDP
; ##################################################
 
; ##################################################
KYBD_HANDLER PROC
; set console flags to appropriate mode to read a single character
; determine if a new character is waiting in the buffer
; if yes, read it in and call the KEY_LISTENER procedure
; reset console flags to their previous values
; bug: all keypresses are executed twice ??
; ##################################################
.data
	saveFlags		dword	?	; default console flags
	bytesRead		dword	?	; number of bytes in buffer
.code
	INVOKE		GetConsoleMode,		stdInhandle, saveFlags
	INVOKE		SetConsoleMode,		stdInHandle, 0
	; ----------
	; peek in buffer and see if anything new is there
	INVOKE	PeekConsoleInput,
		stdInHandle, ADDR buffer, 1, ADDR bytesRead
	; if no, skip to the end
	cmp		bytesRead, 1
	jne		Done
	; ----------
	; if a keypress is waiting in the buffer,
	; put its value into al and call KEY_LISTENER
	; to do something with it
	INVOKE	ReadConsoleInput,
		stdInHandle, ADDR buffer, 1, ADDR bytesRead
	xor		eax, eax
	mov		esi, OFFSET buffer
	add		esi, 14
	mov		al, [esi]
	call	KEY_LISTENER
Done:
	INVOKE		SetConsoleMode,		stdInHandle, saveFlags
	ret
; ##################################################
KYBD_HANDLER ENDP
; ##################################################
 
; ##################################################
PRINT_BLANK PROC
; assume dh and dl have already been set
; position cursor
; print a blank space to the screen
; ##################################################
	mov		al, ' '
	call	Gotoxy
	call	WriteChar
	ret
; ##################################################
PRINT_BLANK ENDP
; ##################################################
 
; ##################################################
PRINT_INVADER PROC
; assume dh and dl have already been set
; position cursor
; set cursor color to the invader color
; print the invader character to the screen
; reset cursor color to the default color
; ##################################################
	call	Gotoxy
	mov		eax, invader_color
	call	SetTextColor
	mov		al, invader_char
	call	WriteChar
	mov		eax, default_color
	call	SetTextColor
	ret
; ##################################################
PRINT_INVADER ENDP
; ##################################################
 
; ##################################################
PRINT_PLAYER PROC
; assume dh and dl have already been set
; position cursor
; set cursor color to the player color
; print the player character to the screen
; reset cursor color to the default color
; ##################################################
	call	Gotoxy
	mov		eax, player_color
	call	SetTextColor
	mov		al, player_char
	call	WriteChar
	mov		eax, default_color
	call	SetTextColor
	ret
; ##################################################
PRINT_PLAYER ENDP
; ##################################################
 
; ##################################################
PRINT_BULLET PROC
; assume dh and dl have already been set
; position cursor
; set cursor color to the bullet color
; print the bullet character to the screen
; reset cursor color to the default color
; ##################################################
	call	Gotoxy
	mov		eax, bullet_color
	call	SetTextColor
	mov		al, bullet_char
	call	WriteChar
	mov		eax, default_color
	call	SetTextColor
	ret
; ##################################################
PRINT_BULLET ENDP
; ##################################################
 
; ##################################################
INIT_GAME PROC
; determine if using default variables
; if not using defaults, call INIT_VARS procedure to prompt for them
; screen print that the game is starting
; call INIT_KEYBOARD procedure to set up the keyboard console input handler
; call CREATE_OBJS procedure to print the player and invaders on the screen
; call RUN_GAME procedure to run the game loop
; return to main when finished
; ##################################################
.data
	msg					byte	"Game starting ...", 0
	msg_howto_left		byte	"F to move left ", 0
	msg_howto_right		byte	"J to move right", 0
	msg_howto_fire		byte	"B to fire      ", 0
	msg_howto_quit		byte	"E to quit game ", 0
.code
	mov		eax, default_color	; set cursor to use default text color
	call	SetTextColor
	.IF use_default == 0		; use user-defined variables?
		call	INIT_VARS			; initialize global variables
		call	DumpRegs			; pretty print out what is in the registers
	.ENDIF
	mov		edx, OFFSET msg
	call	WriteString			; print "Game starting"
	call	Crlf
	; give user directions how to play
	mov		edx, OFFSET msg_howto_left
	call	WriteString
	call	Crlf
	mov		edx, OFFSET msg_howto_right
	call	WriteString
	call	Crlf
	mov		edx, OFFSET msg_howto_fire
	call	WriteString
	call	Crlf
	mov		edx, OFFSET msg_howto_quit
	call	WriteString
	call	Crlf
	mov		eax, 500
	call	Delay				; pause
	call	WaitMsg
	call	Clrscr
	call	INIT_KEYBOARD		; set up keyboard handler to get input
	call	CREATE_OBJS			; create visual objects
	call	RUN_GAME			; run the game
	ret							; game over
; ##################################################
INIT_GAME ENDP
; ##################################################
 
