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12
8246B–AVR–09/11
ATtiny2313A/4313
Figure 4-3.
The X-, Y-, and 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).
4.5
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 loca-
tions 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 0x60. 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 Stack Pointer is implemented as one 8-bit register in the I/O space.
4.6
Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR
CPU is driven by the CPU clock clk
CPU, directly generated from the selected clock source for the
chip. No internal clock division is used.
Figure 4-4 shows the parallel instruction fetches and instruction executions enabled by the Har-
vard architecture and the fast-access Register File concept. This is the basic pipelining concept
to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost,
functions per clocks, and functions per power-unit.
15
XH
XL
0
X-register
70
7
0
R27 (0x1B)
R26 (0x1A)
15
YH
YL
0
Y-register
70
7
0
R29 (0x1D)
R28 (0x1C)
15
ZH
ZL
0
Z-register
70
7
0
R31 (0x1F)
R30 (0x1E)
Bit
7
65
43
21
0
SP7
SP6
SP5
SP4
SP3
SP2
SP1
SP0
SPL
Read/Write
R/W
Initial Value
RAMEND