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UQSUB8

Unsigned Saturating Subtract 8 performs four unsigned 8-bit integer subtractions, saturates the results to the 8-bit unsigned integer range 0 <= x <= 28 - 1, and writes the results to the destination register.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 111101100110RnRd(1)(1)(1)(1)1111Rm
cond

A1

UQSUB8{<c>}{<q>} {<Rd>,} <Rn>, <Rm>

d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);
if d == 15 || n == 15 || m == 15 then UNPREDICTABLE;

T1

15141312111098765432101514131211109876543210
111110101100Rn1111Rd0101Rm

T1

UQSUB8{<c>}{<q>} {<Rd>,} <Rn>, <Rm>

d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);
if d == 15 || n == 15 || m == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rd>

Is the general-purpose destination register, encoded in the "Rd" field.

<Rn>

Is the first general-purpose source register, encoded in the "Rn" field.

<Rm>

Is the second general-purpose source register, encoded in the "Rm" field.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    diff1 = UInt(R[n]<7:0>) - UInt(R[m]<7:0>);
    diff2 = UInt(R[n]<15:8>) - UInt(R[m]<15:8>);
    diff3 = UInt(R[n]<23:16>) - UInt(R[m]<23:16>);
    diff4 = UInt(R[n]<31:24>) - UInt(R[m]<31:24>);
    R[d]<7:0>   = UnsignedSat(diff1, 8);
    R[d]<15:8>  = UnsignedSat(diff2, 8);
    R[d]<23:16> = UnsignedSat(diff3, 8);
    R[d]<31:24> = UnsignedSat(diff4, 8);
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