Does MOP.RR.2 write rd by default?
Yes. The official Zimop rule says it writes zero to rd by default.
MOP.RR.2
RISC-V MOP.RR.2 Instruction Details
Instruction ManualR-typeZimop MOP.RR.2; by default writes zero to rd and may be redefined to read rs1/rs2 and write rd.
Instruction Syntax
mop.rr.2 rd, rs1, rs2
Operand Breakdown
rd: the default semantics write 0 to x[rd]; with rd=x0 the architectural write is discarded and x0 remains 0.
rs1 and rs2: explicit register fields in the recommended assembly syntax; the default Zimop definition reads neither and carries no syntactic dependency from either source to rd.
Bits 31..25 are a fixed MOP.RR.n selector; only a future redefining extension may specify reads of rs1, rs2, or both.
ZimopSystem & Privileged
Instruction Encoding
31..25
fixed n selector
24..20
rs2
19..15
rs1
14..12
funct3
11..7
rd
6..0
opcode
MOP-RR-2 uses opcode 1110011 (0x73), funct3 100, fixed n selector 1001001. MOP.RR.n uses SYSTEM fields: bits 31..25 are the fixed n selector, rs2, rs1, and rd are explicit register fields, funct3=100, and opcode=SYSTEM. The default definition writes only 0 to x[rd]; neither rs1 nor rs2 is a default read source.
Format: Zimop MOP.RR SYSTEM fields (n=2)
opcode: 1110011 (0x73)
funct3: 100 (0x4)
fixed n selector: 1001001 (0x49)
Instruction Behavior
MOP.RR.2 is a Zimop may-be-operation with recommended syntax mop.rr.2 rd, rs1, rs2. If no other extension redefines the encoding, the official default behavior writes 0 to x[rd]. The encoding space allows future extensions to define it as reading x[rs1] and x[rs2] and writing x[rd]. Standard software should not execute a MOP unless deliberately targeting an extension that redefines it.
Quick Understanding & Search Notes
MOP.RR.2 is a three-register Zimop MOP encoding; when not redefined, it writes zero to rd.
The official extension defines 8 MOP.RR.n instructions numbered 0 through 7.
MOP.RR.n allows a future extension to read rs1/rs2 and write rd, but by default it reads no source registers and creates no source-to-destination syntactic dependency.
Common Usage Scenarios
System & Privileged
Understand this scenario with real code like «mop.rr.2 t0, t1, t2».
Extension Compatibility
Understand this scenario with real code like «mop.rr.2 t0, t1, t2».
Pre-Use Checklist
Syntax Check
- Confirm the assembly syntax has only rd, rs1, and rs2; bits 31..25 are the fixed MOP.RR.n selector, not an ordinary R-type funct7 operation.
- Confirm any x-register is valid for all three fields; the default definition reads neither rs1 nor rs2 and carries no syntactic dependency from either source to rd.
Semantic Check
- Confirm that, unless another extension has redefined it, the instruction only writes 0 to x[rd]; with rd=x0, x0 remains 0.
- Do not assume a specific future redefining operation; standard software should execute this MOP only when deliberately targeting that redefining extension.
Pitfalls / Common Confusions
The default is not a true no-register-write NOP; MOP.RR.n writes zero to rd by default.
Default Zimop MOPs carry no syntactic dependency from rs1/rs2 to rd; a redefining extension may specify otherwise.
Zimop defines only MOP.RR.0 through MOP.RR.7.
FAQ
How is MOP.RR.2 different from MOP.R.2?
MOP.RR is reserved for future extensions that may read two source registers; MOP.R is reserved for future extensions that may read one source register.