sbfm

Signed Bitfield Move

SBFM <Wd>, <Wn>, #<immr>, #<imms>

Extracts/Inserts bitfield with sign extension.

Details

Extracts a bitfield from Wn with sign extension and places it in Wd. The bitfield is defined by immr (rotate right amount) and imms (field size), with the field width determined by the difference between imms and immr (plus 1). The extracted value is sign-extended to fill the 32-bit destination. No condition flags are affected. This is an AArch64 instruction.

Pseudocode Operation

width ← imms - immr + 1
if imms >= immr then
  extracted ← ROR(Wn, immr)[width-1:0]
  Wd ← SignExtend(extracted, width)
else
  extracted ← ROR(Wn, immr)[width-1:0]
  Wd ← SignExtend(extracted, width)
N ← unchanged
Z ← unchanged
C ← unchanged
V ← unchanged

Example

SBFM w0, w1, #immr, #imms

Encoding

Binary Layout
0
00
100110
0
immr
imms
Rn
Rd
 
Format Bitfield
Opcode 0x13000000
Extension Base

Operands

  • Wd
    Destination 32-bit integer register
  • Wn
    First source / base 32-bit integer register
  • immr
    Rotate
  • imms
    Size

Reference (Arm A64 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0x13000000 SBFM <Wd>, <Wn>, #<immr>, #<imms> A64 0 | 00 | 100110 | 0 | immr | imms | Rn | Rd
0x93400000 SBFM <Xd>, <Xn>, #<immr>, #<imms> A64 1 | 00 | 100110 | 1 | immr | imms | Rn | Rd

Description

Signed Bitfield Move is usually accessed via one of its aliases, which are always preferred for disassembly. If <imms> is greater than or equal to <immr>, this copies a bitfield of (<imms>-<immr>+1) bits starting from bit position <immr> in the source register to the least significant bits of the destination register. If <imms> is less than <immr>, this copies a bitfield of (<imms>+1) bits from the least significant bits of the source register to bit position (regsize-<immr>) of the destination register, where regsize is the destination register size of 32 or 64 bits. In both cases the destination bits below the bitfield are set to zero, and the bits above the bitfield are set to a copy of the most significant bit of the bitfield.

Operation

bits(datasize) src = X[n, datasize];

// perform bitfield move on low bits
bits(datasize) bot = ROR(src, r) AND wmask;

// determine extension bits (sign, zero or dest register)
bits(datasize) top = Replicate(src<s>, datasize);

// combine extension bits and result bits
X[d, datasize] = (top AND NOT(tmask)) OR (bot AND tmask);