ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Shift and rotate operations
The following types of shift and rotate operations are used in instructions:
Logical Shift Left
Logical Shift Right
Arithmetic Shift Right
Application Level Programmers’ Model
(LSL) moves each bit of a bitstring left by a specified number of bits. Zeros are shifted in at
the right end of the bitstring. Bits that are shifted off the left end of the bitstring are
discarded, except that the last such bit can be produced as a carry output.
(LSR) moves each bit of a bitstring right by a specified number of bits. Zeros are shifted in
at the left end of the bitstring. Bits that are shifted off the right end of the bitstring are
discarded, except that the last such bit can be produced as a carry output.
(ASR) moves each bit of a bitstring right by a specified number of bits. Copies of the leftmost
bit are shifted in at the left end of the bitstring. Bits that are shifted off the right end of the
bitstring are discarded, except that the last such bit can be produced as a carry output.
Rotate Right (ROR) moves each bit of a bitstring right by a specified number of bits. Each bit that is shifted
off the right end of the bitstring is re-introduced at the left end. The last bit shifted off the
right end of the bitstring can be produced as a carry output.
Rotate Right with Extend
(RRX) moves each bit of a bitstring right by one bit. The carry input is shifted in at the left
end of the bitstring. The bit shifted off the right end of the bitstring can be produced as a
carry output.
Pseudocode details of shift and rotate operations
These shift and rotate operations are supported in pseudocode by the following functions:
// LSL_C()
// =======
(bits(N), bit) LSL_C(bits(N) x, integer shift)
assert shift > 0;
extended_x = x : Zeros(shift);
result = extended_x;
carry_out = extended_x;
return (result, carry_out);
// LSL()
// =====
bits(N) LSL(bits(N) x, integer shift)
assert shift >= 0;
if shift == 0 then
result = x;
else
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