#STACK DEFINITION PSP#

This allows the stack for the kernel to be separate from stack memory for the application processes.

In a typical embedded application with an OS, the OS kernel uses the MSP and the application processes use the PSP.

#STACK DEFINITION 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.

Stack definition