In a typical embedded application with an OS, the OS kernel uses the MSP and the application processes use the PSP.
STACKS ON MAIN PSP
The PSP is usually only required when an OS is used in the embedded application.
In many simple applications, only one stack pointer is needed and by default the MSP is used.
Depending on the processor state and the CONTROL register value, the stack pointer accessed can either be the MSP or the PSP. In programming, the stack pointer can be accessed as either R13 or SP in the program codes. For this reason, bit of both stack pointers in the Cortex-M processors are hardwired to zeros and read as zeros. The stack memory accesses in the Cortex-M processors are designed to be always word aligned (address values must be a multiple of 4, for example, 0x0, 0x4, 0x8,…) as this gives the best efficiency for minimum design complexity. The minimum data size to be transferred for each push and pop operations is one word (32-bit) and multiple registers can be pushed or popped in one instruction. This can result in unpredictable behaviors, for example, function return to incorrect addresses. Typically, each register PUSH operation should have a corresponding register POP operation otherwise the stack pointer will not be able to restore registers to their original values. At the beginning of a function, the current contents of the registers used by the calling program are stored onto the stack memory using PUSH operations, and at the end of the function, the data on the stack memory is restored to the registers using POP operations. PUSH and POP are commonly used at the beginning and at the end of a function or subroutine. Stack PUSH and POP in the Cortex ®-M processors. If the stack size is too small for a thread to run, the program will abort with a segmentation fault.Figure 4.12.
STACKS ON MAIN TRIAL
However, it may not be possible to know just how large to set it, except by trial and error, especially if private/local arrays are involved. Setting the thread stack size to a value larger than the default may be necessary for most parallelized Fortran codes. The size is set with the STACKSIZE environment variable:ĭemo% setenv STACKSIZE 8192 <- Set thread stack size to 8 Mb C shell The thread's PRIVATE arrays and variables (local to the thread) are allocated on the thread stack. This stack mimics the main program stack but is unique to the thread. Stacksize 8192 kbytes ulimit -a Korn Shell exampleĮach thread of a multithreaded program has its own thread stack. The limit command displays the current main stack size as well as setting it: (See the discussion of -stackvar in the Fortran User's Guide.) stackvar is required with explicitly parallelized loops containing subprogram calls. Use of -stackvar is recommended with parallelization because it improves the optimizer's ability to parallelize CALLs in loops. However, the -stackvar option forces allocation of all local variables and arrays on the stack (as if they were AUTOMATIC variables). The Fortran compilers normally allocate local variables and arrays as STATIC (not on the stack). The default size of the main stack is about 8 megabytes. Stacks are temporary memory address spaces used to hold arguments and AUTOMATIC variables over subprogram invocations. The executing program maintains a main memory stack for the parent program and distinct stacks for each thread.
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