Does vsetvli x0, x0, ... request VLMAX?
No. Keep current vl form: rd=x0, rs1=x0. VLMAX request form: rs1=x0, rd must be nonzero.
VSETVLI
RISC-V VSETVLI Instruction Details
Instruction ManualOP-V vector-configuration formatConfigure vl/vtype from rs1/rd AVL rules and vtypei, then write the selected vl to rd
Instruction Syntax
vsetvli rd, rs1, vtypei
Operand Breakdown
rd: receives the selected new vl; with rd=x0 it is not saved to an integer register.
rs1: supplies AVL, with special rs1=x0 forms for requesting VLMAX or preserving the current vl.
vtypei: encodes SEW, LMUL, tail policy, and mask policy.
VVector OperationsVector Config
Instruction Behavior
VSETVLI is an RVV vector-configuration instruction. It uses the OP-V opcode space with funct3=111 and derives AVL from the rs1/rd combination. Ordinary AVL form: rs1 is nonzero, AVL=x[rs1]. VLMAX request form: rs1=x0, rd must be nonzero. Keep current vl form: rd=x0, rs1=x0, valid only when the new SEW/LMUL ratio leaves VLMAX unchanged and prior vtype.vill=0. vtypei encodes SEW, LMUL, vta, and vma; the selected vl is written to rd, with rd=x0 discarding the GPR write. Unsupported vtype follows the official vill and vl=0 result.
VSETVLI Decode And Execute Animation
Decode the vset fields, derive AVL and vtype, then show the architectural vl/vtype/rd update.
Step 1 / 6
Encoding
VSETVLI is an OP-V configuration instruction. This step shows the opcode, funct3, and the fields carrying AVL/vtype information.
Instruction input
vsetvli
Execution Context
Scope: this animation shows only the ISA-visible vl/vtype selection rules from the V extension. Demo VLMAX and mid-range vl choice are educational inputs, not claims about a processor's VLEN, ELEN, support matrix, or OS handling.
Encoding fieldsvsetvli / OP-V
31
fixed
0
30..20
vtypei
00011010000
19..15
rs1
01010
14..12
funct3
111
11..7
rd
00101
6..0
opcode
1010111
asm
form
VSETVLI
Configuration data path
AVL source
x10
AVL
40
VLMAX
16
selected vl
16
vtype
e32, m1, ta, ma
x5
not written back
vstart
3
Current configuration summary
vl result
16
policy
ta/ma
vl selection result
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
The demo selects vl=16 from AVL=40 and VLMAX=16. Green cells show the body element range after configuration.
Quick Understanding & Search Notes
VSETVLI is an RVV vector-configuration instruction. It derives AVL from the rs1/rd combination, derives SEW, LMUL, vta, and vma from vtypei, selects vl against the implementation VLMAX, and writes vl to rd. Later vector instructions use that vl/vtype for active elements, tail policy, and masked-off policy.
The OP-V opcode is 1010111 with funct3=111; vtypei encodes vma, vta, vsew, and vlmul.
Ordinary AVL form: rs1 nonzero, AVL=x[rs1]. VLMAX request form: rs1=x0, rd must be nonzero. Keep current vl form: rd=x0, rs1=x0.
Legal forms update vl/vtype and reset vstart to zero; the new vl is written to rd, with rd=x0 discarding the integer-register write.
Unsupported vtype follows the official vill and vl=0 result; the page animation does not model an implementation-specific support matrix.
Common Usage Scenarios
Vector Integer Arithmetic
Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma # configure SEW=32, LMUL=1, write vl to t0».
Widening Operations
Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma # configure SEW=32, LMUL=1, write vl to t0».
Vectorized Loops
Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma # configure SEW=32, LMUL=1, write vl to t0».
Pre-Use Checklist
Syntax Check
- Requires V; use rd, rs1 and vtypei, explicitly specifying ta/tu and ma/mu.
Semantic Check
- Check the AVL source from the rs1/rd combination; the keep-current-vl form also requires unchanged VLMAX and prior vill=0.
- The AVL/VLMAX rules select vl; legal execution updates vl/vtype, resets vstart, and discards GPR writeback when rd=x0.
Pitfalls / Common Confusions
VLMAX request form: rs1=x0, rd!=x0. Keep current vl form: rd=x0, rs1=x0.
V 1.0 assembly should explicitly spell ta/tu and ma/mu.
Unsupported vtype follows the official vill and vl=0 result.
FAQ
Does vsetvli set vl directly from vtypei?
No. vtypei selects SEW/LMUL and policy bits; vl is selected from AVL, VLMAX, and the official vector-length rules.