Zlib is used in the firmware to compress and uncompress bitmap.
See this thread
SztupY found this:
I found these functions inside 60D/1.0.9: zlib_deflateEnd_maybe ROM FF3F84B4 000000FC R . . . . . . zlib_deflateInit2__maybe ROM FF3F8730 00000250 R . . . B . . zlib_deflateInit__maybe ROM FF3F8980 00000040 R . . . . . . zlib_deflate_fast ROM FF3F9B20 00000258 R . . . . . . zlib_deflate_maybe ROM FF3F8C7C 000001BC R . . . . . . zlib_deflate_slow ROM FF3F9D78 00000008 R . . . . . . zlib_deflate_stored ROM FF3F98A8 00000134 R . . . . . . zlib_fill_window ROM FF3F9758 00000150 R . . . . . . zlib_inflate ROM FF1C3780 000003CC R . . . . . . zlib_inflateEnd ROM FF1C1A58 00000074 R . . . . . . zlib_inflateInit_ ROM FF1C1A48 00000010 R . . . . . . zlib_inflateInit2_ ROM FF1C195C 000000EC R . . . . . . zlib_inflate_x ROM FF1C3664 0000011C R . . . . . . zlib_uncompress_maybe ROM FF32E388 000000A4 R . . . . . . zlib_uncompress_maybe2 ROM FF05EE50 000000B4 R . . . . . .
and for 550D/T2i:
Alex found this: Here are the equivalents for 550D/109, which has the same zlib version FF37B298 zlib_deflateEnd_maybe FF37B514 zlib_deflateInit2__maybe FF37B764 zlib_deflateInit__maybe FF37BA60 zlib_deflate_maybe FF1BF538 zlib_inflate(__int32 *strm, int flush) FF1BD810 zlib_inflateEnd(__int32 *strm) FF1BD800 zlib_inflateInit_(void *strm, const char *version, int stream_size) FF1BD714 zlib_inflateInit2_(void *strm, int windowBits, const char *version, int stream_size) FF1BF41C zlib_inflate_x FF2D817C zlib_uncompress_maybe(unsigned __int8 *dest, int destLen, unsigned __int8 *src, int srcLen) FF05B234 zlib_uncompress_maybe2 Indy: FF2D8964 zlib_bitm_decompress(unsigned __int8 *dest, unsigned __int32 dest_size, unsigned __int8 *src) FF37B540 zlib_deflate_init2_maybe(void *strm, int level, int method, int windowsBits, int memLevel, int strategy, const char *version, int stream_size)
used here in BitmapCodec
550d 108 ROM:FF2D836C MOV R3, #9 ; compression level ? ROM:FF2D8370 STR R3, [SP,#0x30+var_30] ROM:FF2D8374 MOV R3, R4 ROM:FF2D8378 MOV R2, R7 ROM:FF2D837C ADD R1, SP, #0x30+var_28 ROM:FF2D8380 MOV R0, R5 ROM:FF2D8384 BL lzc_112