Buck Rogers - Countdown to Doomsday/ROM map

The following article is a ROM map for Buck Rogers - Countdown to Doomsday.

Compression

A compression algorithm is used for most data (gfx, tilemaps, text). It's a variable length encoding, where a code can refer to a regular unencrypted data, or a reference to previous decrypted data.

Description on an example

At the beginning, a new element is 8 or 9 bits. If the value on 8 bits is in a certain range, it's considered as a single regular character. If it's outside, it's an entry in a dictionary coded on 9 bits, so one more bit is read and placed before the 8 bits already read.

The range for regular characters shifts at each new character read. It starts with 0x02 - 0xff.

Let's take the piece of text at 0x433bf:

   59 4F 55 20 4A 4F 49 4E 45 44 20 09 A7 90 2A 07 51 92 51 D2 15 03 E9 08 A4 0A 48 29 A4 09 A8 A9 C8 28 68 A5 C4 00 28 25 05 48 66 84 8A 11 47 20 21 29 24 A2 A3 05 48 01 11 78 CD 0D 21 25 0D 05 1D 3D 48 BE A6 A0 45 45 28 20 D5 0B 80 02 72 28 A6 A8 90 20 24 10 4A 63 0A 28 82 40 44 22 95 8A

• 1st 8bits are 0b01011001=0x59 ; range is 0x02 - 0xff so it's a regular character: 0x59
• 2nd 8bits are 0b01001111=0x4f ; range is 0x03 - 0xff so it's a regular character: 0x4f
• 3rd 8bits are 0b01010101=0x55 ; range is 0x04 - 0xff so it's a regular character: 0x55
• 4th 8bits are 0b00100000=0x20 ; range is 0x05 - 0xff so it's a regular character: 0x20
• 5th 8bits are 0b01001010=0x4a ; range is 0x06 - 0xff so it's a regular character: 0x4a
• 6th 8bits are 0b01001111=0x4f ; range is 0x07 - 0xff so it's a regular character: 0x4f
• 7th 8bits are 0b01001001=0x49 ; range is 0x08 - 0xff so it's a regular character: 0x49
• 8th 8bits are 0b01001110=0x4e ; range is 0x09 - 0xff so it's a regular character: 0x4e
• 9th 8bits are 0b01000101=0x45 ; range is 0x0a - 0xff so it's a regular character: 0x45
• 10th 8bits are 0b01000100=0x44 ; range is 0x0b - 0xff so it's a regular character: 0x44
• 11th 8bits are 0b00100000=0x20 ; range is 0x0c - 0xff so it's a regular character: 0x20
• 12th 8bits are 0b00001001=0x09 ; range is 0x0d - 0xff, we're outside: it's a special code, and one more bit is read and placed before the 8 bits. Next bit is 1 (first bit of 0xa7) so the special code is 0b100001001=0x109
• 13th 8 bits are 0b01001111=0x4f (last 7 bits of 0xa7 and first bit of 0x90) ; range is 0x0e-0xff so it's a regular character: 0x4f

Eventually, the special code range will be 0xff-0xff.