ARM Cortex-M4 - Saturating Instructions

The Cortex-M4 Instruction Set
ARM DUI 0553A Copyright © 2010 ARM. All rights reserved. 3-95
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3.7 Saturating instructions
Table 3-10 shows the saturating instructions:
For signed n-bit saturation, this means that:
• if the value to be saturated is less than −2
n−1
, the result returned is −2
n-1
• if the value to be saturated is greater than 2
n−1
−1, the result returned is 2
n-1
−1
• otherwise, the result returned is the same as the value to be saturated.
For unsigned n-bit saturation, this means that:
• if the value to be saturated is less than 0, the result returned is 0
• if the value to be saturated is greater than 2
n
−1, the result returned is 2
n
−1
• otherwise, the result returned is the same as the value to be saturated.
If the returned result is different from the value to be saturated, it is called saturation. If
saturation occurs, the instruction sets the Q flag to 1 in the APSR. Otherwise, it leaves the Q flag
unchanged. To clear the Q flag to 0, you must use the
MSR instruction, see MSR on page 3-164.
To read the state of the Q flag, use the
MRS instruction, see MRS on page 3-163.
Table 3-10 Saturating instructions
Mnemonic Brief description See
SSAT Signed Saturate SSAT and USAT on page 3-96
SSAT16 Signed Saturate Halfword SSAT16 and USAT16 on page 3-97
USAT Unsigned Saturate SSAT and USAT on page 3-96
USAT16 Unsigned Saturate Halfword SSAT16 and USAT16 on page 3-97
QADD Saturating Add QADD and QSUB on page 3-98
QSUB Saturating Subtract QADD and QSUB on page 3-98
QSUB16 Saturating Subtract 16 QADD and QSUB on page 3-98
QASX Saturating Add and Subtract with Exchange QASX and QSAX on page 3-100
QSAX Saturating Subtract and Add with Exchange QASX and QSAX on page 3-100
QDADD Saturating Double and Add QDADD and QDSUB on page 3-102
QDSUB Saturating Double and Subtract QDADD and QDSUB on page 3-102
UQADD16 Unsigned Saturating Add 16 UQADD and UQSUB on page 3-105
UQADD8 Unsigned Saturating Add 8 UQADD and UQSUB on page 3-105
UQASX Unsigned Saturating Add and Subtract with Exchange UQASX and UQSAX on page 3-103
UQSAX Unsigned Saturating Subtract and Add with Exchange UQASX and UQSAX on page 3-103
UQSUB16 Unsigned Saturating Subtract 16 UQADD and UQSUB on page 3-105
UQSUB8 Unsigned Saturating Subtract 8 UQADD and UQSUB on page 3-105