Does C.MOP.1 write a register by default?
No. The official Zcmop rule says C.MOP.n writes no register by default.
C.MOP.1
RISC-V C.MOP.1 Instruction Details
Instruction ManualC-typeZcmop 16-bit C.MOP.1 writes no register in its base behavior; active Zicfiss uses this encoding for C.SSPUSH x1.
Instruction Syntax
c.mop.1
Operand Breakdown
The recommended syntax c.mop.1 has no explicit operands; its 16-bit encoding is fixed to 0110000010000001.
Base Zcmop behavior writes no register; implicit x1 is used by a redefinition. Active Zicfiss uses this encoding for C.SSPUSH x1.
ZcmopMay-Be-Operations
Instruction Encoding
15..0
0110000010000001
C.MOP.1 has fixed 16-bit encoding 0110000010000001; its assembly syntax has no explicit operands.
Format: Zcmop fixed 16-bit encoding
16-bit encoding: 0110000010000001
Compressed instruction, 16-bit encoding.
Instruction Behavior
C.MOP.1 is a nullary 16-bit may-be-operation in Zcmop 1.0, with fixed encoding 0110000010000001 (0x6081). Unless redefined by another extension, it writes no register; the encoding permits a redefinition to read implicit x1. Zcmop depends on Zca. Zicfiss uses this encoding for C.SSPUSH x1; without implemented and active Zicfiss it reverts to Zcmop behavior. This page describes the base Zcmop behavior.
Quick Understanding & Search Notes
C.MOP.1 is a fixed nullary 16-bit encoding that writes no register under base Zcmop behavior. Active Zicfiss uses the same encoding for C.SSPUSH x1.
Zcmop defines only the eight odd-numbered C.MOP.n instructions from 1 through 15.
C.MOP.1 has fixed encoding 0110000010000001 (0x6081) in the reserved space corresponding to C.LUI x1, 0; the recommended syntax is nullary, and a redefinition may read implicit x1.
The default differs from Zimop MOP.R.n, which writes zero to rd.
With Zicfiss implemented and active, this encoding is C.SSPUSH x1; otherwise it reverts to the Zcmop-defined behavior.
Common Usage Scenarios
Zcmop MOP
Understand this scenario with real code like «c.mop.1».
Extension-defined
Understand this scenario with real code like «c.mop.1».
Pre-Use Checklist
Syntax Check
- Verify Zcmop and its Zca dependency; C.MOP.1 is the fixed nullary 16-bit encoding 0110000010000001.
Semantic Check
- Base Zcmop behavior writes no register; a redefinition may read implicit x1.
- Check whether Zicfiss is implemented and active: this encoding is then C.SSPUSH x1; otherwise Zcmop behavior applies.
Pitfalls / Common Confusions
By default C.MOP.n writes no rd; this differs from Zimop MOP.R/MOP.RR, which write zero to rd.
Do not rely on useful C.MOP behavior in ordinary software unless a target extension explicitly redefines the encoding.
Zicfiss already redefines this encoding as C.SSPUSH x1; do not apply the base Zcmop behavior when that extension is active.
FAQ
What is the implicit register for C.MOP.1?
Zcmop permits a redefinition to read implicit x1. Zicfiss uses this encoding for C.SSPUSH x1; when Zicfiss is absent or inactive, base Zcmop behavior applies.