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BFC

Bit Field Clear clears any number of adjacent bits at any position in a register, without affecting the other bits in the register.

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

A1

313029282726252423222120191817161514131211109876543210
!= 11110111110msbRdlsb0011111
cond

A1

BFC{<c>}{<q>} <Rd>, #<lsb>, #<width>

d = UInt(Rd);  msbit = UInt(msb);  lsbit = UInt(lsb);
if d == 15 then UNPREDICTABLE;

T1

15141312111098765432101514131211109876543210
11110(0)11011011110imm3Rdimm2(0)msb

T1

BFC{<c>}{<q>} <Rd>, #<lsb>, #<width>

d = UInt(Rd);  msbit = UInt(msb);  lsbit = UInt(imm3:imm2);
if d == 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.

<lsb>

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

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

<width>

Is the number of bits to be cleared, in the range 1 to 32-<lsb>, encoded in the "msb" field as <lsb>+<width>-1.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    if msbit >= lsbit then
        R[d]<msbit:lsbit> = Replicate('0', msbit-lsbit+1);
        // Other bits of R[d] are unchanged
    else
        UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If msbit < lsbit, 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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