Ant Attack: Routine at 8000

The machine code's main loop, called from BASIC line 700 (to draw the city before "READY WHEN YOU ARE") and line 720 (to play). It runs GAME_FRAME, CHECK_RESCUED and CHECK_GAME_OVER over and over, returning to BASIC when 1 is pressed (back to the city gate, and "Have another go!") or when FRAMES_LEFT counts out.

FRAMES_LEFT is both the stop signal and a frame budget. BASIC pokes 2 into it before the first call, so that call draws two frames and returns; it is then 0, and the next call decrements that to $FF, which means "run until the game says stop" -- CHECK_GAME_OVER and CHECK_RESCUED stop it by storing a small count again.

The machine code's main loop, called from BASIC line 700 (to draw the city before "READY WHEN YOU ARE") and line 720 (to play). It runs GAME_FRAME, CHECK_RESCUED and CHECK_GAME_OVER over and over, returning to BASIC when 1 is pressed (back to the city gate, and "Have another go!") or when FRAMES_LEFT counts out. FRAMES_LEFT is both the stop signal and a frame budget. BASIC pokes 2 into it before the first call, so that call draws two frames and returns; it is then 0, and the next call decrements that to $FF, which means "run until the game says stop" -- CHECK_GAME_OVER and CHECK_RESCUED stop it by storing a small count again.
PLAY	8000	DI
	8001	LD HL,SPRITE_LIST_END	Mark the end of SPRITE_LIST, one byte past its eighth entry.
	8004	LD (HL),$FF
	8006	CALL PRINT_COUNTERS	Print the grenade count and both energies on the panel.
PLAY_0	8009	LD BC,$F7FE	Key 1 (bit 0 of the 1-5 half-row, low when pressed)...
	800C	IN A,(C)
	800E	BIT 0,A
	8010	JR NZ,PLAY_1
	8012	EI	...goes straight back to BASIC.
	8013	RET	...goes straight back to BASIC.
PLAY_1	8014	CALL GAME_FRAME	One frame of the game: move everything, draw the view.
	8017	CALL CHECK_RESCUED	Have both left the city?
	801A	CALL CHECK_GAME_OVER	Out of time, or out of energy?
	801D	LD A,(FRAMES_LEFT)	$FF: keep playing.
	8020	CP $FF
	8022	JP Z,PLAY_0
	8025	DEC A	Otherwise count a frame off, and stop at zero.
	8026	LD (FRAMES_LEFT),A
	8029	JP NZ,PLAY_0
	802C	LD A,(RANDOM_BITS)	Leave the random number in SEED -- to no end: the RANDOMIZE in the BASIC's RANDOMIZE USR that calls this overwrites SEED with what the USR returns, and the BASIC has no RND (watched: the ROM's write at $1E5A replaces this one).
	802F	LD (SEED),A
	8032	EI
	8033	RET