ARM Software reference manual

Chapter 2
The following four shift-types operate on the Rm register, shifting the
contents by N bits, where register Rsss holds the value of N:
Rsss0001 logical left by Rs
Rsss0011 logical right by Rs
Rsss0101 arithmetic right by Rs
Rsss0111 rotate right by Rs
Shifts specified by Rs require one additional execution cycle. Only the least
significant byte of Rs is used, and signifies a shift of 0 to +255 bits. If
shifting operations are not required, the bit-pattern of the shift field is set to
00000000. This is decoded as logical shift left 0 bits, and the carry bit is not
changed by this action.
The shifting action is as follows, where n represents the number of bits
shifted:
Logical shift left
C C
The n most significant bits become zero, the carry becomes b(n-1). If the
shift amount is zero, no shift is performed and the carry flag is not altered.
If the shift amount lies in the range 1 to 32, the carry flag is set to b(n-1).
If the shift amount is greater than 32, the carry flag is set to zero.
Arithmetic shift right
- - -> xbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb - -> C
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