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UXTB

Unsigned Extend Byte extracts an 8-bit value from a register, zero-extends it to 32 bits, and writes the result to the destination register. The instruction can specify a rotation by 0, 8, 16, or 24 bits before extracting the 8-bit value.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 and T2 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 1111011011101111Rdrotate(0)(0)0111Rm
cond

A1

UXTB{<c>}{<q>} {<Rd>,} <Rm> {, ROR #<amount>}

d = UInt(Rd);  m = UInt(Rm);  rotation = UInt(rotate:'000');
if d == 15 || m == 15 then UNPREDICTABLE;

T1

1514131211109876543210
1011001011RmRd

T1

UXTB{<c>}{<q>} {<Rd>,} <Rm>

d = UInt(Rd);  m = UInt(Rm);  rotation = 0;

T2

15141312111098765432101514131211109876543210
11111010010111111111Rd1(0)rotateRm

T2

UXTB{<c>}.W {<Rd>,} <Rm> // (<Rd>, <Rm> can be represented in T1)

UXTB{<c>}{<q>} {<Rd>,} <Rm> {, ROR #<amount>}

d = UInt(Rd);  m = UInt(Rm);  rotation = UInt(rotate:'000');
if d == 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.

<Rm>

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

<amount> Is the rotate amount, encoded in rotate:
rotate <amount>
00 (omitted)
01 8
10 16
11 24

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    rotated = ROR(R[m], rotation);
    R[d] = ZeroExtend(rotated<7:0>, 32);

Operational information

If CPSR.DIT is 1, this instruction has passed its condition execution check, and does not use R15 as either its source or destination:

  • The execution time of this instruction is independent of:
    • The values of the data supplied in any of its registers.
    • The values of the NZCV flags.
  • The response of this instruction to asynchronous exceptions does not vary based on:
    • The values of the data supplied in any of its registers.
    • The values of the NZCV flags.
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