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BR

Branch to Register branches unconditionally to an address in a register, with a hint that this is not a subroutine return.

313029282726252423222120191817161514131211109876543210
1101011000011111000000Rn00000
ZopAMRm

Integer

BR <Xn>

integer n = UInt(Rn);
BranchType branch_type;
integer m = UInt(Rm);
boolean pac = (A == '1');
boolean use_key_a = (M == '0');
boolean source_is_sp = ((Z == '1') && (m == 31));

if !pac && m != 0 then 
    UNDEFINED;
elsif pac && !HavePACExt() then 
    UNDEFINED;

case op of
    when '00' branch_type = BranchType_INDIR;
    when '01' branch_type = BranchType_INDCALL;
    when '10' branch_type = BranchType_RET;
    otherwise UNDEFINED;

if pac then
    if Z == '0' && m != 31 then
        UNDEFINED;

    if branch_type == BranchType_RET then
        if n != 31 then UNDEFINED;
        n = 30;
        source_is_sp = TRUE;

Assembler Symbols

<Xn>

Is the 64-bit name of the general-purpose register holding the address to be branched to, encoded in the "Rn" field.

Operation

bits(64) target = X[n];
boolean auth_then_branch = TRUE;

if pac then
    bits(64) modifier = if source_is_sp then SP[] else X[m];

    if use_key_a then
        target = AuthIA(target, modifier, auth_then_branch);
    else
        target = AuthIB(target, modifier, auth_then_branch);

if branch_type == BranchType_INDCALL then X[30] = PC[] + 4;

// Value in BTypeNext will be used to set PSTATE.BTYPE
case branch_type of
    when BranchType_INDIR           // BR, BRAA, BRAB, BRAAZ, BRABZ
        if InGuardedPage then
            if n == 16 || n == 17 then
                BTypeNext = '01';
            else
                BTypeNext = '11';
        else
            BTypeNext = '01';
    when BranchType_INDCALL         // BLR, BLRAA, BLRAB, BLRAAZ, BLRABZ
        BTypeNext = '10';
    when BranchType_RET             // RET, RETAA, RETAB 
        BTypeNext = '00';

BranchTo(target, branch_type);