Study the following recursive AVR code. Note: The "..." represents code that is not shown and contains no other important commands that will affect the answer. The ret shown is the only ret in the function.

my_function:
...
call my_function
...
ret

We are told this function takes 3 arguments for its input, where each argument is a 1 byte number.

If we assume that the stack can only store 415 bytes, how many times can the function be called in total before the stack is full?

Question

Study the following recursive AVR code. Note: The "..." represents code that is not shown and contains no other important commands that will affect the answer. The ret shown is the only ret in the function.

my_function:
...
call my_function
...
ret

We are told this function takes 3 arguments for its input, where each argument is a 1 byte number.

If we assume that the stack can only store 415 bytes, how many times can the function be called in total before the stack is full?

BlackTom AI · Accepted Answer

We need to determine how many recursive calls can be pushed onto the stack before it becomes full, given the stack capacity is 415 bytes and each function call involves 3 one-byte arguments.

First, recall what happens on a function call in AVR: the return address must be placed on the stack so the call can return properly. The return address typically takes 2 bytes. In addition, the 3 arguments for the function are passed on the stack, each occupying 1 byte, so that adds 3 more bytes.

Therefore, the per-call stack usage is 2 bytes (return address) + 3 bytes (arguments) = 5 bytes per call.

To find out how many calls fit into a 415-byte stack, divide the total capacity by the per-call usage:
415 bytes ÷ 5 bytes per call = 83 calls.

Thus, the function can be called 83 times in total before the stack is full.

Note: This analysis assumes there are no additional local variables or saved registers on the stack for this particular function, and that the ABI places the return address and arguments on the stack exactly as described. If the compiler or ABI uses a different convention (e.g., some registers are saved elsewhere), the per-call usage would change and the result would differ accordingly.

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