Jet-Set Willy ST Room Format

Version 1.0: 18 December 2005

by John Elliott

Disclaimer: I do not guarantee this information to be correct. This is my best guess at what is going on inside JSW ST, after wandering around inside it with a disassembler.

Atari JSW is a very close replica of Spectrum JSW, coded in C based on a disassembled version of the original JSW48. It is distributed as a .PRG file, the contents of which are:

Offset in file	Load address, hex	Load address, decimal	Details
0x0000	-	-	PRG file header. Format seems to be similar to CP/M-86 CMD; perhaps the same as CP/M-68k CMD. 60 1A -- Magic? 00 00 63 A2 -- Code len? 00 00 68 86 -- Data len? 00 00 06 62 -- BSS len? 00 00 00 ...
0x001C	0x0000	0	Atari code segment. This contains some constants which a game editor would need to change, including the count of items.
0x63BE	0x63A2	25506	Atari data segment starts here.
0x640D	0x63F1	25585	"Items collected 000 Time 00:00 m" message, followed by various strings of digits.
0x643F	0x6423	25635	"GameOver"
0x64EC	0x64D0	25808	"Press ENTER to start" message / scrolling message. For some reason, this is broken up into segments of irregular length in reverse order; each section is terminated by a zero byte. An editor program changing the message would have to break it up into portions of the exact same size when storing, because the game code makes separate references to each section.
0x6626	0x660A	26122	Unknown.
0x669A	0x667E	26238	In-game music.
0x66DA	0x66BE	26302	Title screen music.
0x673E	0x6722	26402	Unknown, but contains the following messages: "Please enter room number: __ " "jetset: Sorry - must use a colour monitor\n" "jetset: malloc failure!\n" "lisa ambrose"
0x6854	0x6838	26680	Spectrum font; there seems to be a transcription error in the } character.
0x6B54	0x6B38	27448	Conveyor animation table
0x6B64	0x6B48	27464	The stripe patterns for the title screen.
0x6B84	0x6B68	27496	The items table: 83 entries. Unlike in Spectrum JSW, the two halves of the table are interleaved; first byte gives item position low 8 bits, second gives room number and item position high bit.
0x6C2A	0x6C0E	27662	Rope movement table
0x6D2A	0x6D0E	27918	Guardian table, in standard JSW format.
0x712A	0x710E	28942	Title screen.
0x732A	0x730E	29454	Rooms 0-61.
0xB12A	0xB10E	45326	Blank; probably corresponds to rooms 62 and 63.
0xB32A	0xB30E	45838	Sprites. The mapping to Spectrum sprite pages is hardcoded in the guardian drawing code: Spectrum range 9C40-9DFF => sprites 0-13 Spectrum range A600-A680 => sprites 14-17 Spectrum range AB00-B300 => sprites 18-81 Spectrum range B400 et seq.=> sprites 82 et seq. There are only 178 sprites (exactly enough to implement the original JSW) whereas the Spectrum JSW48 engine has enough spare space to go up to 240.
0xC96A	0xC94E	51534	6 long words giving sprite addresses for special sprites: The foot The barrel Maria Willy The toilet The flying pig
0xC982	0xC966	51558	Unknown. Possibly we're getting into the C runtime library by this stage.
0xCB3C	0xCB20	52000	This is definitely in the C runtime; it's the array of character characteristics used by <ctype.h> functions.

In order to lift the limits on the number of sprites and/or items, so that arbitrary JSW48 games could be converted to ST format, it would be necessary to make the data segment bigger. Not being familiar with the Atari PRG format, I don't know if this is possible without major patching of the existing code.

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John Elliott

18-12-2005.