Nintendo 64 had a 9-bit RAM (Rambus RDRAM). Only 8 bits of each byte were accessible from the MIPS CPU for obvious reasons; the 9th bit was only used by the GPU (called "RDP") to store extra information while rendering (begin a UMA architecture, the CPU used the same RDRAM used by the CPU). Typically it contained a flag called "coverage" that was used to discriminate pixels on the edge of polygons, that were later subject to antialiasing. By reading back pixels using the CPU, you would be unable to see the coverage flag.