Green Beret

01 · The stack

Bug: die in the fights or win the Bridge 23 times and the game crashes

A death in an end-of-stage fight leaves two bytes on the stack, and so does winning the Bridge. Nothing ever takes them off, not even a new game.

Deaths in the fights and Bridges won0

Page 1 of memory, $0100-$01FF, one cell a byte, from $0100 at the top left. Red: cia2_serial_check. Blue: the tables of the 21 virtual sprites (X, X high bit, colour, shape). Orange: the bytes the deaths and the Bridges have left. Yellow: the stack the game uses below them, down to 17 bytes under the main loop's, as measured in the emulator, and purple where it runs into the sprites' tables. The verdicts are from the emulator, with the stack pointer moved where that many would leave it and the hero walking and stabbing on the Missile Base.

The routine that kills the hero, player_killed, throws away its caller's return address and jumps to lose_life, which restarts the stage. It expects to have been called from a routine the main loop called, so that the address it throws away is the main loop's own call. In play that is right. During an end-of-stage fight the contact tests are one call further down, and two bytes of return address stay on the stack. In the emulator the stack pointer at the top of the main loop went from $FC to $FA after a death in the Missile Base's fight, and stayed there.

Winning the Bridge does the same without a death. Its fight, bridge_fight, calls helis_launch every frame, and when the last helicopter is down helis_launch jumps straight to stage_complete instead of returning, so that call's return address stays behind. The other three fights end the stage from the main loop's own level and leave nothing. In the emulator the stack pointer at the top of the main loop was two bytes lower after the Bridge was won than before. A player who never dies in a fight still loses two bytes each time round the four stages, so 23 times round leave the stack as 23 deaths in the fights do.

Nothing in the game ever sets the stack pointer, so the bytes pile up for as long as the machine is on, through game over and every new game. The stack grows down through page 1, where the game also keeps its virtual sprites' tables, at $0180-$01D3. With 12 of these pairs of bytes left behind, the stack can reach the tables, and then the game's writes to the tables can land on return addresses. With 22 pairs left behind, the game played on for a minute; at 23 it crashed within half a minute, and at every count tried from 24 to 90 it crashed within 13 seconds.

02 · Key 1

Secret: hold 1 and the hero stops in mid-air

While the 1 key is held, the hero stays where he is, even at the top of a jump, and the jump goes on when it is let go.

(or hold the 1 key)

The hero jumping as player_jump moves him along jump_arc. While 1 is held the frame does not move on, as irq_player skips him. In the emulator, held during a jump, his Y stayed at 194 for 50 frames, and the jump went on when the key was let go.

The hero is moved from the raster interrupt, by irq_player, which reads the 1 key on CIA 1 before it moves him. While the key is down it skips his movement, his animation and his stab, so he freezes, on the ground, on a ladder or in the air. The enemies and their shots go on. Nothing else in the game reads that key, and the game has no pause.

A frozen hero can still die: the main loop's tests of what touches him (enemy_contact, shot_contact) go on.

03 · Extra lives

Secret: the extra lives come at 30,000 and then every 70,000

The first comes at 30,000 points, and each one after it 70,000 later. A player with nine lives gets no tenth.

Score0

From new_game, which sets the first at 30,000, and next_life_check, which adds 70,000 each time one is given. In the emulator, a score of 29,990 plus 10 gave a third life and moved the next to 100,000; with nine lives, 99,990 plus 10 left nine and moved the next to 170,000.

The game keeps the score of the next extra life, next_life, in three decimal bytes like the score. When the score reaches it, next_life_check moves it on by 70,000 and adds a life, unless there are nine already.

04 · The jet

Secret: stop moving forward for 36 seconds and a jet comes

A jet flies over and drops a bomb that follows the hero. For a player who stands still, it comes after 1,792 frames.

The jet over the Missile Base, flying left above the hero

The jet, from the emulator, after the hero had stood still since the start of the stage. It came at 1,800 frames, checked every 8.

A counter, idle_time, goes up by one every 256 frames and down by one each time the screen scrolls a character. When it reaches 7 the jet comes (jet_update): 7 times 256 frames is 36 seconds. Walking back and forth does not count, since only the scroll lowers the counter, and the scroll moves only forward.

05 · The hi score

Secret: a new machine's hi score is 3,560

The hi score on the status line is 3,560 before anyone has played.

The Missile Base at the start of a game; the status line reads HI SCORE 0003560

The first game after loading, in the emulator: HI SCORE 0003560.

The hi score is three decimal bytes at hi_score, and the file holds $56 $03 $00 there, 356 with the 0 the status line adds. A new game leaves it alone, and only hi_score_check writes it, so 3,560 stands until a player beats it.