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 Behavior
sha256sig0 implements the SHA-256 σ0 function from Zknh: σ0(x) = ROTR(x, 7) ^ ROTR(x, 18) ^ SHR(x, 3). It is defined for RV32/RV64: RV32 operates on all of X(rs1), while RV64 uses X(rs1)[31:0] and sign-extends the 32-bit result before writing rd.
SHA256SIG0 Decode And Execute Animation
Bounded teaching inputs execute the official SHA-256 fixed-width bit operations; this does not model a complete algorithm or microarchitecture.
rd
rs1
sha256sig0
,
SHA256SIG0 is defined in RV32/RV64; RV64 uses X(rs1)[31:0] and sign-extends the 32-bit result to XLEN.
fixed selector
source
fixed
destination
OP-IMM
31..20
000100000010
19..15
rs1=x11
14..12
001
11..7
rd=x10
6..0
0010011
Execution Data Path
inb
0x89ABCDEF
ror32(inb, 7)
0xDF13579B
ror32(inb, 18)
0xF37BE26A
inb >> 3
0x113579BD
result
0x3D5DCC4C
Architectural Result
a0(x10) = 0x3D5DCC4C
This animation shows only official ISA fields and architectural state; it does not model a complete hash algorithm, pipeline, cache, or timing implementation.
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
- Verify the immediate field is within the valid range.
- Confirm source register rs1 points to the correct operand.
Semantic Check
- Check if the immediate sign-extension matches expectations.
- Ensure the result register rd has a clear purpose.
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 implements one SHA-2 Sigma transform; software still schedules the remaining SHA-2 computation.