rbit

Reverse Bits (Thumb)

RBIT <Rd>, <Rm>

Reverses bits in a 32-bit register.

Pseudocode Operation

for i = 0 to 31
  Rd[i] ← Rm[31-i]

Example

RBIT r0, r2

Encoding

Binary Layout

111110101

31:23

001

22:20

Rn

19:16

1111

15:12

Rd

11:8

10

7:6

10

5:4

Rm

3:0

 

Format Thumb Misc

Opcode 0xFA90F0A0

Extension A32 (Base)

Operands

• Rd
  Destination general-purpose register
• Rm
  Second source / offset general-purpose register

Related

Other forms of rbit

• rbit SVE Reverse Bits
• rbit Reverse Bits
• rbit Reverse Bits (64-bit)
• rbit Reverse Bits (A32)

Across architectures

Count Trailing Zeros : how x86, ARM, RISC-V, and PowerISA each do this.

More in A32 (Base)

Reference

View in Arm A64 ISA Reference ↗ Arm AArch32 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0x06FF0F30	RBIT{<c>}{<q>} <Rd>, <Rm>		A32		cond | 01101 | 1 | 11 | 1 | 1 | 1 | 1 | Rd | 1 | 1 | 1 | 1 | 0 | 011 | Rm
0xFA90F0A0	RBIT{<c>}{<q>} <Rd>, <Rm>		T32		111110101 | 001 | Rn | 1111 | Rd | 10 | 10 | Rm

Description

Reverse Bits reverses the bit order in a 32-bit register.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    bits(32) result;
    for i = 0 to 31
        result<31-i> = R[m]<i>;
    R[d] = result;