Index

DSB

Data Synchronization Barrier is a memory barrier that ensures the completion of memory accesses, see Data Synchronization Barrier.

This instruction is used by the aliases PSSBB, and SSBB.

It has encodings from 2 classes: Memory barrier and Memory nXS barrier

Memory barrier

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1 1 0 1 0 1 0 1 0 0 0 0 0 0 1 1 0 0 1 1 CRm 1 0 0 1 1 1 1 1
opc

DSB <option>|#<imm>

boolean nXS = FALSE;

DSBAlias alias;
case CRm of
    when '0000' alias = DSBAlias_SSBB;
    when '0100' alias = DSBAlias_PSSBB;
    otherwise alias = DSBAlias_DSB;

MBReqDomain domain;
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;

MBReqTypes types;
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;

Memory nXS barrier
(FEAT_XS)

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1 1 0 1 0 1 0 1 0 0 0 0 0 0 1 1 0 0 1 1 imm2 1 0 0 0 1 1 1 1 1 1

DSB <option>nXS

if !IsFeatureImplemented(FEAT_XS) then UNDEFINED;
MBReqTypes types = MBReqTypes_All;
boolean nXS = TRUE;
DSBAlias alias = DSBAlias_DSB;
MBReqDomain domain;

case imm2 of
    when '00' domain = MBReqDomain_OuterShareable;
    when '01' domain = MBReqDomain_Nonshareable;
    when '10' domain = MBReqDomain_InnerShareable;
    when '11' domain = MBReqDomain_FullSystem;

Assembler Symbols

<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", other than the values 0b0000 and 0b0100, that are not listed above 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).

Note

The value 0b0000 is used to encode SSBB and the value 0b0100 is used to encode PSSBB.

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.
<imm>

Is a 4-bit unsigned immediate, in the range 0 to 15, encoded in the "CRm" field.

Alias Conditions

Alias Is preferred when
PSSBB CRm == '0100'
SSBB CRm == '0000'

Operation

case alias of
    when DSBAlias_SSBB
        SpeculativeStoreBypassBarrierToVA();
    when DSBAlias_PSSBB
        SpeculativeStoreBypassBarrierToPA();
    when DSBAlias_DSB
        if IsFeatureImplemented(FEAT_TME) && TSTATE.depth > 0 then
            FailTransaction(TMFailure_ERR, FALSE);
        if !nXS && IsFeatureImplemented(FEAT_XS) then
            nXS = PSTATE.EL IN {EL0, EL1} && IsHCRXEL2Enabled() && HCRX_EL2.FnXS == '1';
        DataSynchronizationBarrier(domain, types, nXS);
    otherwise
        Unreachable();