Write-Twice Register

A write-twice register is a 16-bit register that is 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 from 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 from the CPU's data bus and the buffered 8-bit value both overwrite the old contents of the 16-bit register simultaneously.

References

• https://retrocomputing.stackexchange.com/a/7071