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CS 220 / ARM / ARM1176JZ-S Technical Reference Mmanual.pdf
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Programmer’s Model

ClearExclusiveLocal(processor_id)

2.11.4CLREX

Figure 2-17 shows the format of the Clear Exclusive, CLREX, instruction.

31

28 27

21 20 19

 

16 15

12 11

8

7

4

3

0

1111

 

0 1 0 1

0 1 1

1

 

SBO

 

 

SBO

 

SBZ

0

0 0 1

 

SBO

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2-17 CLREX instruction

The dummy STREX construct specified in ARMv6 is required for correct system behavior. The CLREX instruction replaces the dummy STREX instruction.

This operation in unconditional in the ARM instruction set.

Syntax

CLREX

Operation

ClearExclusiveLocal(processor_id)

2.11.5NOP-compatible hints

Figure 2-18 shows the format of the NOP-compatible hint instruction.

31

28 27

23 22 21 20 19

16 15

 

12 11

8

7

0

Cond

 

0 0 1 1

0

0

1 0

0

0 0 0

 

SBO

 

0

0 0 0

 

Hint

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2-18 NOP-compatible hint instruction

Syntax

<cond> Is the condition when the instruction executes. It produces no useful change in functionality, but is provided to ensure disassembly followed by reassembly always regenerates the original code.

<hint> defaults to zero

hint == 0x0: the instruction is NOP hint == 0x1: the instruction is YIELD

For all other values, RESERVED, the instruction behaves like NOP.

The true NOP for ARM state is equivalent to an MSR to the CPSR with the immed_value redefined as the hint field and no bytes selected. The instruction is fully architecturally defined, with all encodings assigned.

Note

True NOPs are architected for alignment reasons and do not have any timing guarantees with respect to their neighboring instructions.

ARM DDI 0333H

Copyright © 2004-2009 ARM Limited. All rights reserved.

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Programmer’s Model

In an Symmetric Multi-Threading (SMT) design, a yield instruction enables a thread to generate a hint to the processor that runs it. The hint indicates that the current activity of the thread is not important, for example sitting in a spin-lock, and so can yield. On a uniprocessor system, this instruction behaves as a NOP. OSs can use the yielding NOP in those places that require the yield hint, and the non-yielding NOP in other cases.

Operation

The instruction acts as a NOP irrespective of whether the condition passes or fails, effectively the ALWAYS condition. Do not use RESERVED values in software.

ARM DDI 0333H

Copyright © 2004-2009 ARM Limited. All rights reserved.

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