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ST72311 Datasheet(PDF) 15 Page - STMicroelectronics |
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ST72311 Datasheet(HTML) 15 Page - STMicroelectronics |
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15 / 94 page  15/94 ST72311 CENTRAL PROCESSING UNIT (Cont’d) Stack Pointer (SP) Read/Write Reset Value: 01FFh The Stack Pointer is a 16-bit register which is al- ways 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 8th most significant bits are forced by hardware. Following an MCU Reset, or after a Reset Stack Pointer in- struction (RSP), the Stack Pointer contains its re- set 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 a LD in- struction. Note: When the lower limit is exceeded, the Stack Pointer wraps around to the stack upper limit, with- out indicating the stack overflow. The previously stored information is then overwritten and there- fore lost. The stack also wraps in case of an under- flow. The stack is used to save the return address dur- ing 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 instruc- tions. In the case of an interrupt, the PCL is stored at the first location pointed to by the SP. Then the other registers are stored in the next locations as shown in Figure 8. – When an interrupt is received, the SP is decre- mented 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 in- terrupt five locations in the stack area. Figure 8. Stack Manipulation Example 15 8 0 0 0 000 01 70 SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 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 15 |
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