; ##################################################
INIT_VARS PROC
; this is only invoked if use_default=0
; prompt for and populate user-defined value for dodge_bullets
; prompt for and populate user-defined value for invader_count
; prompt for and populate user-defined value for bottom_row
; use bottom_row value to also set the msg_bottom variable
; return to INIT_GAME when finished
; ##################################################
.data
	promptA		byte	"Allow invaders to dodge bullets? (0=no 1=yes) ", 0
	promptB		byte	"Maximum number of invaders? (minimum 14) ", 0
	promptC		byte	"How many screen rows for your monitor? (20 or 45) ", 0
.code
PromptForA:
	mov		edx, OFFSET promptA		; prompt for dodge_bullets
	call	WriteString
	call	ReadInt					; assume user enters a small value
	mov		dodge_bullets, al		;  and prey this doesn't break
	.IF al!=0 && al!=1
		jmp		PromptForA
	.ENDIF
	; ----------
PromptForB:
	mov		edx, OFFSET promptB		; prompt for # of invaders
	call	WriteString
	call	ReadInt
	mov		invader_count, eax
	cmp		eax, 14
	jl		PromptForB
	; ----------
PromptForC:
	mov		edx, OFFSET promptC		; prompt for screen resolution
	call	WriteString
	call	ReadInt
	mov		bottom_row, al
	.IF al!=20 && al!=45
		jmp		PromptForC
	.ENDIF
	inc		al
	mov		msg_bottom, al
	ret
; ##################################################
INIT_VARS ENDP
; ##################################################
 
; ##################################################
RUN_GAME PROC
; expects a bunch of stuff to already be populated
; continuously runs game loop until all invaders accounted for
;  by either having been killed or escaped
; call KYBD_HANDLER procedure to check for a key press
; move existing bullet up
; move a random invader down
; when appropriate, call the END_GAME procedure
; return to INIT_GAME when finished
; ##################################################
.data
	msg_run_game		byte	"Game started   ", 0
.code
	; ##############################
	mov		dh, msg_bottom
	mov		dl, msg_left
	call	Gotoxy
	mov		edx, OFFSET msg_run_game
	call	WriteString
	; ##############################
	call	Randomize				; randomize seed
	mov		game_running, 1			; set game_running flag on
Step:
	mov		eax, delay_time			; pause
	.IF delay_time<20
		mov	delay_time, 20
	.ENDIF
	call	Delay
	mov		dh, 0
	mov		dl, 0
	call	Gotoxy					; position cursor at top left of screen
	mov		eax, counter
	add		eax, points
	cmp		eax, number_invaders	; loop if not all invaders accounted for
	jge		GameOver
	call	KYBD_HANDLER			; continously run
	call	MOVE_BULLET				; step bullet, if exists, up
	call	MOVE_INVADERS			; step invaders down
	call	PRINT_SCORE
	inc		time					; increment game time
	cmp		game_running, 0			; game over if game_running flag switched to off
	je		GameOver
	jmp		Step
GameOver:
	call	END_GAME				; end game
	ret
; ##################################################
RUN_GAME ENDP
; ##################################################
 
