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 scope : MBMaintenanceScope;
var types : MBReqTypes;
case CRm[1:0] of
when '00' =>
types = MBReqTypes_All;
scope = MBMaintenanceScope_OuterShareable;
when '01' =>
types = MBReqTypes_Reads;
scope = MBMaintenanceScope_None;
when '10' =>
types = MBReqTypes_Writes;
scope = MBMaintenanceScope_None;
when '11' =>
types = MBReqTypes_All;
case CRm[3:2] of
when '00' => scope = MBMaintenanceScope_OuterShareable;
when '01' => scope = MBMaintenanceScope_Nonshareable;
when '10' => scope = MBMaintenanceScope_InnerShareable;
when '11' => scope = MBMaintenanceScope_OuterShareable;
end;
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 scope : MBMaintenanceScope;
case imm2 of
when '00' => scope = MBMaintenanceScope_OuterShareable;
when '01' => scope = MBMaintenanceScope_Nonshareable;
when '10' => scope = MBMaintenanceScope_InnerShareable;
when '11' => scope = MBMaintenanceScope_OuterShareable;
end;
<option>
For the "Memory barrier" variant: specifies the limitation on the Memory Effects and maintenance operations ordered by
the barrier operation, and is encoded in "CRm".
The value 0b0000 is used to encode SSBB and the value 0b0100
is used to encode PSSBB.
For more information on whether an access is before or after a barrier instruction,
see Data Synchronization Barrier (DSB).
CRm
<option>
Description
0001
OSHLD
This option has the same behavior as LD and is deprecated.
0010
OSHST
This option has the same behavior as ST and is deprecated.
0011
OSH
This option has the same behavior as SY.
0101
NSHLD
This option has the same behavior as LD and is deprecated.
0110
NSHST
This option has the same behavior as ST and is deprecated.
0111
NSH
Non-shareable is the required maintenance scope. Reads and writes are
the required access types, both before and after the barrier instruction.
1x00
RESERVED
1001
ISHLD
This option has the same behavior as LD and is deprecated.
1010
ISHST
This option has the same behavior as ST and is deprecated.
1011
ISH
Inner Shareable is the required maintenance scope. Reads and writes are
the required access types, both before and after the barrier instruction.
1101
LD
Reads are the required access type before the barrier instruction.
Reads and writes are the required access types after the barrier instruction.
1110
ST
Writes are the required access type, both before and after the
barrier instruction.
1111
SY
Outer Shareable is the required maintenance scope. Reads and writes are
the required access types, both before and after the barrier instruction.
All encodings of "CRm" that are listed as reserved can be encoded using the #<imm> syntax.
All reserved options have the same behavior as SY, but software must not rely on this behavior.
<option>
For the "Memory nXS barrier" variant: specifies the limitation on the barrier operation, and is
imm2
<option>
Description
00
OSH
This option has the same behavior as SY.
01
NSH
Non-shareable is the required maintenance scope.
Reads and writes are the required access types, both before and after the barrier instruction.
10
ISH
Inner Shareable is the required maintenance scope.
Reads and writes are the required access types, both before and after the barrier instruction.
11
SY
Outer Shareable is the required maintenance scope.
Reads and writes are the required access types, both before and after the barrier instruction.
<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(scope, types, nXS);
otherwise =>
unreachable;
end;
2026-03-26 20:27:25
2026-03_rel