INSTRUCTION SET
SHIFTS
S3C4510B
When the second operand is specified to be a shifted register, the operation of the barrel shifter is controlled by
the shift field in the instruction. This field indicates the type of shift to be performed (logical left or right, arithmetic
right or rotate right). The amount by which the register should be shifted may be contained in an immediate field
in the instruction, or in the bottom byte of another register (other than R15). The encoding for the different shift
types is shown in Figure 3-5.
11
7
6
5
4
11
8
7
6
5
4
0
RS
0
1
[6:5] Shift Type
00 = logical left
10 = arithmetic right
[6:5] Shift Type
00 = logical left
10 = arithmetic right
01 = logical right
11 = rotate right
01 = logical right
11 = rotate right
[11:7] Shift Amount
5 bit unsigned integer
[11:8] Shift Register
Shift amount specified in bottom-byte of Rs
Figure 3-5. ARM Shift Operations
Instruction Specified Shift Amount
When the shift amount is specified in the instruction, it is contained in a 5 bit field which may take any value from
0 to 31. A logical shift left (LSL) takes the contents of Rm and moves each bit by the specified amount to a more
significant position. The least significant bits of the result are filled with zeros, and the high bits of Rm which do
not map into the result are discarded, except that the least significant discarded bit becomes the shifter carry
output which may be latched into the C bit of the CPSR when the ALU operation is in the logical class (see
above). For example, the effect of LSL #5 is shown in Figure 3-6.
31
27 26
0
Contents of Rm
carry out
Value of Operand 2
0
0
0
0
0
Figure 3-6. Logical Shift Left
NOTE
LSL #0 is a special case, where the shifter carry out is the old value of the CPSR C flag. The contents of
Rm are used directly as the second operand. A logical shift right (LSR) is similar, but the contents of Rm
are moved to less significant positions in the result. LSR #5 has the effect shown in Figure 3-7.
3-12
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