; ##################################################
END_GAME PROC
; assumes points has been populated with # of invaders killed
; assumes counter has been populated with # of invaders escaped
; assumes time has been populated with # of game loop iterations
; clears the screen
; pretty prints points, counter, time variables to the screen
; return to end of RUN_GAME when finished
; ##################################################
.data
	msg_points		byte	"Invader Death Counter: ",0
	msg_counter		byte	"Invaders That Escaped: ",0
	msg_time		byte	"Game length: ", 0
.code
	call	Clrscr
	; ----------
	mov		dh, 10
	mov		dl, 10
	call	Gotoxy
	mov		edx, OFFSET msg_points		; print points
	call	WriteString
	mov		eax, points
	call	WriteDec
	; ----------
	mov		dh, 15
	mov		dl, 10
	call	Gotoxy
	mov		edx, OFFSET msg_counter		; print counter
	call	WriteString
	mov		eax, counter
	call	WriteDec
	; ----------
	mov		dh, 20
	mov		dl, 10
	call	Gotoxy
	mov		edx, OFFSET msg_time		; print timer
	call	WriteString
	mov		eax, time
	call	WriteDec
	; ----------
	mov		dh, 30
	mov		dl, 10
	call	Gotoxy
	ret
; ##################################################
END_GAME ENDP
; ##################################################
 
; ##################################################
PRINT_SCORE PROC
; pretty print current game score at bottom right of screen
; ##################################################
.data
	msg_score		byte	"Score: ", 0
	msg_speed		byte	"Game Speed: ", 0
.code
	mov		dh, msg_bottom
	mov		dl, msg_right
	call	Gotoxy						; position cursor
	mov		edx, OFFSET msg_score
	call	WriteString					; write "Score: "
	mov		eax, score
	call	WriteInt					; write the value
	; ----------
	mov		dh, msg_bottom
	inc		dh
	mov		dl, msg_left
	call	Gotoxy						; position cursor
	mov		edx, OFFSET msg_speed		; write "Game Speed: "
	call	WriteString
	mov		eax, 70
	sub		eax, delay_time
	call	WriteDec					; write the value
	ret
; ##################################################
PRINT_SCORE ENDP
; ##################################################
 
; ##################################################
CREATE_OBJS PROC
; self-explanatory
; return to INIT_GAME when finished
; ##################################################
	call	Clrscr
	call	CREATE_PLAYER
	call	CREATE_INVADERS
	ret
; ##################################################
CREATE_OBJS ENDP
; ##################################################
 
; ##################################################
CREATE_PLAYER PROC
; draw player at bottom center screen location
; populate player coordinate to correct value
; return to CREATE_OBJS when finished
; ##################################################
	mov		dh, bottom_row		; row current_bottom
	dec		dh
	mov		dl, 40				; column 40
	call	PRINT_PLAYER
	mov		current_loc, 40		; set current_loc variable
	ret
; ##################################################
CREATE_PLAYER ENDP
; ##################################################
 
; ##################################################
CREATE_INVADERS PROC
; traverse a loop to draw 14 invaders at starting locations
; starting locations are read from byte arrays
; return to CREATE_OBJS when finished
; ##################################################
	mov		icounter, 0					; initialize loop
PrintInvader:
	mov		esi, OFFSET invader_col		; get col
	add		esi, icounter
	mov		dl, [esi]					; set column value
	mov		esi, OFFSET invader_row		; get row
	add		esi, icounter
	mov		dh, [esi]					; set row value
	call	PRINT_INVADER				; print invader to the screen
	inc		icounter					; increment loop counter
	cmp		icounter, 14					; did we do all 14 invaders?
	jl		PrintInvader				;  if not, do next invader
	ret
; ##################################################
CREATE_INVADERS ENDP
; ##################################################
 
