How is VSSRA.VX different from VSRA?
VSSRA.VX rounds shifted-out bits according to vxrm during the right shift; VSRA is an ordinary right shift and does not use vxrm.
VSSRA.VX
RISC-V VSSRA.VX Instruction Details
Instruction ManualOP-V / OPIVXVSSRA.VX performs an element-wise fixed-point rounding arithmetic right shift using a shift count from the x[rs1] scalar value.
Instruction Syntax
vssra.vx vd, vs2, rs1, vm
Operand Breakdown
vd/vs2 are EEW=SEW, EMUL=LMUL vector groups with legal aligned starts. Equal-width source/destination overlap is allowed; equal sources read equal initial values. rs1 selects an integer register: XLEN>SEW takes low SEW bits, XLEN<SEW sign-extends, and x0 supplies zero.
32-bit OP-V: opcode=1010111, funct6=101011, funct3=100; vm at bit25, vs2 at bits24:20, rs1 at bits19:15, vd at bits11:7. Omit v0.t for vm=1; include it for vm=0.
d uses only low lg2(SEW) count bits. Arithmetic right-shift a two’s-complement signed value, then apply vxrm as (v >> d)+r: rnu nearest-up, rne nearest-even, rdn down, rod odd. d=0 retains the input. No saturation or vxsat change.
Require RV32/RV64 V 1.0, enabled vector state, vill=0 and supported vtype/SEW. Only active vstart<=i<vl elements compute; prestart is unchanged, inactive/tail follow vma/vta, retaining old values or becoming all ones. vstart>=vl (including vl=0) updates no destination elements. Masked vd excludes v0; v0 cannot supply both EEW=1 mask and SEW data. Normal completion resets vstart=0; implementations may reject nonzero vstart values they cannot produce.
VVector Operations
Instruction Behavior
VSSRA.VX is a RISC-V V extension fixed-point scaling shift. It reads active vs2 elements, uses the low shift-count bits from the x[rs1] scalar value, rounds shifted-out bits according to vxrm, and writes the SEW-width result to vd.
VSSRA.VX Decode And Execute Animation
Starts from OP-V encoding fields, then shows how VSSRA takes the low shift-count bits, rounds with vxrm, and preserves sign through arithmetic right shift.
Instruction input
vssra.vx
Teaching context: V 1.0, enabled vector state, valid supported vtype/SEW, LMUL=m1, VLEN=4096, vstart=0, VLMAX=4096/16. Equal-width source/destination overlap is legal; equal source registers require equal elements. With v0.t, vd/vs2/vs1 cannot be v0. Use low log2(SEW) shift-count bits and keep low SEW result bits; inactive/tail values are not specified here. Enter nonnegative SEW bit patterns; interpret VSSRA as two’s-complement signed and VSSRL as unsigned, then round using vxrm; vxsat is unchanged. d=0 applies no rounding. x0 supplies zero; normal completion resets vstart=0. VL=0 leaves the destination unchanged; stages are not processor cycles.
Execution Context
Shift-count rule: only the low lg2(SEW) bits of the x[rs1] value are used.
OP-V encodingvssra.vx
funct6
101011
vm
0
vs2
01000
rs1
01011
funct3
100
vd
00100
opcode
1010111
lane
0
1
2
3
4
5
6
7
v8
0x0035
0x800c
0x0080
0x0123
0x801b
0x0019
0x0035
0x802a
round>>
round>>
round>>
round>>
round>>
round>>
round>>
round>>
x11
0x0013
0x0013
0x0013
0x0013
0x0013
0x0013
0x0013
0x0013
v0.t
1
0
1
1
1
0
1
1
v4
...
-
...
...
...
-
...
...
Current step
Show OP-V 32-bit encoding fields
vssra.vx uses OP-V encoding. The animation places fixed fields, register fields, vm, and the rs1 field in one encoding strip.
This animation shows OP-V decode, low-bit shift-count selection, per-element right shifts, and destination writeback; it does not model pipelines or timing.
Quick Understanding & Search Notes
VSSRA.VX is an RVV fixed-point scaling operation: active lanes take values from vs2, take the shift count from the x[rs1] scalar value, apply vxrm rounding, and write vd.
Official syntax is `vssra.vx vd, vs2, rs1, vm`; vm selects unmasked execution or v0.t masking.
VSSRA uses the V extension fixed-point rounding-mode CSR vxrm, with rnu, rne, rdn, and rod modes.
After the arithmetic right shift, apply vxrm rounding and write SEW bits. Only active vstart<=i<vl elements compute; prestart is unchanged and inactive/tail follow vma/vta. vl=0 or vstart>=vl updates no destination elements.
