# Calling Convention — Output Explanation

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

---

## 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`.

---

## TEST 2: 8 arguments, some overflow to the stack

**Function signature:** `result = func(a, b, c, d, e, f, g, h)`

**Output:**
```
===== TEST 2: 8 arguments, some on the stack =====
=== State after do_function_call ===
Registers used: ['RA', 'RB', 'RC', 'RD', 'R8', 'R9']
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}, {'type': 'param', 'name': 'g', 'size': 64, 'value': 0}, {'type': 'param', 'name': 'h', 'size': 64, 'value': 0}]
RS points to: 6
=== State after undo_function_call ===
Clean stack: []
RS: 0
Collected result: {'result': {'type': 'result', 'value': 99}}
```

**Explanation:**

Spider provides 6 registers for passing arguments: RA, RB, RC, RD, R8, R9.
The first 6 arguments (a through f) fill all available registers.
Arguments `g` and `h`, which are 64-bit values, have no registers left and
are pushed onto the stack instead. `RS` points to 6, reflecting the 4
caller-saved entries plus 2 stack parameters. After `undo_function_call`,
all stack entries are cleaned and the result `99` is collected from `RA`.

---

## TEST 3: Boolean arguments

**Function signature:** `ok = func(flag1, flag2, flag3, x)`

**Output:**
```
===== TEST 3: Boolean arguments =====
=== State after do_function_call ===
Registers used: ['RA', 'RB']
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}]
...
=== State after undo_function_call ===
Clean stack: []
RS: 0
Collected result: {'ok': {'type': 'result', 'value': True}}
```

**Explanation:**

Booleans (1-bit values) are not placed individually into registers.
Instead they are accumulated into a queue and packed together before
occupying a single register. The 3 boolean flags (flag1, flag2, flag3)
are packed together and placed in `RB` as a `bool_pack_padded` entry.
The regular argument `x` (8 bits) is placed in `RA` as normal.
No arguments overflow to the stack. After `undo_function_call`, the result
`True` is successfully collected from `RA`.

---

## General observations

- The stack always starts and ends with the same state, proving the
  algorithm correctly preserves the machine's previous context.
- Caller-saved registers (R0-R3) are always pushed before the call
  and restored after, regardless of the number of arguments.
- `RS` accurately tracks the stack size at every step.