Samsung SC32442B54, Руководство пользователя

Страница: 71 / 647

ARM INSTRUCTION SET SC32442B RISC MICROPROCESSOR
3-12
SHIFTS
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 contain 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.
0
[6:5] Shift type
00 = logical left 01 = logical right
10 = arithmetic right 11 = rotate right
[11:7] Shift amount
5 bit unsigned integer
[6:5] Shift type
00 = logical left 01 = logical right
10 = arithmetic right 11 = rotate right
[11:8] Shift register
Shift amount specified in bottom-byte of Rs
4 5 6 7 11
1
4 5 6 7 11 8
0 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 can 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
Contents of Rm
Value of Operand 2
carry out
0
0
0 0 0 0
Figure 3-6. Logical Shift Left
NOTES
LSL #0 is a special case, where the shifter carries 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 .