Datasheet

SAM4E [DATASHEET]
Atmel-11157D-ATARM-SAM4E16-SAM4E8-Datasheet_12-Jun-14
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The Cortex-M4 includes an exclusive access monitor, that tags the fact that the processor has executed a Load-
Exclusive instruction. If the processor is part of a multiprocessor system, the system also globally tags the memory
locations addressed by exclusive accesses by each processor.
The processor removes its exclusive access tag if:
 It executes a CLREX instruction
 It executes a Store-Exclusive instruction, regardless of whether the write succeeds.
 An exception occurs. This means that the processor can resolve semaphore conflicts between different
threads.
In a multiprocessor implementation:
 Executing a CLREX instruction removes only the local exclusive access tag for the processor
 Executing a Store-Exclusive instruction, or an exception, removes the local exclusive access tags, and all
global exclusive access tags for the processor.
For more information about the synchronization primitive instructions, see “LDREX and STREX” and “CLREX” .
12.4.2.8 Programming Hints for the Synchronization Primitives
ISO/IEC C cannot directly generate the exclusive access instructions. CMSIS provides intrinsic functions for
generation of these instructions:
The actual exclusive access instruction generated depends on the data type of the pointer passed to the intrinsic
function. For example, the following C code generates the required LDREXB operation:
__ldrex((volatile char *) 0xFF);
12.4.3 Exception Model
This section describes the exception model.
12.4.3.1 Exception States
Each exception is in one of the following states:
Inactive
The exception is not active and not pending.
Pending
The exception is waiting to be serviced by the processor.
An interrupt request from a peripheral or from software can change the state of the corresponding interrupt to
pending.
Table 12-8. CMSIS Functions for Exclusive Access Instructions
Instruction CMSIS Function
LDREX uint32_t __LDREXW (uint32_t *addr)
LDREXH uint16_t __LDREXH (uint16_t *addr)
LDREXB uint8_t __LDREXB (uint8_t *addr)
STREX uint32_t __STREXW (uint32_t value, uint32_t *addr)
STREXH uint32_t __STREXH (uint16_t value, uint16_t *addr)
STREXB uint32_t __STREXB (uint8_t value, uint8_t *addr)
CLREX void __CLREX (void)