DSB
Data synchronization barrier
This instruction is a memory barrier that ensures the completion
of memory accesses, see Data Synchronization Barrier.
This instruction is used by the aliases
PSSBB
CRm == '0100'
SSBB
CRm == '0000'
See
below for details of when each alias is preferred.
It has encodings from 2 classes:
Memory barrier
and
Memory nXS barrier
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DSB (<option>|#<imm>)
var nXS : boolean = FALSE;
var alias : DSBAlias;
case CRm of
when '0000' => alias = DSBAlias_SSBB;
when '0100' => alias = DSBAlias_PSSBB;
otherwise => alias = DSBAlias_DSB;
end;
var domain : MBReqDomain;
case CRm[3:2] of
when '00' => domain = MBReqDomain_OuterShareable;
when '01' => domain = MBReqDomain_Nonshareable;
when '10' => domain = MBReqDomain_InnerShareable;
when '11' => domain = MBReqDomain_FullSystem;
end;
var types : MBReqTypes;
case CRm[1:0] of
when '00' => types = MBReqTypes_All; domain = MBReqDomain_FullSystem;
when '01' => types = MBReqTypes_Reads;
when '10' => types = MBReqTypes_Writes;
when '11' => types = MBReqTypes_All;
end;
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DSB <option>nXS
if !IsFeatureImplemented(FEAT_XS) then EndOfDecode(Decode_UNDEF); end;
let types : MBReqTypes = MBReqTypes_All;
var nXS : boolean = TRUE;
let alias : DSBAlias = DSBAlias_DSB;
var domain : MBReqDomain;
case imm2 of
when '00' => domain = MBReqDomain_OuterShareable;
when '01' => domain = MBReqDomain_Nonshareable;
when '10' => domain = MBReqDomain_InnerShareable;
when '11' => domain = MBReqDomain_FullSystem;
end;
<option>
For the "Memory barrier" variant: specifies the limitation on the barrier operation. Values are:
SY
Full system is the required shareability domain, reads and writes are the required access types, both before and after the barrier instruction. This option is referred to as the full system barrier. Encoded as CRm = 0b1111.
ST
Full system is the required shareability domain, writes are the required access type, both before and after the barrier instruction. Encoded as CRm = 0b1110.
LD
Full system is the required shareability domain, reads are the required access type before the barrier instruction, and reads and writes are the required access types after the barrier instruction. Encoded as CRm = 0b1101.
ISH
Inner Shareable is the required shareability domain, reads and writes are the required access types, both before and after the barrier instruction. Encoded as CRm = 0b1011.
ISHST
Inner Shareable is the required shareability domain, writes are the required access type, both before and after the barrier instruction. Encoded as CRm = 0b1010.
ISHLD
Inner Shareable is the required shareability domain, reads are the required access type before the barrier instruction, and reads and writes are the required access types after the barrier instruction. Encoded as CRm = 0b1001.
NSH
Non-shareable is the required shareability domain, reads and writes are the required access, both before and after the barrier instruction. Encoded as CRm = 0b0111.
NSHST
Non-shareable is the required shareability domain, writes are the required access type, both before and after the barrier instruction. Encoded as CRm = 0b0110.
NSHLD
Non-shareable is the required shareability domain, reads are the required access type before the barrier instruction, and reads and writes are the required access types after the barrier instruction. Encoded as CRm = 0b0101.
OSH
Outer Shareable is the required shareability domain, reads and writes are the required access types, both before and after the barrier instruction. Encoded as CRm = 0b0011.
OSHST
Outer Shareable is the required shareability domain, writes are the required access type, both before and after the barrier instruction. Encoded as CRm = 0b0010.
OSHLD
Outer Shareable is the required shareability domain, reads are the required access type before the barrier instruction, and reads and writes are the required access types after the barrier instruction. Encoded as CRm = 0b0001.
All other encodings of "CRm" that are not listed, other than the values 0b0000 and 0b0100, are reserved and can be encoded using the #<imm> syntax. All unsupported and reserved options must execute as a full system barrier operation, but software must not rely on this behavior. For more information on whether an access is before or after a barrier instruction, see Data Memory Barrier (DMB) or see Data Synchronization Barrier (DSB).
The value 0b0000 is used to encode SSBB and the value 0b0100 is used to encode PSSBB.
CRm
<option>
0001
OSHLD
0010
OSHST
0011
OSH
0101
NSHLD
0110
NSHST
0111
NSH
1x00
RESERVED
1001
ISHLD
1010
ISHST
1011
ISH
1101
LD
1110
ST
1111
SY
<option>
For the "Memory nXS barrier" variant: specifies the limitation on the barrier operation. Values are:
SY
Full system is the required shareability domain, reads and writes are the required access types, both before and after the barrier instruction. This option is referred to as the full system barrier. Encoded as imm2 = 0b11.
ISH
Inner Shareable is the required shareability domain, reads and writes are the required access types, both before and after the barrier instruction. Encoded as imm2 = 0b10.
NSH
Non-shareable is the required shareability domain, reads and writes are the required access, both before and after the barrier instruction. Encoded as imm2 = 0b01.
OSH
Outer Shareable is the required shareability domain, reads and writes are the required access types, both before and after the barrier instruction. Encoded as imm2 = 0b00.
imm2
<option>
00
OSH
01
NSH
10
ISH
11
SY
<imm>
Is a 4-bit unsigned immediate, in the range 0 to 15, encoded in the "CRm" field.
Alias Conditions
case alias of
when DSBAlias_SSBB =>
SpeculativeStoreBypassBarrierToVA();
when DSBAlias_PSSBB =>
SpeculativeStoreBypassBarrierToPA();
when DSBAlias_DSB =>
if !nXS && IsFeatureImplemented(FEAT_XS) then
nXS = PSTATE.EL IN {EL0, EL1} && IsHCRXEL2Enabled() && HCRX_EL2().FnXS == '1';
end;
DataSynchronizationBarrier(domain, types, nXS);
otherwise =>
unreachable;
end;
2026-03-12 12:23:09
2025-09_rel_asl1