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Write-Twice Register: Difference between revisions

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This is accomplished via an additional, hidden 8-bit buffer.
This is accomplished via an additional, hidden 8-bit buffer.


* On the first write, the 8-bit value from the CPU's data bus is written to the hidden buffer, but the contents of the 16-bit register are not updated yet.
* 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.
* 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.



Revision as of 05:17, 16 July 2023

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