ATxmega128D4-AU ATMEL [ATMEL Corporation], ATxmega128D4-AU Datasheet - Page 10

• Manufacturer Part Number: ATxmega128D4-AU
• Description: 8/16-bit Atmel XMEGA D4 Microcontroller
• Manufacturer: ATMEL [ATMEL Corporation]

6.5
6.6
6.7
8135L–AVR–06/12
Program Flow
Status Register
Stack and Stack Pointer
• Multiplication of signed integers
• Multiplication of a signed integer with an unsigned integer
• Multiplication of unsigned fractional numbers
• Multiplication of signed fractional numbers
• Multiplication of a signed fractional number with an unsigned one
A multiplication takes two CPU clock cycles.
After reset, the CPU starts to execute instructions from the lowest address in the flash program-
memory ‘0.’ The program counter (PC) addresses the next instruction to be fetched.
Program flow is provided by conditional and unconditional jump and call instructions capable of
addressing the whole address space directly. Most AVR instructions use a 16-bit word format,
while a limited number use a 32-bit format.
During interrupts and subroutine calls, the return address PC is stored on the stack. The stack is
allocated in the general data SRAM, and consequently the stack size is only limited by the total
SRAM size and the usage of the SRAM. After reset, the stack pointer (SP) points to the highest
address in the internal SRAM. The SP is read/write accessible in the I/O memory space,
enabling easy implementation of multiple stacks or stack areas. The data SRAM can easily be
accessed through the five different addressing modes supported in the AVR CPU.
The status register (SREG) contains information about the result of the most recently executed
arithmetic or logic instruction. This information can be used for altering program flow in order to
perform conditional operations. Note that the status register is updated after all ALU operations,
as specified in the instruction set reference. This will in many cases remove the need for using
the dedicated compare instructions, resulting in faster and more compact code.
The status register is not automatically stored when entering an interrupt routine nor restored
when returning from an interrupt. This must be handled by software.
The status register is accessible in the I/O memory space.
The stack is used for storing return addresses after interrupts and subroutine calls. It can also be
used for storing temporary data. The stack pointer (SP) register always points to the top of the
stack. It is implemented as two 8-bit registers that are accessible in the I/O memory space. Data
are pushed and popped from the stack using the PUSH and POP instructions. The stack grows
from a higher memory location to a lower memory location. This implies that pushing data onto
the stack decreases the SP, and popping data off the stack increases the SP. The SP is auto-
matically loaded after reset, and the initial value is the highest address of the internal SRAM. If
the SP is changed, it must be set to point above address 0x2000, and it must be defined before
any subroutine calls are executed or before interrupts are enabled.
During interrupts or subroutine calls, the return address is automatically pushed on the stack.
The return address can be two or three bytes, depending on program memory size of the device.
For devices with 128KB or less of program memory, the return address is two bytes, and hence
the stack pointer is decremented/incremented by two. For devices with more than 128KB of pro-
gram memory, the return address is three bytes, and hence the SP is decremented/incremented
XMEGA D4
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