10 “Zicsr”, Control and Status Register (CSR) Instructions, Version 2.0
RISC-V defines a separate address space of 4096 Control and Status registers associated with each hart. This chapter defines the full set of CSR instructions that operate on these CSRs.
10.1 CSR Instructions
All CSR instructions atomically read-modify-write a single CSR, whose CSR specifier is encoded in the 12-bit csr field of the instruction held in bits 31–20. The immediate forms use a 5-bit zero-extended immediate encoded in the rs1 field.
The CSRRW (Atomic Read/Write CSR) instruction atomically swaps values
in the CSRs and integer registers. CSRRW reads the old value of the
CSR, zero-extends the value to XLEN bits, then writes it to integer
register rd. The initial value in rs1 is written to the
CSR. If rd=x0, then the instruction shall not read the CSR
and shall not cause any of the side effects that might occur on a CSR
read.
The CSRRS (Atomic Read and Set 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 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.
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.
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 exceptions on accesses to read-only CSRs.
Both CSRRS and CSRRC always read the addressed CSR and cause any read
side effects regardless of rs1 and rd fields. Note that
if rs1 specifies a register holding a zero value other than x0, the instruction will still attempt to write the unmodified
value back to the CSR and will cause any attendant side effects. A
CSRRW with rs1=x0 will attempt to write zero to the
destination CSR.
The CSRRWI, CSRRSI, and CSRRCI variants are similar to CSRRW, CSRRS,
and CSRRC respectively, except they update the CSR using an XLEN-bit
value obtained by zero-extending a 5-bit unsigned immediate (uimm[4:0]) field
encoded in the rs1 field instead of a value from an integer
register. For CSRRSI and CSRRCI, if the uimm[4:0] field is zero, then
these instructions will not write to the CSR, and shall not cause any
of the side effects that might otherwise occur on a CSR write, nor raise
illegal instruction exceptions on accesses to read-only CSRs.
For CSRRWI, if rd=x0, then the instruction shall not read the
CSR and shall not cause any of the side effects that might occur on a
CSR read. Both CSRRSI and CSRRCI will always read the CSR and cause
any read side effects regardless of rd and rs1 fields.
Table 1.1 summarizes the behavior of the CSR instructions with respect to whether they read and/or write the CSR.
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 write. Indirect effects of a CSR write are not considered by the RISC-V ISA to be side effects of that write.
An example of side effects for CSR accesses would be if reading from a specific CSR causes a light bulb to turn on, while writing an odd value to the same CSR causes the light to turn off. Assume writing an even value has no effect. In this case, both the read and write have side effects controlling whether the bulb is lit, as this condition is not determined solely from the CSR value. (Note that after writing an odd value to the CSR to turn off the light, then reading to turn the light on, writing again the same odd value causes the light to turn off again. Hence, on the last write, it is not a change in the CSR value that turns off the light.)
On the other hand, if a bulb is rigged to light whenever the value of a particular CSR is odd, then turning the light on and off is not considered a side effect of writing to the CSR but merely an indirect effect of such writes.
More concretely, the RISC-V privileged architecture defined in Volume II specifies that certain combinations of CSR values cause a trap to occur. When an explicit write to a CSR creates the conditions that trigger the trap, the trap is not considered a side effect of the write but merely an indirect effect.
Standard CSRs do not have any side effects on reads. Standard CSRs may have side effects on writes. Custom extensions might add CSRs for which accesses have side effects on either reads or writes.
Some CSRs, such as the instructions-retired counter, instret,
may be modified as side effects of instruction execution. In these
cases, if a CSR access instruction reads a CSR, it reads the value
prior to the execution of the instruction. If a CSR access
instruction writes such a CSR, the write is done instead of the
increment. In particular, a value written to instret by one
instruction will be the value read by the following instruction.
The assembler pseudoinstruction to read a CSR, CSRR rd, csr, is encoded as CSRRS rd, csr, x0. The assembler pseudoinstruction to write a CSR, CSRW csr, rs1, is encoded as CSRRW x0, csr, rs1, while CSRWI csr, uimm, is encoded as CSRRWI x0, csr, uimm.
Further assembler pseudoinstructions are defined to set and clear bits in the CSR when the old value is not required: CSRS/CSRC csr, rs1; CSRSI/CSRCI csr, uimm.
CSR Access Ordering
Each RISC-V hart normally observes its own CSR accesses, including its implicit CSR accesses, as performed in program order. In particular, unless specified otherwise, a CSR access is performed after the execution of any prior instructions in program order whose behavior modifies or is modified by the CSR state and before the execution of any subsequent instructions in program order whose behavior modifies or is modified by the CSR state. Furthermore, an explicit CSR read returns the CSR state before the execution of the instruction, while an explicit CSR write suppresses and overrides any implicit writes or modifications to the same CSR by the same instruction.
Likewise, any side effects from an explicit CSR access are normally observed to occur synchronously in program order. Unless specified otherwise, the full consequences of any such side effects are observable by the very next instruction, and no consequences may be observed out-of-order by preceding instructions. (Note the distinction made earlier between side effects and indirect effects of CSR writes.)
For the RVWMO memory consistency model (Chapter [ch:memorymodel]), CSR accesses are weakly ordered by default, so other harts or devices may observe CSR accesses in an order different from program order. In addition, CSR accesses are not ordered with respect to explicit memory accesses, unless a CSR access modifies the execution behavior of the instruction that performs the explicit memory access or unless a CSR access and an explicit memory access are ordered by either the syntactic dependencies defined by the memory model or the ordering requirements defined by the Memory-Ordering PMAs section in Volume II of this manual. To enforce ordering in all other cases, software should execute a FENCE instruction between the relevant accesses. For the purposes of the FENCE instruction, CSR read accesses are classified as device input (I), and CSR write accesses are classified as device output (O).
Informally, the CSR space acts as a weakly ordered memory-mapped I/O region, as defined by the Memory-Ordering PMAs section in Volume II of this manual. As a result, the order of CSR accesses with respect to all other accesses is constrained by the same mechanisms that constrain the order of memory-mapped I/O accesses to such a region.
These CSR-ordering constraints are imposed to support ordering main
memory and memory-mapped I/O accesses with respect to CSR accesses that
are visible to, or affected by, devices or other harts.
Examples include the time, cycle, and mcycle
CSRs, in addition to CSRs that reflect pending interrupts, like mip and
sip.
Note that implicit reads of such CSRs (e.g., taking an interrupt because of
a change in mip) are also ordered as device input.
Most CSRs (including, e.g., the fcsr) are not visible to other harts;
their accesses can be freely reordered in the global memory order with respect
to FENCE instructions without violating this specification.
The hardware platform may define that accesses to certain CSRs are strongly ordered, as defined by the Memory-Ordering PMAs section in Volume II of this manual. Accesses to strongly ordered CSRs have stronger ordering constraints with respect to accesses to both weakly ordered CSRs and accesses to memory-mapped I/O regions.
The rules for the reordering of CSR accesses in the global memory order should probably be moved to Chapter [ch:memorymodel] concerning the RVWMO memory consistency model.
While CSRs are primarily used by the privileged architecture, there are several uses in unprivileged code including for counters and timers, and for floating-point status.
The counters and timers are no longer considered mandatory parts of the standard base ISAs, and so the CSR instructions required to access them have been moved out of Chapter [rv32] into this separate chapter.