PM0214 datasheet(125/245 Pages) STMICROELECTRONICS

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PM0214

The STM32 Cortex-M4 instruction set

244

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 64)

•

‘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 0

≤x ≤

2n-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.

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