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Specify the interaction between CSRRS/C and "reads as" bits #1581

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Specify the interaction between CSRRS/C and "reads as" bits #1581

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10 changes: 10 additions & 0 deletions src/zicsr.adoc
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Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,16 @@ treated as a bit mask that specifies bit positions to be cleared in the
CSR. Any bit that is high in _rs1_ will cause the corresponding bit to
be cleared in the CSR, if that CSR bit is writable.

[NOTE]
====
Since CSRRS and CSRRC perform a read-modify-write, any bits that read as
a different value to their underlying value may be modified by these
instructions even if the corresponding bit is not set in _rs1_. For example
pmpaddr[G-1] may have an underlying value of 1 but read as 0. Executing
CSRRC or CSRRS to modify a different bit will cause 0 to be read from
pmpaddr[G-1] and then written back, updating the underlying value to 0.
====

For both CSRRS and CSRRC, if _rs1_=`x0`, then the instruction will not
write to the CSR at all, and so shall not cause any of the side effects
that might otherwise occur on a CSR write, nor raise illegal-instruction
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