; ##################################################
MOVE_PLAYER PROC
; invoked from within KEY_LISTENER
; assumes a keyboard char to move the player has been pressed
; prints empty character over player's current location
; moves player's current location variable one space left or right
; prints player character to screen at the new location
; doesn't allow player to move out of bounds
; return to KEY_LISTENER when finished
; ##################################################
.data
	temp_dl					byte	?
	temp_dh					byte	?
	msg_move_player_left	byte	"Moving left    ", 0
	msg_move_player_right	byte	"Moving right   ", 0
.code
	inc		bug_fix_counter			; bug workaround!!
	.IF bug_fix_counter==2
		mov		bug_fix_counter, 0
		jmp		Done
	.ENDIF							; end bug fix :(
	mov		dh, bottom_row			; erase from old location
	dec		dh
	mov		dl, current_loc
	call	PRINT_BLANK
	mov		temp_dh, dh
	mov		temp_dl, dl
	; ----------
	.IF direction == 'j'			; determine direction
		cmp		dl, 70
		jge		RightBound			; handle out of bounds
		inc		temp_dl				; increment dl
		; ##############################
		mov		dh, msg_bottom
		mov		dl, msg_left
		call	Gotoxy
		mov		edx, OFFSET msg_move_player_right
		call	WriteString
		; ##############################	
	.ELSE
		cmp		dl, 5
		jle		LeftBound			; handle out of bounds
		dec		temp_dl				; decrement dl
		; ##############################
		mov		dh, msg_bottom
		mov		dl, msg_left
		call	Gotoxy
		mov		edx, OFFSET msg_move_player_left
		call	WriteString
		; ##############################
	.ENDIF
		jmp		Move
LeftBound:
	mov		dl, 5
	jmp		Move
RightBound:
	mov		dl, 70
Move:
	mov		dl, temp_dl
	mov		dh, temp_dh
	mov		current_loc, dl		; position cursor at new location
	call	PRINT_PLAYER		; print the player all pretty-like
Done:
	ret
; ##################################################
MOVE_PLAYER ENDP
; ##################################################
 
; ##################################################
KEY_LISTENER PROC
; assumes al is populated with a character pressed from the keyboard
; initiates FIRE procedure if 'b' key pressed
; sets game_running flag to off if 'e' key is pressed
;  otherwise, initiates MOVE_PLAYER procedure
; returns to KYBD_HANDLER when finished
; ##################################################
	mov		direction, al		; stick it in direction
	.IF direction == 'b'		; if entered 'b' fire bullet
		call	FIRE
	.ELSEIF direction == 'e'
		mov		game_running, 0
	.ELSE
		call	MOVE_PLAYER			;   else, move player
	.ENDIF
Done:
	ret
; ##################################################
KEY_LISTENER ENDP
; ##################################################
 
; ##################################################
FIRE PROC
; assumes 'b' key has been pressed to fire a bullet
; determine starting location of bullet
; draw bullet to screen if another bullet isn't in progress
; return to KEY_LISTENER when finished
; ##################################################
.data
	msg_fire			byte	"Bullet fired   ", 0
	msg_fire_inprogress	byte	"Cannot Fire    ", 0
.code
	; ##############################
	mov		dh, msg_bottom
	mov		dl, msg_left
	call	Gotoxy
	mov		edx, OFFSET msg_fire
	call	WriteString
	; ##############################
	cmp		bullet_col, 0					; is a bullet currently in progress?
	jne		ExistingBullet					;  if yes, skip down and do nothing
	mov		dh, bottom_row
	sub		dh, 2
	mov		bullet_row, dh					; position bullet's starting location
	mov		al, current_loc					;  from player's current location
	mov		bullet_col, al
	mov		dh, bullet_row					; set bullet location
	mov		dl, bullet_col
	call	PRINT_BULLET					; print bullet to screen
	sub		score, 50
	jmp		Done
ExistingBullet:
	; ##############################
	mov		dh, msg_bottom					; only one bullet allowed in the air at a time
	mov		dl, msg_left
	call	Gotoxy
	mov		edx, OFFSET msg_fire_inprogress
	call	WriteString
	; ##############################
Done:
	ret
; ##################################################
FIRE ENDP
; ##################################################
 
