Rainbow-electronics ATmega8HVD Uživatelský manuál stáhnout pdf (Strana 10)

8052B–AVR–09/08

ATmega4HVD/8HVD

Figure 6-3. The X-, Y-, and Z-registers

In the different addressing modes these address registers have functions as fixed displace-

ment, automatic increment, and automatic decrement (see the instruction set reference for

details).

6.5 Stack Pointer

The Stack is mainly used for storing temporary data, for storing local variables and for storing

return addresses after interrupts and subroutine calls. The Stack Pointer Register always

points to the top of the Stack. Note that the Stack is implemented as growing from higher

memory locations to lower memory locations. This implies that a Stack PUSH command

decreases the Stack Pointer.

The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt

Stacks are located. This Stack space 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 0x100. The Stack Pointer is decremented by one when data is pushed onto

the Stack with the PUSH instruction, and it is decremented by two when the return address is

pushed onto the Stack with subroutine call or interrupt. The Stack Pointer is incremented by

one when data is popped from the Stack with the POP instruction, and it is incremented by two

when data is popped from the Stack with return from subroutine RET or return from interrupt

RETI.

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 implementa-

tions of the AVR architecture is so small that only SPL is needed. In this case, the SPH

6.5.1 SPH and SPL – Stack pointer High and Low Register

15 XH XL 0

X-register 707 0

R27 (0x1B) R26 (0x1A)

15 YH YL 0

Y-register 707 0

R29 (0x1D) R28 (0x1C)

15 ZH ZL 0

Z-register 70 7 0

R31 (0x1F) R30 (0x1E)

Bit 151413121110 9 8

SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH

SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL

76543210

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000010

11111111

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 167 168

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Rainbow-electronics ATmega8HVD Uživatelský manuál Strana 10