8.5 I/O memory
The I/O space definition of the Atmel
ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P is shown in ”Register summary” on page 628.
All ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P I/Os and peripherals are placed in the I/O
space. All I/O locations may be accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data
between the 32 general purpose working registers and the I/O space. I/O Registers within the address range
0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the value of single
bits can be checked by using the SBIS and SBIC instructions. Refer to the instruction set section for more
details. When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used.
When addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these
addresses. The ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P is a complex microcontroller with
more peripheral units than can be supported within the 64 location reserved in Opcode for the IN and OUT
instructions. For the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD
instructions can be used.
For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory
addresses should never be written.
Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most other AVRs, the
CBI and SBI instructions will only operate on the specified bit, and can therefore be used on registers containing
such Status Flags. The CBI and SBI instructions work with registers 0x00 to 0x1F only.
The I/O and peripherals control registers are explained in later sections.
The ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P contains three General Purpose I/O
Registers, see ”Register Description” on page 24. These registers can be used for storing any information, and
they are particularly useful for storing global variables and Status Flags. General Purpose I/O Registers within
the address range 0x00 - 0x1F are directly bit-accessible using the SBI, CBI, SBIS, and SBIC instructions.