11
8331B–AVR–03/12
Atmel AVR XMEGA AU
The SP is decremented by one when data are pushed on the stack with the PUSH instruction,
and incremented by one when data is popped off the stack using the POP instruction.
To prevent corruption when updating the stack pointer from software, a write to SPL will auto-
matically disable interrupts for up to four instructions or until the next I/O memory write.
3.9 Register File
The register file consists of 32 x 8-bit general purpose working registers with single clock cycle
access time. The register file supports the following input/output schemes:
• One 8-bit output operand and one 8-bit result input
• Two 8-bit output operands and one 8-bit result input
• Two 8-bit output operands and one 16-bit result input
• One 16-bit output operand and one 16-bit result input
Six of the 32 registers can be used as three 16-bit address register pointers for data space
addressing, enabling efficient address calculations. One of these address pointers can also be
used as an address pointer for lookup tables in flash program memory.
Figure 3-4. AVR CPU general purpose working registers.
The register file is located in a separate address space, and so the registers are not accessible
as data memory.
3.9.1 The X-, Y-, and Z- Registers
Registers R26..R31 have added functions besides their general-purpose usage.
These registers can form 16-bit address pointers for addressing data memory. These three
address registers are called the X-register, Y-register, and Z-register. The Z-register can also be
used as an address pointer to read from and/or write to the flash program memory, signature
rows, fuses, and lock bits.
7 0 Addr.
R0 0x00
R1 0x01
R2 0x02
…
R13 0x0D
General R14 0x0E
Purpose R15 0x0F
Working R16 0x10
Registers R17 0x11
…
R26 0x1A X-register Low Byte
R27 0x1B X-register High Byte
R28 0x1C Y-register Low Byte
R29 0x1D Y-register High Byte
R30 0x1E Z-register Low Byte
R31 0x1F Z-register High Byte
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