This is an implementation of the heapsort algorithm in AVR assembly. This algorithm lends itself to a relatively simple assembly implementation because it is both iterative and in place, requiring no recursive calls or extra space to complete.
You will need some sort of AVR simulator and assembler. Microchip, a manufacturer of AVR instruction set microprocessors, has released the Atmel Studio simulator for free on their website.
This program is Windows only, and is quite large. You easily can find other AVR simulators and assemblers that are smaller or will work on other platforms on the internet through a quick Google search.
Since the registers on the Atmega328P are 8 bit, you can only work with numbers between -127 and 127, accounting for the 8th sign bit. This also means you can sort at most 127 numbers at a time.