Rainbow Electronics ATmega128RFA1 - Manual (Page 13)

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8266A-MCU Wireless-12/09
ATmega128RFA1
Figure 7-2. The X-, Y-, Z-registers
In the different addressing modes these address registers have functions as fixed
displacement, automatic increment, and automatic decrement (see the instruction set
reference for details).
7.6 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 0x0200. The initial value of the stack pointer is
the last address of the internal SRAM.
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.
When the FLASH memory exceeds 128Kbyte one additional cycle is required. In this
case the Stack Pointer is decremented by three when the return address is pushed onto
the Stack with subroutine call or interrupt and is incremented by three when data is
popped from the Stack with return from subroutine RET or return from interrupt RETI.
7.6.1 SPH – Stack Pointer High
Bit 7 6 5 4 3 2 1 0
$3E ($5E) SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH
Read/Write RW RW RW RW RW RW RW RW
Initial Value 0 0 1 0 0 0 0 1
The AVR Stack Pointer is implemented as two 8-bit registers SPL and SPH 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.
• Bit 7:0 – SP15:8 - Stack Pointer High Byte