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PM0056

The Cortex-M3 instruction set

3.7

Saturating instructions

This section describes the saturating instructions, 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 the following:

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 57)

•

‘Rd’ is 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’ is 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:

-2n–1

≤ x ≤ 2n–1-1.

The USAT instruction applies the specified shift, then saturates to the unsigned range:

≤ 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 -2n-1

•

If the value to be saturated is greater than 2n-1-1, the result returned is 2n-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 on

page 101.

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