AVR Assembly Language Code

Question

I am stuck on the following code and don't have a basic understand of how it works?

Determine the decimal value of r22 after the following line of code is executed:

clr r22  

dec r22

push r22

dec r22

pop r5

value of r22 = ?

Answer 1

Well, just work through each instruction one at a time and figure out what each one does.

1. clr r22: This sets the register to 0.
2. dec r22: This decrements it, so it underflows from 0 to 255.
3. push r22: This pushes 255 (r22) onto the stack.
4. dec r22: This decrements r22, so r22 is now 254.
5. pop r5: This pops a value (255) off the stack and stores it in r5, not affecting r22.

So, unless I got something wrong, the answer is r22 = 254.

Answer 2

The register(in 8bit) goes from 0 to FF(dec r22) to FE(dec r22)

If r22 is 4 bit then E = 14

If r22 is 8 bit then FE = 254

if r22 is 16 bit then FFFE = 65534

and so on

A processor register is like a car mileometer When it reaches 99,999 miles it goes forwards to 00000

A register reaches F-F-F-F, 15-15-15-15 (not 9-9-9-9 because it's hex not decimal) and when we +1 it returns to 0-0-0-0