VSSRA does not set vxsat, unlike the saturating narrowing behavior of VNCLIP/VNCLIPU.
Vector Execution Context
vd/vs2 are EEW=SEW, EMUL=LMUL vector groups with legal aligned starts. Equal-width source/destination overlap is allowed; equal sources read equal initial values. rs1 selects an integer register: XLEN>SEW takes low SEW bits, XLEN<SEW sign-extends, and x0 supplies zero.
32-bit OP-V: opcode=1010111, funct6=101011, funct3=100; vm at bit25, vs2 at bits24:20, rs1 at bits19:15, vd at bits11:7. Omit v0.t for vm=1; include it for vm=0.
d uses only low lg2(SEW) count bits. Arithmetic right-shift a two’s-complement signed value, then apply vxrm as (v >> d)+r: rnu nearest-up, rne nearest-even, rdn down, rod odd. d=0 retains the input. No saturation or vxsat change.
Require RV32/RV64 V 1.0, enabled vector state, vill=0 and supported vtype/SEW. Only active vstart<=i<vl elements compute; prestart is unchanged, inactive/tail follow vma/vta, retaining old values or becoming all ones. vstart>=vl (including vl=0) updates no destination elements. Masked vd excludes v0; v0 cannot supply both EEW=1 mask and SEW data. Normal completion resets vstart=0; implementations may reject nonzero vstart values they cannot produce.
Common Usage Scenarios
Fixed-point scaling
Assume enabled V/VS, valid e8,m1,tu,mu, VLEN=4096, XLEN=64, vl=4, vstart=0, vxrm=rne, v0.mask[0..3]=1111, v12=[1,127,128,255], and x5=3. d=3; nearest-even rounding gives v8=[0,16,240,0] (0,16,-16,0). Sources, inactive/tail values and vxsat stay unchanged; normal completion resets vstart=0.
Vector shifts
Assume enabled V/VS, valid e8,m1,tu,mu, VLEN=4096, XLEN=64, vl=4, vstart=0, vxrm=rne, v0.mask[0..3]=1111, v12=[1,127,128,255], and x5=3. d=3; nearest-even rounding gives v8=[0,16,240,0] (0,16,-16,0). Sources, inactive/tail values and vxsat stay unchanged; normal completion resets vstart=0.
DSP data paths
Assume enabled V/VS, valid e8,m1,tu,mu, VLEN=4096, XLEN=64, vl=4, vstart=0, vxrm=rne, v0.mask[0..3]=1111, v12=[1,127,128,255], and x5=3. d=3; nearest-even rounding gives v8=[0,16,240,0] (0,16,-16,0). Sources, inactive/tail values and vxsat stay unchanged; normal completion resets vstart=0.
Pre-Use Checklist
Syntax Check
- vd/vs2 are EEW=SEW, EMUL=LMUL vector groups with legal aligned starts. Equal-width source/destination overlap is allowed; equal sources read equal initial values. rs1 selects an integer register: XLEN>SEW takes low SEW bits, XLEN<SEW sign-extends, and x0 supplies zero.
- 32-bit OP-V: opcode=1010111, funct6=101011, funct3=100; vm at bit25, vs2 at bits24:20, rs1 at bits19:15, vd at bits11:7. Omit v0.t for vm=1; include it for vm=0.
Semantic Check
- d uses only low lg2(SEW) count bits. Arithmetic right-shift a two’s-complement signed value, then apply vxrm as (v >> d)+r: rnu nearest-up, rne nearest-even, rdn down, rod odd. d=0 retains the input. No saturation or vxsat change.
- Require RV32/RV64 V 1.0, enabled vector state, vill=0 and supported vtype/SEW. Only active vstart<=i<vl elements compute; prestart is unchanged, inactive/tail follow vma/vta, retaining old values or becoming all ones. vstart>=vl (including vl=0) updates no destination elements. Masked vd excludes v0; v0 cannot supply both EEW=1 mask and SEW data. Normal completion resets vstart=0; implementations may reject nonzero vstart values they cannot produce.
Pitfalls / Common Confusions
VSSRA rounds shifted-out bits with vxrm; it is not an ordinary VSRA right shift.
The instruction does not saturate or set vxsat; use VNCLIP/VNCLIPU for saturating narrowing shifts.
The VX form takes the shift count from the x[rs1] scalar value; the effective shift count uses the SEW-selected low bits.
FAQ
Does VSSRA.VX set vxsat?
No. VSSRA.VX is a fixed-point rounding scaling shift, not a saturating clip instruction.