; ##################################################
MOVE_BULLET PROC
; determine if there are any bullets in progress
; if yes, move bullet up
; update bullet location
; erase bullet from old location and draw to new location
; initiate procedure to determine if bullet hit an invader
; if bullet reached top of screen, update variables to
;  indicate no bullets are in progress
; return to loop in RUN_GAME when finished
; ##################################################
	cmp		bullet_row, 0		; determine if bullet is in progress
	je		NoBullets			;  if no, there is nothing to move
	mov		dh, bullet_row
	mov		dl, bullet_col
	call	PRINT_BLANK			; write empty string in current location
	mov		al, invader_steps	; move bullet_row up
	sub		bullet_row, al
	mov		dh, bullet_row
	call	PRINT_BULLET
	call	HIT_INVADER			; see if we hit any invaders
	.IF dodge_bullets==1
		call	SHIFT_INVADERS	; shift invaders to the side
	.ENDIF
	; ----------
	cmp		bullet_row, 1		; see if we reached the top of the screen
	jle		HideBullet
	jmp		NoBullets
HideBullet:
	mov		bullet_row, 0		; set value to default (no bullets in progress)
	mov		bullet_col, 0		; set value to default (no bullets in progress)
	sub		score, 100
NoBullets:
	ret
; ##################################################
MOVE_BULLET ENDP
; ##################################################
 
; ##################################################
HIT_INVADER PROC
; traverse through each column an invader is in
; determine if bullet is in the same column
;  if yes, check to see if rows match too
; if everything matches, invoke KILL_INVADER procedure
; return to MOVE_BULLET when finished
; ##################################################
	mov		icounter, 0
CheckCol:
	mov		esi, OFFSET invader_col		; find the invader column array
	add		esi, icounter
	mov		dl, [esi]					; get column location for invader
	cmp		bullet_col, dl				; compare to column location for bullet
	je		CheckRow					; if a match, check the row
	inc		icounter
	cmp		icounter, 14					; loop for all invaders
	jl		CheckCol
	jmp		Done
CheckRow:
	mov		esi, OFFSET invader_row		; find the invader row array
	add		esi, icounter
	mov		dh, [esi]					; get row location for invader
	mov		dl, bullet_row				; get row location for bullet
	sub		dl, dh						; difference in location
	.IF dl<=invader_steps && dl>=0 && dh!=bottom_row
		call	KILL_INVADER			; if we got here, he's dead
	.ENDIF
Done:	
	ret
; ##################################################
HIT_INVADER ENDP
; ##################################################
 
; ##################################################
KILL_INVADER PROC
; delete bullet from screen
; draw an X where invader was killed
; pause, and then draw a blank space
;  (need to handle this separately in case going at different speeds)
; move bullet location to default (none in progress)
; set invader row to bottom_row
; if we want another invader, create it!
; increase points value
; return to HIT_INVADER when finished
; ##################################################
.data
	msg_kill_invader		byte	"Invader Killed ", 0
.code
	; ##############################
	mov		dh, msg_bottom
	mov		dl, msg_left
	call	Gotoxy
	mov		edx, OFFSET msg_kill_invader
	call	WriteString
	; ##############################
	add		score, 500
	mov		dh, bullet_row
	mov		dl, bullet_col
	call	PRINT_BLANK					; delete bullet from screen
	; ----------
	mov		esi, OFFSET invader_row		; find the invader row array
	add		esi, icounter
	mov		dh, [esi]
	mov		esi, OFFSET invader_col		; find the invader col array
	add		esi, icounter
	mov		dl, [esi]
	; ----------
	mov		al, 'X'						; X marks the spot
	call	Gotoxy
	call	WriteChar					; draw an X really quickly
	mov		eax, 100
	call	Delay
	call	PRINT_BLANK					; pause, and then draw a blank space over the X
	; ----------
	mov		bullet_col, 0				; move bullet column location to default
	mov		bullet_row, 0				; move bullet row location to default
	mov		esi, OFFSET invader_row		; find the invader row array
	add		esi, icounter
	mov		dh, bottom_row
	mov		eax, invader_count
	.IF number_invaders < eax			; do we want more invaders?
		mov		dh, 2					;  if yes, create a new one
		inc		number_invaders
	.ENDIF		
	mov		[esi], dh					; set invader row to bottom_row
	inc		points						; increase points value
	sub		delay_time, 2				; game runs faster as it progresses
	ret
; ##################################################
KILL_INVADER ENDP
; ##################################################
 
