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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
15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
1 | 1 | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 1 | 0 | 1 | Rn | (1) | (1) | (1) | (1) | (0) | (0) | (0) | (0) | 0 | 0 | 0 | H | Rm |
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> |
<q> |
<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. |
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);