Atmel Xplained Pro Evaluation Kit ATSAM4E-XPRO ATSAM4E-XPRO Data Sheet
Product codes
ATSAM4E-XPRO
SAM4E [DATASHEET]
Atmel-11157D-ATARM-SAM4E16-SAM4E8-Datasheet_12-Jun-14
178
12.6.10.4 TBB and TBH
Table Branch Byte and Table Branch Halfword.
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.
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.
LSL #1 doubles the value in Rm to form the right offset into the table.
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.
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.
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.
Condition Flags
These instructions do not change the flags.
Examples
ADR.W
R0, BranchTable_Byte
TBB
[R0, R1]
; R1 is the index, R0 is the base address of the
; branch table
Case1
; an instruction sequence follows
Case2
; an instruction sequence follows
Case3
; an instruction sequence follows
BranchTable_Byte
DCB
0
; Case1 offset calculation
DCB
((Case2-Case1)/2)
; Case2 offset calculation
DCB
((Case3-Case1)/2)
; Case3 offset calculation
TBH
[PC, R1, LSL #1]
; R1 is the index, PC is used as base of the
; branch table
BranchTable_H
DCI
((CaseA - BranchTable_H)/2)
; CaseA offset calculation
DCI
((CaseB - BranchTable_H)/2)
; CaseB offset calculation
DCI
((CaseC - BranchTable_H)/2)
; CaseC offset calculation
CaseA