; ##################################################
MOVE_INVADERS PROC
; find a random invader
; if the invader is already gone, choose another one
; move the chosen invader down
; if invader reached the bottom, delete the invader
; if we want another invader, create it!
; return to loop in RUN_GAME when finished
; ##################################################
.data
	prevent_loop	byte	0			; just in case ...
.code
	mov		prevent_loop, 0
ChooseInvader:							; prevent an infinite loop if
	inc		prevent_loop				;  no invaders are available
	cmp		prevent_loop, 14
	jg		Done
	; ----------
	mov		eax, 14						; generate random number between 0-13
	call	RandomRange
	mov		icounter, eax				; put random number into icounter
PrintInvader:
	mov		esi, OFFSET invader_col		; get invader row array
	add		esi, icounter
	mov		dl, [esi]					; set dl to current column of icounter invader
	mov		esi, OFFSET invader_row		; get invader column array
	add		esi, icounter
	mov		dh, [esi]					; set dh to current row of icounter invader
	; ----------
	cmp		dh, bottom_row				; if invader is gone already, choose another invader
	jge		ChooseInvader
	; ----------
	call	PRINT_BLANK					; delete invader from current location
	; ----------
	add		dh, invader_steps
	mov		al, bottom_row
	dec		al
	cmp		dh, al						; did we reach bottom?
	jge		HideInvader					;  if yes, delete invader
	call	PRINT_INVADER
Continue:
	mov		[esi], dh					; update invader array
	jmp		Done
HideInvader:
	sub		score, 100
	mov		dh, bottom_row				; position below player
	inc		counter						; add to invaders that escaped counter
	call	PRINT_BLANK
	mov		eax, invader_count
	.IF number_invaders < eax			; do we want more invaders?
		mov		dh, 2					;  if yes, create a new one
		inc		number_invaders
	.ENDIF		
	jmp		Continue	
Done:
	ret
; ##################################################
MOVE_INVADERS ENDP
; ##################################################
 
; ##################################################
SHIFT_INVADERS PROC
; only when a bullet is in the air:
; every few iterations, move all invaders right or left
; don't let invaders go out of bounds
; return to MOVE_BULLET when finished
; ##################################################
	mov		icounter, 0
ShiftInvaders:
	mov		esi, OFFSET invader_row		; find the invader row array
	add		esi, icounter
	mov		dh, [esi]					; get row location for invader
	mov		esi, OFFSET invader_col		; find the invader column array
	add		esi, icounter
	mov		dl, [esi]					; get column location for invader
	cmp		dh, bottom_row
	jge		NextInvader
	; ----------
	call	PRINT_BLANK					; write a blank space to current location
	; ----------
	.IF inv_direction == 5				; wobble back and forth
		add	dl, invader_steps
	.ELSEIF inv_direction == 10
		sub	dl, invader_steps
	.ENDIF
	.IF dl < 5							; don't let invaders go out of bounds
		mov	dl, 5
	.ELSEIF dl > 70
		mov	dl, 70
	.ENDIF
	mov		[esi], dl
	; ----------
	mov		esi, OFFSET invader_row		; get invader row array
	add		esi, icounter
	mov		dh, [esi]					; set dh to current row of icounter invader
	mov		esi, OFFSET invader_col		; get invader column array
	add		esi, icounter
	mov		dl, [esi]					; set dh to current column of icounter invader
	call	PRINT_INVADER
	; ----------
NextInvader:
	inc		icounter
	cmp		icounter, 14				; loop for all invaders
	jl		ShiftInvaders
	.IF inv_direction < 15				; don't wobble every time
		inc	inv_direction
	.ELSE
		mov	inv_direction, 0
	.ENDIF
	ret
; ##################################################
SHIFT_INVADERS ENDP
; ##################################################
 
;---------------------------------------------------
main proc
; our glorious starting point
;---------------------------------------------------
	call	Clrscr				; clear the screen
	call	INIT_GAME			; initialize game variables and start game
	call	WaitMsg
	exit						; game over!
main  endp
end main
;---------------------------------------------------