Signed bitfield move
This instruction 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.
This instruction is used by the aliases ASR (immediate), SBFIZ, SBFX, SXTB, SXTH, and SXTW.
| 31 | 30 | 29 | 28 | 27 | 26 | 25 | 24 | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| sf | 0 | 0 | 1 | 0 | 0 | 1 | 1 | 0 | N | immr | imms | Rn | Rd | ||||||||||||||||||
| opc | |||||||||||||||||||||||||||||||
if sf == '1' && N != '1' then EndOfDecode(Decode_UNDEF); end; if sf == '0' && (N != '0' || immr[5] != '0' || imms[5] != '0') then EndOfDecode(Decode_UNDEF); end; let d : integer{} = UInt(Rd); let n : integer{} = UInt(Rn); let datasize : integer{} = 32 << UInt(sf); let s : integer = UInt(imms); let r : integer = UInt(immr); var wmask : bits(datasize); var tmask : bits(datasize); (wmask, tmask) = DecodeBitMasks{datasize}(N, imms, immr, FALSE);
| <Wd> |
Is the 32-bit name of the general-purpose destination register, encoded in the "Rd" field. |
| <Wn> |
Is the 32-bit name of the general-purpose source register, encoded in the "Rn" field. |
| <Xd> |
Is the 64-bit name of the general-purpose destination register, encoded in the "Rd" field. |
| <Xn> |
Is the 64-bit name of the general-purpose source register, encoded in the "Rn" field. |
| Alias | Of variant | Is preferred when |
|---|---|---|
| ASR (immediate) | 32-bit | imms == '011111' |
| ASR (immediate) | 64-bit | imms == '111111' |
| SBFIZ | UInt(imms) < UInt(immr) | |
| SBFX | BFXPreferred(sf, opc[1], imms, immr) | |
| SXTB | immr == '000000' && imms == '000111' | |
| SXTH | immr == '000000' && imms == '001111' | |
| SXTW | immr == '000000' && imms == '011111' |
let src : bits(datasize) = X{}(n); // Perform bitfield move on low bits let bot : bits(datasize) = ROR(src, r) AND wmask; let top : bits(datasize) = Replicate{}(src[s]); // Combine extension bits and result bits X{datasize}(d) = (top AND NOT(tmask)) OR (bot AND tmask);
This instruction is a data-independent-time instruction as described in About PSTATE.DIT.
2026-03_rel 2026-03-26 20:48:11
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