Does SHA256SIG0 use floating-point or vector registers?
No. These scalar crypto extension instructions use integer X registers.
SHA256SIG0
RISC-V SHA256SIG0 Instruction Details
Instruction ManualI-typeSHA-256 σ0 function: ROTR(rs1,7) ^ ROTR(rs1,18) ^ SHR(rs1,3)
Instruction Syntax
sha256sig0 rd, rs1
Operand Breakdown
Destination rd: general-purpose register receiving the result.
Source rs1: register holding the first operand.
Immediate imm: 12-bit signed value, sign-extended before operation with rs1.
ZknhCrypto & Security
Instruction Encoding
31..20
imm[11:0]
19..15
rs1
14..12
funct3
11..7
rd
6..0
opcode
SHA256SIG0 uses opcode 0010011 (0x13), funct3 001, funct7 0001000. The rs1 field selects the source register, the 12-bit immediate provides the second operand, and rd selects the destination.
Format: I-type
opcode: 0010011 (0x13)
funct3: 001 (0x1)
funct7: 0001000 (0x08)
Instruction Behavior
sha256sig0 implements the SHA-256 σ0 function (lowercase sigma) from Zknh: σ0(x) = ROTR(x, 7) ^ ROTR(x, 18) ^ SHR(x, 3). Reads rs1, computes σ0, writes to rd. Used in SHA-256 message schedule word expansion preprocessing.
Quick Understanding & Search Notes
SHA256SIG0 is a Zknh scalar cryptography instruction for SHA-256 transform. This page is checked against the official scalar crypto extension, avoiding confusion among round functions, key schedule steps, and operand sources.
sha256sig0 performs the SHA-256 lowercase sigma message-schedule transform using the official fixed rotate-right and logical-shift combination.
Scalar crypto instructions use integer X registers, and the official spec requires the relevant crypto instructions to be implemented with data-independent execution latency.
Common Usage Scenarios
Crypto & Security
Understand this scenario with real code like «sha256sig0 a0, a1».
Hash Algorithms
Understand this scenario with real code like «sha256sig0 a0, a1».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is I-type.
- Confirm the operand order matches the example.
Semantic Check
- Ensure the destination register usage is compatible with the calling convention.
- Confirm this is not the lower-level form of a pseudo-instruction expansion.
Pitfalls / Common Confusions
Distinguish from Σ0 (sha256sum0, uppercase sigma): σ0 for message schedule, Σ0 for compression.
sha256sig0 uses logical right shift (SHR), not arithmetic.
FAQ
Is SHA256SIG0 a complete algorithm implementation?
No. It is a low-level step from AES, SHA, SM3, or SM4; software still combines instructions with the algorithm schedule, round constants, or round keys.