Cortex-M3 Devices Generic User Guide: 3.7.1. SSAT and USAT

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.

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 ≤ 2ⁿ−1.

For signed n-bit saturation using SSAT, this means that:

if the value to be saturated is less than −2ⁿ⁻¹, the result returned is −2^n-1
if the value to be saturated is greater than 2ⁿ⁻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 using USAT, 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 2ⁿ−1, the result returned is 2ⁿ−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.

To read the state of the Q flag, use the MRS instruction, see MRS.

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.

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