Datasheet

91
SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
10.11.7 Conditional execution
Most data processing instructions can optionally update the condition flags in the Application Program Status
Register (APSR) according to the result of the operation, see “Application Program Status Register” on page 54.
Some instructions update all flags, and some only update a subset. If a flag is not updated, the original value is
preserved. See the instruction descriptions for the flags they affect.
You can execute an instruction conditionally, based on the condition flags set in another instruction, either:
 immediately after the instruction that updated the flags
 after any number of intervening instructions that have not updated the flags.
Conditional execution is available by using conditional branches or by adding condition code suffixes to
instructions. See Table 10-16 on page 92 for a list of the suffixes to add to instructions to make them conditional
instructions. The condition code suffix enables the processor to test a condition based on the flags. If the condition
test of a conditional instruction fails, the instruction:
 does not execute
 does not write any value to its destination register
 does not affect any of the flags
 does not generate any exception.
Conditional instructions, except for conditional branches, must be inside an If-Then instruction block. See “IT” on
page 137 for more information and restrictions when using the IT instruction. Depending on the vendor, the
assembler might automatically insert an IT instruction if you have conditional instructions outside the IT block.
Use the CBZ and CBNZ instructions to compare the value of a register against zero and branch on the result.
This section describes:
 “The condition flags”
 “Condition code suffixes” .
10.11.7.1 The condition flags
The APSR contains the following condition flags:
N Set to 1 when the result of the operation was negative, cleared to 0 otherwise.
Z Set to 1 when the result of the operation was zero, cleared to 0 otherwise.
C Set to 1 when the operation resulted in a carry, cleared to 0 otherwise.
V Set to 1 when the operation caused overflow, cleared to 0 otherwise.
For more information about the APSR see “Program Status Register” on page 52.
A carry occurs:
 if the result of an addition is greater than or equal to 2
32
 if the result of a subtraction is positive or zero
 as the result of an inline barrel shifter operation in a move or logical instruction.
Overflow occurs if the result of an add, subtract, or compare is greater than or equal to 2
31
, or less than –2
31
.
Most instructions update the status flags only if the S suffix is specified. See the instruction descriptions for more
information.