Rainbow Electronics ATmega128RFA1 - Manual (Page 12)

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8266A-MCU Wireless-12/09
ATmega128RFA1
stored when entering an interrupt routine and restored when returning from an interrupt
routine. This must be handled by software.
7.5 General Purpose Register File
The Register File is optimized for the AVR Enhanced RISC instruction set. In order to
achieve the required performance and flexibility, the following input/output schemes are
supported by the Register File:
• 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
Figure 7-1 below shows the structure of the 32 general purpose working registers in the
CPU.
Figure 7-1. AVR CPU General Purpose Working Registers
Most of the instructions operating on the Register File have direct access to all
registers, and most of them are single cycle instructions.
As shown in Figure 7-1 above on page 12, each register is also assigned a data
memory address, mapping them directly into the first 32 locations of the user Data
Space. Although not being physically implemented as SRAM locations, this memory
organization provides great flexibility in access of the registers, as the X-, Y- and Z-
pointer registers can be set to index any register in the file.
7.5.1 The X-register, Y-register, and Z-register
The registers R26...R31 have some added functions to their general purpose usage.
These registers are 16-bit address pointers for indirect addressing of the data space.
The three indirect address registers X, Y, and Z are defined as described in
Figure 7-2
on page 13 .