qdsub

Saturating Double and Subtract

QDSUB<c> <Rd>, <Rm>, <Rn>

Doubles the second operand, subtracts from first, and saturates.

Details

Saturates the doubling of Rm, then subtracts the result from Rn with saturation, writing the saturated difference to Rd. The Q flag is set if saturation occurred during either the doubling or subtraction; other flags are unaffected. A32-only instruction requiring the Saturating Arithmetic extension; must be executed in privileged mode for certain CPSR modifications.

Pseudocode Operation

doubled ← SignedSat(Rm << 1, 32)
result ← SignedSat(Rn - doubled, 32)
Rd ← result
if (overflow during doubling or subtraction) then Q ← 1

Example

QDSUB r0, r2, r1

Encoding

Binary Layout
cond
00010
11
0
Rn
Rd
0
0
0
0
0101
Rm
 
Format Data Proc
Opcode 0x01600050
Extension A32 (Sat)

Operands

  • Rd
    Destination general-purpose register
  • Rm
    Second source / offset general-purpose register
  • Rn
    First source / base general-purpose register

Reference (Arm AArch32 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0x01600050 QDSUB{<c>}{<q>} {<Rd>,} <Rm>, <Rn> A32 cond | 00010 | 11 | 0 | Rn | Rd | 0 | 0 | 0 | 0 | 0101 | Rm
0xFA80F0B0 QDSUB{<c>}{<q>} {<Rd>,} <Rm>, <Rn> T32 111110101 | 000 | Rn | 1111 | Rd | 10 | 11 | Rm

Description

Saturating Double and Subtract subtracts a doubled register value from another register value, and writes the result to the destination register. Both the doubling and the subtraction have their results saturated to the 32-bit signed integer range -231 <= x <= 231 - 1. If saturation occurs in either operation, it sets PSTATE.Q to 1.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    (doubled, sat1) = SignedSatQ(2 * SInt(R[n]), 32);
    boolean sat2;
    (R[d], sat2)  = SignedSatQ(SInt(R[m]) - SInt(doubled), 32);
    if sat1 || sat2 then
        PSTATE.Q = '1';