Looks like the author fixed the C translation following your comments: y = rndval & 0x000FF; /* Y = low 8 bits */ x = (rndval & 0x1FF00) >> 8; /* X = Hi

Looks like the author fixed the C translation following your comments: y = rndval & 0x000FF; /* Y = low 8 bits */ x = (rndval & 0x1FF00) >> 8; /* X = High 9 bits */ However, given that the assembly is verbatim from id-software's git, I guess those extra instructions are part of history now.