Delta: the source

Current truth for this game. How the understanding developed is in agent-history.md. Every fact names the routine or table it comes from (names as in symbols.json). Unless marked live, a fact comes from reading the code in work/entry.vsf, the hand-over snapshot that orientation.md describes. Live facts were checked in VICE x64sc (v3.13.1 release, PAL) on 1 October 2026; the tests are listed at the end.

Build

• The image is a crack by the Reflex Cracking Squad with its own packer and intro (orientation.md); the game's entry is game_entry $1770.
• No version string. The title texts the game stores say "(c)MCMLXXXVI" and "(c)1986 THALAMUS" (title_texts $F7B0); the title screen the player sees is a picture that says MCMLXXXVII and (c)1987.
• $9FF4 holds the PETSCII text S:DEMO DELTA, a disk-drive command to scratch a file called DEMO DELTA, left in memory by the developer's tools; nothing reads it (leftover_9ff2).

Memory layout

All of RAM is used. VIC bank 1 ($DD00 = $96), screen $4400, character set $4800; $01 = $35 during play (bank_io_only).

The frame

• The main loop (main_loop $17C9) only waits for raster line $E4 and runs the sound (sfx_queue_service $9D60). Everything else runs in a chain of raster interrupts.
• irq_handler $1800 (hardware vector $FFFE) saves the registers and jumps through $1000/$1001; each band handler sets the next band's line and handler (page $18) and ends at irq_exit $1768.
• Play frame, from $1888 (line $2E): 38 columns, then irq_band_top (screen on, extended colour), the multiplexer, the hazard-row bands, irq_band_bottom (line $F3, the icon row) and irq_band_frame_top (line $F9: enemy_fire_setup, pause, stars, panel). Live: in play the handlers $1888, $180E, $18A0, $18D0 each ran 50 times in 50 frames; on the title $180E, $1864, $18A3, $18A6, $18A9, $18AC every frame and $185E, $1861, $18C5 on alternate ones.
• NMI points at an RTI (nmi_rti $FFF8): RESTORE does nothing (live: pressing it in play changed nothing).

Text

The game's alphabet (found by letter differences in one pass, 30-text):

Glyphs: banner_font $FC60, 20 bytes each (39 glyphs), drawn at half width into sprites by banner_draw_step $3B80; the panel's score digits are separate 8-byte glyphs at score_digits $FB20.

Strings: title_texts (BY / STAVROS / FASOULAS / (c)MCMLXXXVI / (c)1986 THALAMUS), the high-score table (STAVROS, ANDREW, GARY, THALAMUS), TOP SCORES, DEMO, SHOOT ALIENS AND COLLECT WEAPONRY, GAME OVER (title_texts_b); DELTA MISSION COMPLETE, PLAYER n, FASTEN YOUR SEATBELT, GAME OVER (messages $FB70); the stage words (stage_words $EB80).

Stages

32 stages (stage_advance $91EF; after stage 32, game_completed plays tune 12, shows DELTA MISSION COMPLETE and starts again at stage 1 with the next difficulty). Each banner is three words from stage_words, chosen by stage_banner_table $F880; live: poking the stage-clear state in stage 1 brought up "ENTERING ROCKS OF DEATH / STAGE 02".

The words LOST and DRAGONS are in the table but no stage uses them.

The ENTERING stages (except FURTHER SPACE) have hazard rows: one or two rows of large two-sprite shapes that scroll in along the top and bottom of the play area (stage_hazard_table $F900: row count, colours, spawn-list page). The rows come from spawn lists under the I/O area (hz_spawn_lists $D300), a new hazard every eighth frame (hazard_rows_step $2080), animated from hz_anim_lists $D200 (hazard_frames_step $9480), moving 3 pixels a frame. The LEAVING stages, FURTHER SPACE and stage 1 have none. Between stages the hazard shapes in the VIC bank are exchanged with the second set at $E000 (stage_gfx_sequence $9580), 128 bytes a frame.

Attack waves

• Each stage is a list of groups (stage_waves_a $B800 for stages 1-21, stage_waves_b $B900 from stage 22), each an index into the 8-byte wave_records $AB00 (enemy_spawn $811F decodes them: hit points, animation, colour, side, points, path, start Y, delay).
• Up to seven enemies at once (sprites 1-7). Each follows a path (path_lists $B500) of segments (path_segments $A880) that run velocity lists (velocity_lists $B000) of (dx, dy) steps, mirrored as the record says (enemy_move_1 $82CC and six copies).
• Hit points: wave_hits_table $A100 → $1241-$1247; 0 or 1 dies at the first hit, $FF cannot be destroyed. An enemy can change into a new form instead of dying (bit 7 of its segment's byte 3).
• When slot 1 holds an indestructible enemy and slot 7's enemy is destroyed, the whole wave explodes (enemies_wave_timers $9024): shooting the leader destroys the escort.
• Credits: when a group ends and none of its enemies escaped, a group marked to carry a credit (most are) adds one (wave_spawn_step $8010, $80B5); a group marked with bit 6 takes one away.

