Does XPERM4 access memory?
No. It only rearranges bit fields or performs register lookup within integer registers.
XPERM4
RISC-V XPERM4 Instruction Details
Instruction ManualR-typenibble crossbar permutation (Zbkx).
Instruction Syntax
xperm4 rd, rs1, rs2
Syntax: xperm4 rd, rs1, rs2; integer x0–x31 or ABI names, without an assembly immediate.
32-bit OP: funct7=0010100, funct3=010, opcode=0110011.
RV32/RV64 Zbkx; rs1 holds XLEN/4 4-bit entries, each rs2 element supplies an index, and out-of-range results are zero.
An x0 source is zero and writes to x0 are discarded. Overlap reads values before writeback; equal source registers share one value. No memory access.
ZbkxBit Manipulation
Instruction Behavior
XPERM4 (Zbkx) treats rs1 as a table of XLEN/4 4-bit elements and each 4-bit element of rs2 as an index. At each result position, an index below XLEN/4 selects the corresponding 4-bit element from rs1; otherwise the result element is zero.
XPERM4 Decode And Execute Animation
Each 4-bit rs2 element indexes the rs1 element table; an out-of-range index produces a zero result element.
Execution Context
x0 is always zero; source values are read before writeback. Teaching steps are not processor cycles.
funct7
31..25
0010100rs2
24..20
01100rs1
19..15
01011funct3
14..12
010rd
11..7
01010opcode
6..0
0110011Current calculation
0: 8 -> 0 0
1: 9 -> 0 0
2: A -> 0 0
3: B -> 0 0
4: C -> 0 0
5: D -> 0 0
6: E -> 0 0
7: F -> 0 0
x[rs2] indexes 8 entries in x[rs1]; result = 0x00000000
Step: decode
Shows the instruction's fixed and register fields.
x[a0] <- 0x00000000
1/4: decode
Quick Understanding & Search Notes
XPERM4 performs register lookup: rs1 holds XLEN/4 nibble entries and each nibble of rs2 selects an entry; out-of-range indexes produce zero.
Available on RV32 and RV64; each table entry and index is 4 bits.
Index 0 selects the least-significant element; out-of-range indexes produce zero without modulo wrapping.
Common Usage Scenarios
Bit Manipulation
RV32 Zbkx, x11=0x89ABCDEF, x12=0x76543210: xperm4 x10, x11, x12 gives x10=0x89ABCDEF.
Register Lookup
RV32 Zbkx, x11=0x89ABCDEF, x12=0x76543210: xperm4 x10, x11, x12 gives x10=0x89ABCDEF.
Pre-Use Checklist
Syntax Check
- Syntax: xperm4 rd, rs1, rs2; integer x0–x31 or ABI names, without an assembly immediate.
- 32-bit OP: funct7=0010100, funct3=010, opcode=0110011.
Semantic Check
- RV32/RV64 Zbkx; rs1 holds XLEN/4 4-bit entries, each rs2 element supplies an index, and out-of-range results are zero.
- An x0 source is zero and writes to x0 are discarded. Overlap reads values before writeback; equal source registers share one value. No memory access.
Pitfalls / Common Confusions
Indexes come from each 4-bit element of rs2; rs1 is the table source.
Out-of-range indexes produce zero; they do not wrap modulo the element count.
The count of valid indexes is determined by the XLEN-wide number of 4-bit elements.
FAQ
Which extension contains XPERM4?
It belongs to the Zbkx crossbar-permutation extension.