SBFM

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

32-bit encoding

(sf == 0 && N == 0)

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

64-bit encoding

(sf == 1 && N == 1)

SBFM <Xd>, <Xn>, #<immr>, #<imms>

Decode (all encodings)

if sf == '1' && N != '1' then EndOfDecode(Decode_UNDEF); if sf == '0' && (N != '0' || immr<5> != '0' || imms<5> != '0') then EndOfDecode(Decode_UNDEF); constant integer d = UInt(Rd); constant integer n = UInt(Rn); constant integer datasize = 32 << UInt(sf); constant integer s = UInt(imms); constant integer r = UInt(immr); bits(datasize) wmask; bits(datasize) tmask; (wmask, tmask) = DecodeBitMasks(N, imms, immr, FALSE, datasize);

Assembler Symbols

<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.

<immr>	For the "32-bit" variant: is the right rotate amount, in the range 0 to 31, encoded in the "immr" field.
	For the "64-bit" variant: is the right rotate amount, in the range 0 to 63, encoded in the "immr" field.

<imms>	For the "32-bit" variant: is the leftmost bit number to be moved from the source, in the range 0 to 31, encoded in the "imms" field.
	For the "64-bit" variant: is the leftmost bit number to be moved from the source, in the range 0 to 63, encoded in the "imms" 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 Conditions

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'

Operation

constant bits(datasize) src = X[n, datasize]; // Perform bitfield move on low bits constant bits(datasize) bot = ROR(src, r) AND wmask; constant bits(datasize) top = Replicate(src<s>, datasize); // Combine extension bits and result bits X[d, datasize] = (top AND NOT(tmask)) OR (bot AND tmask);

Operational information

This instruction is a data-independent-time instruction as described in About PSTATE.DIT.

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