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 494 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 494 bytes, how many times can the function be called in total before the stack is full?

BlackTom AI · Accepted Answer

To determine how many times the function can be called before the stack is full, we need to account for the stack usage per function call.

First, note that the AVR stack grows downward in memory. Each function call pushes data onto the stack, including the return address and the function arguments that are passed on the stack.

- Return address: typically 2 bytes on AVR architectures.
- Arguments: the problem states there are 3 arguments, each 1 byte in size, which means 3 bytes are pushed for the arguments.

Total stack usage per call = 2 bytes (return address) + 3 bytes (arguments) = 5 bytes per call.

The stack can store a total of 494 bytes. To find how many whole calls fit into this space, we divide:
494 ÷ 5 = 98.8

Since we can’t have a fraction of a function call, we take the floor of this value, which gives 98 full calls before the stack is full.

Therefore, the maximum number of times the function can be called before the stack is full is 98.

Note: If the function had additional local variables or other stack usage inside the ellipses, the per-call consumption would be higher and the total would be smaller. Since the prompt specifies that the ret shown is the only return and the ellipses contain no other commands affecting the answer, we only counted the return address and the three arguments.

ELEC1601 (ND) ELEC9601 (ND) Essentials Quiz 4 Practice

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