UBFM

Unsigned 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 and above the bitfield are set to zero.

Encoding: With zeros to left and right

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
	1	0	1	0	0	1	1	0
sf	opc								N	immr						imms						Rn					Rd

32-bit (sf == 0 && N == 0)

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

64-bit (sf == 1 && N == 1)

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

Decoding algorithm

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);

Operation

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

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

// Combine extension bits and result bits
X[d, datasize] = bot AND tmask;

Explanations

<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.
<immr>: 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.
<imms>: 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.

Operational Notes

If PSTATE.DIT is 1:

• 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.