Merge pull request #1256 from riscv/csr-fix

Clarify behavior of CSR access side effects

src/zicsr.adoc

@@ -38,16 +38,14 @@ of the CSR, zero-extends the value to XLEN bits, and writes it to
 integer register _rd_. The initial value in integer register _rs1_ is
 treated as a bit mask that specifies bit positions to be set in the CSR.
 Any bit that is high in _rs1_ will cause the corresponding bit to be set
-in the CSR, if that CSR bit is writable. Other bits in the CSR are not
-explicitly written.
+in the CSR, if that CSR bit is writable.
 
 The CSRRC (Atomic Read and Clear Bits in CSR) instruction reads the
 value of the CSR, zero-extends the value to XLEN bits, and writes it to
 integer register _rd_. The initial value in integer register _rs1_ is
 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. Other bits in the
-CSR are not explicitly written.
+be cleared in the CSR, if that CSR bit is writable.
 
 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
@@ -105,6 +103,19 @@ CSR
 <<csrsideeffects>> summarizes the behavior of the CSR
 instructions with respect to whether they read and/or write the CSR.
 
+In addition to side effects that occur as a consequence of reading or
+writing a CSR, individual fields within a CSR might have side effects
+when written.  The CSRRW[I] instructions action side effects for all
+such fields within the written CSR.  The CSRRS[I] an CSRRC[I] instructions
+only action side effects for fields for which the _rs1_ or _uimm_ argument
+has at least one bit set corresponding to that field.
+[NOTE]
+====
+As of this writing, no standard CSRs have side effects on field writes.
+Hence, whether a standard CSR access has any side effects can be determined
+solely from the opcode.
+====
+
 For any event or consequence that occurs due to a CSR having a
 particular value, if a write to the CSR gives it that value, the
 resulting event or consequence is said to be an _indirect effect_ of the