Write-Twice Register: Difference between revisions

Latest revision as of 16:57, 14 August 2024

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

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-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.
+'''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.
+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 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 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 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.<sup>[2]</sup>
 === References ===
-* https://retrocomputing.stackexchange.com/a/7071
+# https://retrocomputing.stackexchange.com/a/7071
+# https://problemkaputt.de/fullsnes.htm#snesmemorycgramaccesspalettememory
 [[Category:SNES Hardware]]
 [[Category:Registers]]
 [[Category:Buffers]]