• Recorded (the game's code run in the kit's machine with nobody playing, waves.html; all 32 stages compared with VICE frame by frame; stage 29 differs in places, TODO.md):
• stage 1 ends with a boss, record $0D: slot 7 has 8 hit points and parks at X $CF, Y $8C while six indestructible escorts circle it. The group waits for all seven ($1298) and the escorts never leave, so the stage ends only when the boss is destroyed; destroying slot 7 while the wave timer $129E runs and a weapon has hit ($129D) explodes the escort (enemies_wave_timers, $904D);
• across the 32 stages, 36 of the 424 groups stay on screen until shot; the rest fly off on their own;
• the shop is one of records $45-$4D, 30 shops in the 32 stages;
• an enemy shows the empty sprite $9B for its first one to three frames and at the end of its explosion, and at no other time.

The shop

• The shop is a group like the others ($1288, bit 4 of the record's byte 2): its sprites are the weapon icons, flying through the play area. In stage 1 it is the second group (stage_waves_a entry 1, record $45). Live: it arrived about 150 frames into stage 1.
• An icon's price is its number, 1-7 credits (shop_try_buy $2261). Icons the player can afford are light blue, the rest grey (icons_update $0808).
• Touching an icon that costs more than the credits, or a second icon in the same shop, counts as being hit ($1087): the grey icons are deadly, and one purchase per shop. Live: touching icon 5 with 2 credits set the hit flag.
• Buying spends all the credits: the price, and the rest is paid out as 100 points each (shop_spend_step $2298). Live: icon 4 bought with 7 credits left a score of 300, credits 0.
• The bought icon blinks 14 times (8 frames each) in the bottom row and then takes effect (icons_update). Live: icon 4's frame alternated $87/$8D and $0F03 became 1 after the blink.

Weapons

Icon 0 shows the credits ($107C). The owned weapons also change the enemies' fire: the shield brings bombs, and the double laser and slow-down change the aiming (enemy_fire_setup $9100). All weapons are lost when the ship dies (ship_death_weapons $397E). Live: with all seven poked on, one press fired the laser, the three-way burst, both beams and the orbiter (reference/weapons-poked.png).

The laser: up to three bolts, each a pair of characters moving one cell a frame (lasers_move $356F), fire delay 8 frames, one press per shot (ship_fire).

The character set and its colours

The character set at $4800 is inverted and the screen runs in extended-colour mode ($D011 = $5C). Colour RAM is black everywhere (clear_screen $0DDD), so every set bit is black; a clear bit shows one of the four background colours $D021-$D024, picked by the top two bits of the character code, and those four colours cycle every fourth frame (colour_cycle_step $1DDD, play_colours_set $363B). The blank character $2E is solid; each star glyph has a single clear bit and the three layers use codes $30, $6F and $B1 to take three different colours; the enemy fire and the player's weapons use the codes with both top bits set ($C0-$FF) or clear ($32-$34). Every enemy shot and bomb is a four-pixel ball cut out of the glyphs beneath it with one mask table (shot_masks $FF70, eight positions across two cells).

Hits

Every shot in the game, the player's and the enemies', is drawn with characters, and every hit between a sprite and a shot is found by reading the screen under the sprite, not by the video chip:

• an enemy is hit when one of the four cells under its sprite holds a glyph of $32-$3F, the player's weapons; the glyph's low nibble says which weapon, and that one is used up (enemies_hit_by_weapons $2B0A, weapon_hit_handlers $2B7C); the orbiter (glyph $35) is not used up;
• the ship is hit when the cell under its middle holds a glyph below $2E, the enemy fire's glyphs (ship_hit_by_fire $2BBC);
• sprite against sprite (the ship and the enemies, the hazard rows, the shop's icons) uses the video chip's collision register $D01E, read in the bands (wave_step $2C22, ship_touch_icons $2BD2).

