ATMega registers and memory ranges

Sun, 10 Apr 2011 16:28:50 GMT

The AVR family has a reasonably complex register and memory architecture, with:

Multiple address spaces
Registers that can be accessed directly and via the memory address space
Register pairs that are used as memory indexes
Restrictions on which registers can be loaded directly with constant values
Some IO ports that appear in both the memory and IO address spaces, but with different addresses in each
Some IO ports that are only accessible via the memory address space

In addition, the gcc compiler has conventions governing which registers it uses for what purpose, for example:

Which registers it uses for purposes such as storing temporary values and zero
Which registers it uses for passing and returning arguments
Which registers it uses as a stack frame pointer
Which registers are saved by the caller on entry to a subroutine and which ones have to be saved by the callee

When I'm reading the assembler output of gcc I find it helpful to have a table to remind me which registers are being used for what, and why. The information is not exactly well-explained in the AVR documentation and I've not been able to find all the information in once place on the Internet, so I've drawn up the following quick-reference table to remind me of what goes where:

Click on the image for a PDF copy of the table. I hope you find this useful, corrections and suggestions welcome - please leave a comment below!

Alan's Ramblings - instruction set tag

ATMega registers and memory ranges

References