Datasheet

135
11011B–ATARM–21-Feb-12
SAM3N
10.17.4 TBB and TBH
Table Branch Byte and Table Branch Halfword.
10.17.4.1 Syntax
TBB [Rn, Rm]
TBH [Rn, Rm, LSL #1]
where:
Rn is the register containing the address of the table of branch lengths. If Rn is PC,
then the address of the table is the address of the byte immediately following the TBB or TBH
instruction.
Rm is the index register. This contains an index into the table. For halfword tables,
LSL #1 doubles the value in Rm to form the right offset into the table.
10.17.4.2 Operation
These instructions cause a PC-relative forward branch using a table of single byte offsets for
TBB, or halfword offsets for TBH. Rn provides a pointer to the table, and Rm supplies an index
into the table. For TBB the branch offset is twice the unsigned value of the byte returned from
the table. and for TBH the branch offset is twice the unsigned value of the halfword returned
from the table. The branch occurs to the address at that offset from the address of the byte
immediately after the TBB or TBH instruction.
10.17.4.3 Restrictions
The restrictions are:
• Rn must not be SP
• Rm must not be SP and must not be PC
• when any of these instructions is used inside an IT block, it must be the last instruction of the
IT block.
10.17.4.4 Condition flags
These instructions do not change the flags.