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ATtiny261A-MU Datasheet(PDF) 11 Page - ATMEL Corporation |
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ATtiny261A-MU Datasheet(HTML) 11 Page - ATMEL Corporation |
11 / 292 page 11 8197A–AVR–10/09 ATtiny261A/461A/861A Figure 4-3. The X-, Y-, and Z-registers In different addressing modes these address registers function as automatic increment and automatic decrement (see the instruction set reference for details). 4.5 Stack Pointer The Stack is mainly used for storing temporary data, local variables and return addresses for interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack, in the data SRAM Stack area where the subroutine and interrupt stacks are located. The Stack in the data SRAM must be defined by the program before any subroutine calls are executed or interrupts are enabled. The Stack Pointer must be set to point above start of the SRAM (see Figure 5-2 on page 16). The initial Stack Pointer value equals the last address of the internal SRAM. Note that the Stack is implemented as growing from higher to lower memory locations. This means a Stack PUSH command decreases the Stack Pointer. See Table 4-1. The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is implementation dependent. Note that the data space in some implementations of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register will not be present. 15 XH XL 0 X-register 7 0 7 0 R27 (0x1B) R26 (0x1A) 15 YH YL 0 Y-register 7 0 7 0 R29 (0x1D) R28 (0x1C) 15 ZH ZL 0 Z-register 7 0 7 0 R31 (0x1F) R30 (0x1E) Table 4-1. Stack Pointer instructions Instruction Stack pointer Description PUSH Decremented by 1 Data is pushed onto the stack CALL ICALL RCALL Decremented by 2 Return address is pushed onto the stack with a subroutine call or interrupt POP Incremented by 1 Data is popped from the stack RET RETI Incremented by 2 Return address is popped from the stack with return from subroutine or return from interrupt |
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