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SUB (immediate, from PC)

Subtract from PC subtracts an immediate value from the Align(PC, 4) value to form a PC-relative address, and writes the result to the destination register. Arm recommends that, where possible, software avoids using this alias.

This is an alias of ADR. This means:

  • The encodings in this description are named to match the encodings of ADR.
  • The description of ADR gives the operational pseudocode for this instruction.

It has encodings from the following instruction sets: A32 ( A2 ) and T32 ( T2 ) .

A2

313029282726252423222120191817161514131211109876543210
!= 1111001001001111Rdimm12
cond

A2

SUB{<c>}{<q>} <Rd>, PC, #<const>

is equivalent to

ADR{<c>}{<q>} <Rd>, <label>

and is the preferred disassembly when imm12 == '000000000000'.

T2

15141312111098765432101514131211109876543210
11110i10101011110imm3Rdimm8

T2

SUB{<c>}{<q>} <Rd>, PC, #<imm12>

is equivalent to

ADR{<c>}{<q>} <Rd>, <label>

and is the preferred disassembly when i:imm3:imm8 == '000000000000'.

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rd>

For encoding A2: is the general-purpose destination register, encoded in the "Rd" field. If the PC is used, the instruction is a branch to the address calculated by the operation. This is an interworking branch, see Pseudocode description of operations on the AArch32 general-purpose registers and the PC.

For encoding T2: is the general-purpose destination register, encoded in the "Rd" field.

<label>

For encoding A2: the label of an instruction or literal data item whose address is to be loaded into <Rd>. The assembler calculates the required value of the offset from the Align(PC, 4) value of the ADR instruction to this label.

If the offset is zero or positive, encoding A1 is used, with imm32 equal to the offset.

If the offset is negative, encoding A2 is used, with imm32 equal to the size of the offset. That is, the use of encoding A2 indicates that the required offset is minus the value of imm32.

Permitted values of the size of the offset are any of the constants described in Modified immediate constants in A32 instructions.

For encoding T2: the label of an instruction or literal data item whose address is to be loaded into <Rd>. The assembler calculates the required value of the offset from the Align(PC, 4) value of the ADR instruction to this label.

If the offset is zero or positive, encoding T3 is used, with imm32 equal to the offset.

If the offset is negative, encoding T2 is used, with imm32 equal to the size of the offset. That is, the use of encoding T2 indicates that the required offset is minus the value of imm32.

Permitted values of the size of the offset are 0-4095.

<imm12>

Is a 12-bit unsigned immediate, in the range 0 to 4095, encoded in the "i:imm3:imm8" field.

<const>

An immediate value. See Modified immediate constants in A32 instructions for the range of values.

Operation

The description of ADR gives the operational pseudocode for this instruction.

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