Write-Twice Register

Write-twice registers need to be written to twice before their value is updated. They are 16-bit registers that are mapped to only 8-bits in an address space. The motivation behind write-twice registers is that they prevent glitches caused by software accidentally only updating half of a 16-bit register. This is accomplished via an additional, hidden 8-bit buffer:

• On the first write, the 8-bit value on the CPU Data Bus is written to the hidden buffer, but the contents of the 16-bit register are not updated yet.
• On the second write, both the new 8-bit value on the data bus and the buffered 8-bit value both overwrite the old contents of the 16-bit register simultaneously.

Fullsnes says a flipflop keeps track of whether we are on the 1st or 2nd access for CGRAM writes.^[2]

References

1. https://retrocomputing.stackexchange.com/a/7071
2. https://problemkaputt.de/fullsnes.htm#snesmemorycgramaccesspalettememory