dsb

Data Synchronization Barrier (A32)

DSB <option>

Ensures completion of memory accesses.

Pseudocode Operation

if ConditionPassed() then
  DataSynchronizationBarrier(option)

Example

DSB option

Encoding

Binary Layout

111101010111

31:20

0100

7:4

option

3:0

Format System

Opcode 0xF57FF040

Extension A32 (Base)

Operands

option
SY, ISH, etc

Other forms of dsb

dsb Data Synchronization Barrier
dsb Data Synchronization Barrier (Thumb)

Across architectures

Memory Barrier / Fence : how x86, ARM, RISC-V, and PowerISA each do this.

Reference

View in Arm A64 ISA Reference ↗ Arm AArch32 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0xF57FF040	DSB{<c>}{<q>} {<option>}		A32		111101010111 \| 1 \| 1 \| 1 \| 1 \| 1 \| 1 \| 1 \| 1 \| 0 \| 0 \| 0 \| 0 \| 0100 \| option
0xF3BF8F40	DSB{<c>}{<q>} {<option>}		T32		111100111011 \| 1 \| 1 \| 1 \| 1 \| 10 \| 0 \| 0 \| 1 \| 1 \| 1 \| 1 \| 0100 \| option

Description

Data Synchronization Barrier is a memory barrier that ensures the completion of memory accesses, see Data Synchronization Barrier (DSB). An AArch32 DSB instruction does not require the completion of any AArch64 TLB maintenance instructions, regardless of the nXS qualifier, appearing in program order before the AArch32 DSB.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    boolean nXS;
    if HaveFeatXS() then
        nXS = (PSTATE.EL IN {EL0, EL1} && !ELUsingAArch32(EL2) &&
            IsHCRXEL2Enabled() && HCRX_EL2.FnXS == '1');
    else
        nXS = FALSE;
    MBReqDomain domain;
    MBReqTypes types;
    case option of
        when '0001'  domain = MBReqDomain_OuterShareable;  types = MBReqTypes_Reads;
        when '0010'  domain = MBReqDomain_OuterShareable;  types = MBReqTypes_Writes;
        when '0011'  domain = MBReqDomain_OuterShareable;  types = MBReqTypes_All;
        when '0101'  domain = MBReqDomain_Nonshareable;    types = MBReqTypes_Reads;
        when '0110'  domain = MBReqDomain_Nonshareable;    types = MBReqTypes_Writes;
        when '0111'  domain = MBReqDomain_Nonshareable;    types = MBReqTypes_All;
        when '1001'  domain = MBReqDomain_InnerShareable;  types = MBReqTypes_Reads;
        when '1010'  domain = MBReqDomain_InnerShareable;  types = MBReqTypes_Writes;
        when '1011'  domain = MBReqDomain_InnerShareable;  types = MBReqTypes_All;
        when '1101'  domain = MBReqDomain_FullSystem;      types = MBReqTypes_Reads;
        when '1110'  domain = MBReqDomain_FullSystem;      types = MBReqTypes_Writes;
        otherwise
            assert !(option IN {'0x00'});
            domain = MBReqDomain_FullSystem;  types = MBReqTypes_All;

    if PSTATE.EL IN {EL0, EL1} && EL2Enabled() then
        if HCR.BSU == '11' then
            domain = MBReqDomain_FullSystem;
        if HCR.BSU == '10' && domain != MBReqDomain_FullSystem then
            domain = MBReqDomain_OuterShareable;
        if HCR.BSU == '01' && domain == MBReqDomain_Nonshareable then
            domain = MBReqDomain_InnerShareable;

    DataSynchronizationBarrier(domain, types, nXS);

dsb