Does MOP.RR.5 write rd by default?
Yes, when the default Zimop definition applies; with rd=x0, x0 remains zero. Use the redefining extension’s semantics when applicable.
MOP.RR.5
RISC-V MOP.RR.5 Instruction Details
Instruction ManualZimop MOP.RR SYSTEM fieldsMOP.RR.5 writes zero to rd under its default Zimop definition; a redefining extension may use rs1 and rs2.
Instruction Syntax
mop.rr.5 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.5 uses opcode 1110011 (0x73), funct3 100, fixed n selector 1100011. 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=5)
opcode: 1110011 (0x73)
funct3: 100 (0x4)
fixed n selector: 1100011 (0x63)
Instruction Behavior
MOP.RR.5 is a Zimop may-be-operation with recommended syntax mop.rr.5 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. In RV32/RV64, rd may be x0, in which case x0 remains zero; register overlap does not turn the default behavior into a copy or computation.
Quick Understanding & Search Notes
MOP.RR.5 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.
In the 32-bit SYSTEM encoding, bits 31..25 are fixed to 1100011; rs2, rs1, and rd are explicit 5-bit fields, funct3=100, and opcode=1110011.
The Zimop 1.0 default semantics apply to RV32/RV64: the result is XLEN-bit zero, with no arithmetic on the source fields.
Common Usage Scenarios
System & Privileged
Assume RV32/RV64 Zimop 1.0 is implemented and no redefinition applies: mop.rr.5 x5, x6, x7 writes x5=0 regardless of old x6 and x7; x6 and x7 are unchanged. Overlapping rd with a source field still writes zero; rd=x0 leaves x0 zero. Standard software should execute it only when deliberately targeting an extension that redefines this MOP.
Extension Compatibility
Assume RV32/RV64 Zimop 1.0 is implemented and no redefinition applies: mop.rr.5 x5, x6, x7 writes x5=0 regardless of old x6 and x7; x6 and x7 are unchanged. Overlapping rd with a source field still writes zero; rd=x0 leaves x0 zero. Standard software should execute it only when deliberately targeting an extension that redefines this MOP.
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.5 different from MOP.R.5?
Both write zero by default. The recommended MOP.RR syntax has rd, rs1, and rs2, allowing a redefining extension to use two sources; MOP.R has rd and rs1, allowing one source.