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UBFX

Unsigned Bit Field Extract extracts any number of adjacent bits at any position from a register, zero-extends them to 32 bits, and writes the result to the destination register.

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

A1

313029282726252423222120191817161514131211109876543210
!= 11110111111widthm1Rdlsb101Rn
cond

A1

UBFX{<c>}{<q>} <Rd>, <Rn>, #<lsb>, #<width>

d = UInt(Rd);  n = UInt(Rn);
lsbit = UInt(lsb);  widthminus1 = UInt(widthm1);
if d == 15 || n == 15 then UNPREDICTABLE;

T1

15141312111098765432101514131211109876543210
11110(0)111100Rn0imm3Rdimm2(0)widthm1

T1

UBFX{<c>}{<q>} <Rd>, <Rn>, #<lsb>, #<width>

d = UInt(Rd);  n = UInt(Rn);
lsbit = UInt(imm3:imm2);  widthminus1 = UInt(widthm1);
if d == 15 || n == 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 general-purpose source register, encoded in the "Rn" field.

<lsb>

For encoding A1: is the bit number of the least significant bit in the field, in the range 0 to 31, encoded in the "lsb" field.

For encoding T1: is the bit number of the least significant bit in the field, in the range 0 to 31, encoded in the "imm3:imm2" field.

<width>

Is the width of the field, in the range 1 to 32-<lsb>, encoded in the "widthm1" field as <width>-1.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    msbit = lsbit + widthminus1;
    if msbit <= 31 then
        R[d] = ZeroExtend(R[n]<msbit:lsbit>, 32);
    else
        UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If msbit > 31, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The value in the destination register is unknown.

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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