Can SB.RL use offset(rs1)?
No. The official syntax is sb.rl rs2, (rs1), with no offset immediate; the address comes directly from rs1.
SB.RL
RISC-V SB.RL Instruction Details
Instruction ManualR-typeZalasr atomic store of 1 byte with release-RCsc ordering.
Instruction Syntax
sb.rl rs2, (rs1)
Operand Breakdown
Source rs2: supplies the low bits written to memory; the B/H/W/D form selects the width.
Address register rs1: directly supplies the memory address; Zalasr syntax has no offset immediate.
Encoding field rd is fixed to 00000/x0; rl must be 1, and aq/rl only annotate ordering.
ZalasrCache & Synchronization
Instruction Behavior
SB.RL is a 1-byte ordered atomic store in the Zalasr standalone load-acquire/store-release family. It atomically writes the low 8 bits of x[rs2] to the address in x[rs1]; the syntax has no offset field, and rd is fixed to x0 in the encoding. The address is naturally aligned to the access width by default unless relaxed by the misaligned atomicity granule PMA.
SB.RL Zalasr Decode And Execute Animation
Shows the Zalasr standalone store-release AMO encoding, no-offset address syntax, 8-bit data transfer, and aq/rl ordering annotation.
rs2
rs1
sb.rl,()
31..27
26
25
24..20
19..15
14..12
11..7
6..0
00111
funct5
0
aq
1
rl
01011
rs2
01010
rs1
000
funct3
00000
rd=00000
0101111
opcode
Zalasr Data Path
instruction
0x3AB5002F
opcode
0101111 -> AMO major opcode
funct5
00111 -> SB.RL
aq / rl
0/1 -> release
operands
rs2=a1(x11), rs1=a0(x10), rd=x0
address
a0(x10) -> 0x00001000 (no offset)
store source
low 8 bits of a1(x11) -> 0x7F
execute
atomic store 8 bits at 0x00001000
writeback
mem[0x00001000] = 0x7F
8-bit Transfer View
7
6
5
4
3
2
1
0
0
1
1
1
1
1
1
1
rs2 low 8
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-
-
-
-
-
-
-
SB.RL writes only the low 8 bits of rs2 to memory; higher rs2 bits are not part of the stored value.
This animation shows ISA-visible Zalasr atomic load/store behavior and aq/rl ordering annotations; it does not model LR/SC reservations, AMO read-modify-write old-value return, caches, buses, or pipeline timing.
Quick Understanding & Search Notes
SB.RL uses rs1 directly as the address and atomically writes the low 8 bits of rs2; aq/rl bits provide ordering annotations and do not change the stored data.
The syntax is sb.rl rs2, (rs1), with no offset field; the address comes from x[rs1].
funct5=00111 selects the Zalasr store-release class, funct3=000 selects the SB.RL access width, the AMO major opcode is 0101111, the rl bit must be 1 for Zalasr stores, and rd is fixed to x0.
The stored value comes from the low 8 bits of x[rs2].
aq/rl describe memory ordering only; they do not change the stored data and do not return the old memory value.
Common Usage Scenarios
Atomic & Sync
Understand this scenario with real code like «sb.rl x5, (x6)».
Concurrency & Multi-core
Understand this scenario with real code like «sb.rl x5, (x6)».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is R-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
The syntax has no offset immediate; the address comes directly from rs1.
The store uses only the low 8 bits of rs2; higher bits are not part of the written value.
Zalasr store encodings without rl set are RESERVED.
Zalasr is a single ordered atomic load/store instruction, not an LR/SC reservation protocol or an AMO read-modify-write old-value return.
FAQ
Is SB.RL an LR/SC or AMO read-modify-write operation?
No. These Zalasr instructions are single ordered atomic load/store operations; the animation does not show reservations or old-value return.
Do aq/rl bits change the data result?
No. aq/rl are ordering annotations; the stored value is still the low 8 bits of rs2.