[giovannibajo1]

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.

[klodolph]

The 9th bit was also used by the depth buffer.

To add to this, the reason that the RAM was available with 9 bits in the first place is so that it could be used to make systems with ECC. It's just that you didn't have to use that 9th bit for error correction, you could use it for extra data, if you designed the system to use it that way.

[phaedrus]

Earlier today I was idly wondering: if aliens invented computers and went through the whole 8-bit era, 16-bit era etc. - would they perhaps have developed architectures which had additional "shadow" or "tag" bits on every word? What might they use them for?

The idea being maybe an intelligent race with different balance of motivations might not do only the minimum economical thing, instead being willing to trade X% of memory bits or X% of instructions per second or X% more chips for other purposes. For example extra tag bits might be used to encode where the data came from or its datatype, or additional clock cycles between instructions might be used to run a reliability check on the program as it executes, etc.

[wolfram74]

Yet another weird bit of N64 lore, I'm amazed mupen64 works as well as it does.

[oh_sigh]

> weird bit

Quite literally