Compression Algorithm - Nintendo GAME BOY Programming Manual

Game Boy Programming Manual

9. COMPRESSION ALGORITHM

Compressed data essentially consist of control codes specifying the data type and length and the actual
data.
Control code 1 + raw data
Control code 2 + loop data
Control Code
...
7Fh
RAW Data
Control code 1 + Raw data
7Fh: Next 80h bytes are raw data
0h-7Eh (N): Next < N + 1 data items (01h-7Fh) are raw data
Control code 2 + Loop data
FFh: Repeat the next < 1 byte of data for 81h bytes
80h-FEh: Repeat the next < 1 byte of data for 2 (80) – 80h (FE) items
Example:
09h A0h A1h
10h bytes of raw data
Control Code Control Code
...
7Eh FFh
RAW Data Loop Data
...
A2h A3h 7Fh
AAh
80h bytes of raw data
Control Code Control Code
... ...
80h FEh
Loop Data Loop Data
...
80h 81h 82h FFh
250
... ...
FFh 55h 80h
AAh
81h items of 55h, 02h items of
AAh