Logical & bitwise instructions#

Document conventions#

Notation: Rd, Rn denote ARM registers R0-R7 except in the case of the special instructions where R0-R15 may be used. Rn<a-b> denotes an ARM register whose contents must lie in range a <= contents <= b. In the case of instructions with two register arguments, it is permissible for them to be identical. For example the following will zero R0 (Python R0 ^= R0) regardless of its initial contents.

• eor(r0, r0)

These instructions affect the condition flags except where stated.

Logical instructions#

• and_(Rd, Rn) Rd &= Rn
• orr(Rd, Rn) Rd |= Rn
• eor(Rd, Rn) Rd ^= Rn
• mvn(Rd, Rn) Rd = Rn ^ 0xffffffff i.e. Rd = 1's complement of Rn
• bic(Rd, Rn) Rd &= ~Rn bit clear Rd using mask in Rn

Note the use of "and_" instead of "and", because "and" is a reserved keyword in Python.

Shift and rotation instructions#

• lsl(Rd, Rn\<0-31>) Rd <<= Rn
• lsr(Rd, Rn\<1-32>) Rd = (Rd & 0xffffffff) >> Rn Logical shift right
• asr(Rd, Rn\<1-32>) Rd >>= Rn arithmetic shift right
• ror(Rd, Rn\<1-31>) Rd = rotate_right(Rd, Rn) Rd is rotated right Rn bits.

A rotation by (for example) three bits works as follows. If Rd initially contains bits b31 b30..b0 after rotation it will contain b2 b1 b0 b31 b30..b3

Special instructions#

Condition codes are unaffected by these instructions.

• clz(Rd, Rn) Rd = count_leading_zeros(Rn)

count_leading_zeros(Rn) returns the number of binary zero bits before the first binary one bit in Rn.

• rbit(Rd, Rn) Rd = bit_reverse(Rn)

bit_reverse(Rn) returns the bit-reversed contents of Rn. If Rn contains bits b31 b30..b0 Rd will be set to b0 b1 b2..b31

Trailing zeros may be counted by performing a bit reverse prior to executing clz.