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I have been trying to grasp a better understand of registers and port manipulation. I have been looking through the ATmega 328P datasheet to look at the different registers.

For some of the registers, I see that there are two addresses listed. For example, for PORTB is listed at 0x05 (0x25). Similarly, for its DDRB and PINB, they are listed at 0x04 (0x24), and 0x03 (0x23), respectively. What does this mean for the register? Thank you.

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One (the lower number) is the IO port address which is used when using the IN or OUT op-code. The other (higher number) is the memory mapped address.

Different methods of access are more programatically efficient depending on how you are manipulating the values. For instance IN and OUT are more efficient at setting or reading the entire register value, but SBI and CBI on the memory mapped address are more efficient when manipulating individual bits.

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