Source: aOrante's blog

Inspired by this article on SMS Power! - and specifically the type of run-length-encoding used in Wonder Boy on the Sega Master System, aOrante wrote a decompression routine that can be used in assembly (asMSX) or C (SDCC) that can extract files compressed in this 'RLE WB' format.

Relevant link: unRLEWB 1.1