Delta

01 · The copy studied

A cracked single file, with the loader gone

These pages describe one program file, a crack by the Reflex Cracking Squad. Its own depacker and intro run first, then two unpacking layers, and the game starts at $1770 (About has the steps). The crack replaced the original loader, so Mix-E-Load is not in it, and nothing in it says Delta Patrol.

The game itself fills the whole of the C64's memory, from the sound effects at $0400 to the vectors at $FFFA, including the RAM under the I/O chips. Every byte is described on the Source code tab.

02 · Left in the code

Secret: a trainer nothing calls

At $BA00 sits a routine that no instruction reaches. It was written to run in place of the jump to the ship's movement: holding F3 sets the lives back to 3, and F5 toggles two patches, one making the enemies' fire harmless ($2BC9) and one switching off the collision with the shop's icons ($2BDA). It ends by jumping to the ship's movement, so hooking it in would be a single changed address. It is probably the developer's own cheat for testing, left in the finished game.

03 · A store one page off

Bug: a timer cleared one page too low

The wave routine at $9024 keeps a wave timer at $129E. At $9057 it clears a timer with STY $029E, one page below, an address nothing else reads or writes. The neighbouring code uses $129E throughout, so the store was almost certainly meant for it: the timer is not cleared there. The next instruction writes $129F, which nothing reads either.

04 · Randomness

Secret: the random numbers are the game's own code

Delta has no random number generator. When it needs a random choice for the enemies' fire or the boss's aim, enemy_ai_step ($99AC) walks through the bytes at $3000, using each one to find the next. Those bytes are the game's own code: the routines that move the enemies' bullets and shots. Any change to that code would change how the enemies fire.

05 · Leftovers

Secret: 1986 and a disk command in memory

• The title texts stored in the game say "(c)MCMLXXXVI" and "(c)1986 THALAMUS" ($F7B0). The title screen the player sees is a picture saying MCMLXXXVII and 1987.
• $9FF4 holds the text S:DEMO DELTA, a disk-drive command to delete a file called DEMO DELTA, left in memory by the developer's tools.
• Code no instruction reaches: $175E (switches the ROMs back in and restarts BASIC), $3D13, and a test of a stage flag at $9D77 padded out with sixteen NOPs.

06 · Open questions

What we have not settled

• Mix-E-Load. The loading-music remixer belonged to the original tape loader, which this crack does not contain. An original tape or disk image would show it.
• Delta Patrol. Whether the American release differs from this one in code is not known: the copy studied says nothing about it.
• Taking credits away. A wave record can be marked to take a credit away when the wave ends (bit 6 of its byte 3, $80B5). Which waves carry the mark, if any reach play, was not checked.