lopez-repo/calling-convention/Output-explanation.md

Calling Convention — Output Explanation

This document explains the output of the calling convention algorithm implemented in calling-convention.ipynb, tested across 3 scenarios.

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TEST 1: A single 1-byte argument

Function signature: result = func(x) where x is 8 bits (1 byte)

Output:

===== TEST 1: a 1 byte argument =====
=== State after do_function_call ===
Registers used: ['RA']
Stack: [{'type': 'caller_saved', 'reg': 'R0', 'value': 0}, {'type': 'caller_saved', 'reg': 'R1', 'value': 0}, {'type': 'caller_saved', 'reg': 'R2', 'value': 0}, {'type': 'caller_saved', 'reg': 'R3', 'value': 0}]
RS points to: 4
=== State after undo_function_call ===
Clean stack: []
RS: 0
Collected result: {'result': {'type': 'result', 'value': 42}}

Explanation:

Since there is only one argument and it fits in a single register, it is placed directly in RA (the first available argument register). Before the call, the 4 caller-saved registers (R0-R3) are pushed onto the stack to preserve their values. RS points to position 4, reflecting those 4 entries. After undo_function_call, the stack is empty, RS returns to 0, and the result value 42 is successfully collected from RA.