Enemy fire

Three kinds, all drawn with characters (enemy_fire_decide $9208):

• aimed shots, six at most, travelling along a row at a speed set by the angle to the ship (move_enemy_shot_0-5, glyphs rebuilt each frame by draw_enemy_shot_n with masks, so the stars show through);
• bombs, two at most, 2x2 characters that home on the ship (move_enemy_bomb_0/1, bombs_home_step $98C0);
• bullets, seven at most, the character $ED moving in one of eight directions (bullet_move_0-6).

New fire must keep the total in flight (shot 5, bomb 10, bullet 1) under the stage's budget (fire_budget_levels $9F08 11-36 by fire_budget_by_stage); each enemy fires when its counter reaches the fire rate (difficulty_fire_rate $9F00). The pseudo-random numbers for the boss and the fire come from walking the game's own code at $3000 as a table (enemy_ai_step $99AC).

Scoring and lives

• Points per enemy: 5, 7, 10, 15, 20, 25, 50 or 100 ($11D8), shown ten times larger: the panel prints a 0 after the five digits. Live: each enemy shot added 5 internally and 50 on screen.
• An extra life each time the score's ten-thousands digit goes up (extra_life_check $9CD6), at most 9 lives. Live: poking that digit from 0 to 1 raised the lives from 3 to 4.
• Three lives each (game_start_scores $1600).

Difficulty

difficulty_set $9D35 loads one of three 32-byte tables (difficulty_by_loop $FA20) by the number of completed passes through the game, capped at two: the enemies fire sooner and more fire is allowed on screen on later passes.

Controls

• Joystick in either port: both are read and ORed (read_joysticks $3375).
• Keyboard control (F3 on the title): W up, X down, A left, D right, RETURN fire (read_keys $0C40).
• Title: F1 one or two players, F3 joystick or keys, F5 sound effects or music in play, SPACE shows the options, fire starts (irq_title_keys $0C90). Live: F1, F3 and F5 toggled $12CA, $0FFC and $0FFD and the three option sprites appeared (reference/title-options.png).
• RUN/STOP pauses; fire resumes; T during the pause quits to game over (pause_check $9A30). Live: RUN/STOP set the pause flag and the sprites stayed put for 25 frames; fire cleared it.
• Two players take turns, the turn passing at each death (swap_players $1650). Live: turns alternated and each player's lives were kept.
• The attack-mode demo plays recorded input: 63 joystick bytes (demo_input $9F40), one every eight frames, read by pointing the joystick read at $0F90 (demo_joystick_patch). Live: at the read, the operand was $0F90 and the first byte $17.

Sound

• Two players share the SID. sfx_play_frame $0406 runs 21 effects of two 8-byte records each (pitch scripts, sweeps, alternating waveforms). music_play $BDE4 is a Hubbard-style driver: 13 tunes (music_tune_table $C4F4), 109 patterns, 22 instruments with vibrato, pulse sweeps, drums and filter sweeps.
• In play the default is the effects ($0FFD = 1); F5 on the title switches to the in-game music. Tunes are requested through $117D, which holds the tune's number plus one (sfx_queue_service, DEY before set_tune $C357): tune 0 when play starts and at each player's turn (request 1 at $16FA and next_player_tune; with the effects on, that request only gives the voices back to the effects), tune 11 on the title (called directly at $17C3) and at game over and name entry (request $0C), tune 12 when the game is completed (request $0D, game_completed). Tunes 1-10 are in the driver and nothing requests them.
• Register census: the game writes $D400-$D406 (indexed, all three voices), $D416, $D417, $D418; it never writes the filter cutoff's low bits ($D415).

Video register census

Never written: $D017, $D01B, $D01D (no expansion, no priority changes) and $D01F (background collisions): hits between sprites and characters are found by reading the screen under each sprite (see Hits).

Corner cases

• enemies_wave_timers writes STY $029E at $9057, one page below the wave timer $129E that the neighbouring code uses; nothing reads $029E. Probably a slip; the effect is that the timer is not cleared there.
• $129F is written ($905C) and never read.
• Buying the speed icon at its top level sets it back to the slowest (icons_update, $08CE); the code that might have done something else there is two NOPs.
• $12D5 is read, its value written back unchanged ($08D6), and copied into the code at $34B7; nothing changes it.
• unreached_cheat $BA00: a routine nothing calls that would refill the lives on F3 and switch off the enemy fire's hits on F5; a trainer left in the code.
• Code no instruction reaches: $175E (bank the ROMs in and warm-start the KERNAL), $3D13, $9D77; tables nothing reads: $A000-$A07F.

Live tests

Cheats

Pokes that change the game within its own parameters. Each one names the variable it changes and whether it has been tested live. Untested pokes are labelled as candidates.