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ST72321BXXX-AUTO Datasheet(PDF) 43 Page - STMicroelectronics |
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ST72321BXXX-AUTO Datasheet(HTML) 43 Page - STMicroelectronics |
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43 / 247 page  ST72321Bxxx-Auto Central processing unit (CPU) 43/247 The stack pointer is a 16-bit register which is always pointing to the next free location in the stack. It is then decremented after data has been pushed onto the stack and incremented before data is popped from the stack (see Figure 8). Since the stack is 256 bytes deep, the 8 most significant bits are forced by hardware. Following an MCU Reset, or after a reset stack pointer instruction (RSP), the stack pointer contains its reset value (the SP7 to SP0 bits are set) which is the stack higher address. The least significant byte of the stack pointer (called S) can be directly accessed by an LD instruction. Note: When the lower limit is exceeded, the stack pointer wraps around to the stack upper limit, without indicating the stack overflow. The previously stored information is then overwritten and therefore lost. The stack also wraps in case of an underflow. The stack is used to save the return address during a subroutine call and the CPU context during an interrupt. The user may also directly manipulate the stack by means of the PUSH and POP instructions. In the case of an interrupt, the PCL is stored at the first location pointed to by the SP. The other registers are then stored in the next locations as shown in Figure 8. ● When an interrupt is received, the SP is decremented and the context is pushed on the stack. ● On return from interrupt, the SP is incremented and the context is popped from the stack. A subroutine call occupies two locations and an interrupt five locations in the stack area. Figure 8. Stack manipulation example PCH PCL SP PCH PCL SP PCL PCH X A CC PCH PCL SP PCL PCH X A CC PCH PCL SP PCL PCH X A CC PCH PCL SP SP Y CALL Subroutine Interrupt Event PUSH Y POP Y IRET RET or RSP @ 01FFh @ 0100h Stack Higher Address = 01FFh Stack Lower Address = 0100h |
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