ARM Cortex-M4 - SSAT and USAT

The Cortex-M4 Instruction Set
ARM DUI 0553A Copyright © 2010 ARM. All rights reserved. 3-96
ID121610 Non-Confidential
3.7.1 SSAT and USAT
Signed Saturate and Unsigned Saturate to any bit position, with optional shift before saturating.
Syntax
op{cond} Rd, #n, Rm {, shift #s}
where:
op Is one of:
SSAT Saturates a signed value to a signed range.
USAT Saturates a signed value to an unsigned range.
cond Is an optional condition code, see Conditional execution on page 3-18.
Rd Specifies the destination register.
n Specifies the bit position to saturate to:
•
n ranges from 1 to 32 for SSAT
• n ranges from 0 to 31 for USAT .
Rm Specifies the register containing the value to saturate.
shift #s Is an optional shift applied to Rm before saturating. It must be one of the following:
ASR #s where s is in the range 1 to 31
LSL #s where s is in the range 0 to 31.
Operation
These instructions saturate to a signed or unsigned
n -bit value.
The
SSAT instruction applies the specified shift, then saturates to the signed range
−2
n–1
≤ x ≤ 2
n–1
−1.
The
USAT instruction applies the specified shift, then saturates to the unsigned range
0 ≤ x ≤ 2
n
−1.
Restrictions
Do not use SP and do not use PC
.
Condition flags
These instructions do not affect the condition code flags.
If saturation occurs, these instructions set the Q flag to 1.
Examples
SSAT R7, #16, R7, LSL #4 ; Logical shift left value in R7 by 4, then
; saturate it as a signed 16-bit value and
; write it back to R7
USATNE R0, #7, R5 ; Conditionally saturate value in R5 as an
; unsigned 7 bit value and write it to R0.