spectrum-scubadive-rev

Reversing Scuba Dive game for ZX Spectrum, 1983 Durell Software.

Status: work in progress.

Original

The original binary used for the disasm is .TAP file from World of Spectrum, 39428 bytes length.

.TAP file contains four blocks:

   Name         Type   Addr    Size  SizeHex   RangeHex
  "SCUBA     "  BASIC 
  "DIVE      "  CODE   16384,  7150   $1BEE   $4000-$5BED
  "S         "  CODE   24576, 25856   $6500   $6000-$C4FF
  "D         "  CODE   55552,  6144   $1800   $D900-$F0FF

Spectrum 48K ROM usage

The BASIC loader code is the following:

1 CLEAR 24575: INK 0: PAPER 0: BORDER 1: CLS : LOAD ""CODE: PRINT AT 2,0;: LOAD ""CODE: PRINT AT 2,0;: LOAD ""CODE
2 IF USR 60895THEN SAVE "S/D SCORE"CODE 59866,160: GO TO 2
3 LOAD "S/D SCORE"CODE : GO TO 2

Line 1 loads the game binaries. Line 2 starts the assemmbly code at 60895 ($EDDF); in the game menu user could press S to save score table, in this case we're returning from the assembly code and going to THEN on line 2. Or, user could press L on the menu, and in this case we're going to line 3, loading the score table. Anyway, after load or save we're returning to line 2 and re-entering the same starting point again.

The game code uses some ROM subroutines: to clear screen, print text on the screen, set border color, keyboard scan and sound generation. Also the code uses some system variables like LAST-K and ATTR-P. ROM font used in text output subroutines.

The reversed code

The reversed code structure:

• basloader.bas - text representation for the BASIC code used for game loading, and also for score table saving and loading
• scubacoda.asm - block $4000-$5BED
  • scubascrn.dat - loading screen in raw binary format
• scubacodb.asm - block $6000-$C4FF
  • scubasprt.asm - sprites $6000-$9C4F
  • scubarelf.asm - relief tiles and world mini-map
• scubacodc.asm - block $D900-$F0FF

All three .asm code files are compiled with pasmo cross-assembler, and compared to original binary blocks byte-to-byte, no differences.

The game map

The game labyrinth "warped up": first, the AC5D table (the "mini-map") contains 32x32 = 1024 block indices; second, blocks at A4DD contains 8x8 tiles each; and finally, tiles at 9134 are 8x8 pixels each.
So the whole world is 256x256 tiles = 2048x2048 pixels. We always see only 24x24 tiles on the screen.

The game map created in a procedural way, changing block numbers in the AC5D table. The map depth depends on the game level 1..4.

There's an interesting way to put a big Octopus on the map. The Octopus always guarding a way leading down the map. So when we need an Octopus, we just put blocks number $1C and $1D next to each other, and that's used as a placeholder for the Octopus.

Tools for the tools folder

• bas2tap.exe, bin2tap.exe utilities https://sourceforge.net/projects/zxspectrumutils/files/

• pasmo.exe cross-assembler http://pasmo.speccy.org/

Links

• Scuba Dive disasm in SkoolKit HTML
• nzeemin/skoolkit-game-revs repo for some game disasm using SkoolKit