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TBB, TBH

Table Branch Byte or Halfword causes a PC-relative forward branch using a table of single byte or halfword offsets. A base register provides a pointer to the table, and a second register supplies an index into the table. The branch length is twice the value returned from the table.

T1

15141312111098765432101514131211109876543210
111010001101Rn(1)(1)(1)(1)(0)(0)(0)(0)000HRm

Byte (H == 0)

TBB{<c>}{<q>} [<Rn>, <Rm>] // (Outside or last in IT block)

Halfword (H == 1)

TBH{<c>}{<q>} [<Rn>, <Rm>, LSL #1] // (Outside or last in IT block) 

n = UInt(Rn);  m = UInt(Rm);  is_tbh = (H == '1');
if m == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13
if InITBlock() && !LastInITBlock() then UNPREDICTABLE;

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rn>

Is the general-purpose base register holding the address of the table of branch lengths, encoded in the "Rn" field. The PC can be used. If it is, the table immediately follows this instruction.

<Rm>

For the byte variant: is the general-purpose index register, encoded in the "Rm" field. This register contains an integer pointing to a single byte in the table. The offset in the table is the value of the index.

For the halfword variant: is the general-purpose index register, encoded in the "Rm" field. This register contains an integer pointing to a halfword in the table. The offset in the table is twice the value of the index.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    if is_tbh then
        halfwords = UInt(MemU[R[n]+LSL(R[m],1), 2]);
    else
        halfwords = UInt(MemU[R[n]+R[m], 1]);
    BranchWritePC(PC + 2*halfwords, BranchType_INDIR);
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