The Fourth Protocol

01 · The screen

Seven raster interrupts draw one room

Episode three is another program by another programmer. It shows a single room, a picture with a text window under it, and changes the video chip's settings seven times on the way down the screen.

The page draws this from one frame of episode three, recorded in the emulator with the drawer open and the alarm ringing: the video chip's registers at the top of the frame and the 49 writes the interrupts make on the way down, run through the site's model of the video chip. All 104,448 pixels match the emulator's own picture of the same frame. The text window's letters come from the Commodore's character ROM, and the page carries only the 23 letters it shows, from the ROM 901225-01.

The room is a multicolour bitmap at $6000, 160 double-width pixels across and 160 rows down (picture). Everything that moves is a sprite over it: the door, the guard, the drawer and the bomb with its three coloured lights. The video chip has only eight sprites, so the interrupts use some of them twice, once in the top half of the screen and again lower down (irq_guard_lower, irq_door_lower). At line $D7 (215), irq_text switches to the character screen at $0400, where each byte is a letter, for the five rows of text. The same interrupt moves the guards, sounds the siren and reads the keyboard, once a frame.

02 · The parser

The parser finds one verb and one noun anywhere in the line

You type what Preston does, and the game looks for one verb and one noun in the line. Type a line, or press an example.

The page reads the line as parse ($A00C) and find_word ($A182) read it, with the word lists and tables from episode three's listing. A test beside the page runs the game's own parser on 3,530 lines, every verb with every noun and 2,000 lines of random words and pieces of words, and checks that the page finds the same verb, the same noun and the same routine.

The verb is the first word of the line that begins with one of the 18 verbs in verb_words ($A058), so OPENING counts as OPEN and THE is skipped. The noun is looked for after the verb, starting with the character after the verb's last letter, so OPENDOOR finds both. Five verbs need no noun: INVENTORY, EIOK, LISTEN, HIDE and the number 12051987. EIOK is the one that describes the room, and LOOK is not a word the game knows. For each of the others, a table gives one routine for each of the 28 nouns (noun_tables, $B03E); ATTACK, KICK, HIT and KILL share one table, and PUSH and PRESS another.

03 · The bomb

Five lines defuse the bomb

The bomb is in the second drawer of a filing cabinet. Typed in the right order, the five steps below defuse it. Otherwise it goes off on the 144th line you type, whatever you are doing.

Type	What happens	Where
USE SECOND	Chooses the second keypad, for one line. FIRST and THIRD choose the wrong ones.	`$B488`
12051987	On the second keypad the drawer opens and the alarm rings. On the others the bomb goes off.	`$B298`
CUT CABLE	Silences the alarm for good.	`$B362`
PUSH RED, PUSH YELLOW, PUSH GREEN	In that order the panel slides away and the timer shows. A button out of turn sets the bomb off.	`$B4BF`
CUT RED, CUT YELLOW, CUT GREEN	In that order the bomb is defused. A wire out of turn, or the blue one, sets it off.	`$B54E`

The bomb's state is one byte (bomb_state, $8784): 0 while the cabinet is sealed, 2 with the drawer open, 4 with the timer showing, and $80 once it is defused. Attacking, kicking, hitting, destroying or pushing the cabinet or any part of the bomb sets it off, and so does a guard who kills you. The guards come in while the alarm rings, up to five of them, and with each one in the room your chance of being killed after a line goes up: 1 in 16 with one or two guards, then 1 in 8, 1 in 4 and 1 in 2 (attack_chances, $B66C).

How it ends depends on the count of lines typed (commands_typed, $879A). The bomb defused brings an alliance government in the 1987 election and early retirement. An explosion on an odd count brings an epilogue from the annals of the Australo-Indonesian empire, and on an even count one in which the blast at Faslane is blamed on a NATO bomb going off by accident. The 144th line always ends on the even one. The Discoveries tab has the stranger corners of the parser.