ARM Cortex-M4 - Lsr; Lsl

The Cortex-M4 Instruction Set
ARM DUI 0553A Copyright © 2010 ARM. All rights reserved. 3-14
ID121610 Non-Confidential
You can use the ASR #n operation to divide the value in the register Rm by 2
n
, with the result being
rounded towards negative-infinity.
When the instruction is
ASRS or when ASR #n is used in Operand2 with the instructions MOVS , MVNS ,
ANDS , ORRS , ORNS , EORS , BICS , TEQ or TST , the carry flag is updated to the last bit shifted out, bit[ n -1],
of the register
Rm .
Note
• If n is 32 or more, then all the bits in the result are set to the value of bit[31] of Rm .
• If
n is 32 or more and the carry flag is updated, it is updated to the value of bit[31] of Rm .
Figure 3-1 ASR #3
LSR
Logical shift right by
n bits moves the left-hand 32 - n bits of the register Rm , to the right by n
places, into the right-hand
32 - n bits of the result. And it sets the left-hand n bits of the result to
0. See Figure 3-2.
You can use the
LSR #n operation to divide the value in the register Rm by 2
n
, if the value is
regarded as an unsigned integer.
When the instruction is
LSRS or when LSR #n is used in Operand2 with the instructions MOVS , MVNS ,
ANDS , ORRS , ORNS , EORS , BICS , TEQ or TST , the carry flag is updated to the last bit shifted out, bit[ n -1],
of the register
Rm .
Note
• If n is 32 or more, then all the bits in the result are cleared to 0.
• If
n is 33 or more and the carry flag is updated, it is updated to 0.
Figure 3-2 LSR #3
LSL
Logical shift left by
n bits moves the right-hand 32 - n bits of the register Rm , to the left by n places,
into the left-hand
32 - n bits of the result. And it sets the right-hand n bits of the result to 0. See
Figure 3-3 on page 3-15.
Carry
Flag
031 5 4 3 2 1
Carry
Flag
031 5 4 3 2 1
00
0