From c77ae06d150225ed2a82cb61c9b0bcb4b4586fe2 Mon Sep 17 00:00:00 2001
From: Rozelette <Rozelette@users.noreply.github.com>
Date: Sun, 19 Apr 2020 00:09:06 -0500
Subject: [PATCH] Created Decompiling Your First Function (markdown)

---
 Decompiling-Your-First-Function.md | 84 ++++++++++++++++++++++++++++++
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 create mode 100644 Decompiling-Your-First-Function.md

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+Let's take the following function from z_actor and convert it to C.
+
+```
+glabel Actor_SetChestFlag
+/* 016500 0x800B5C90 8C8E1E68 */ lw	$t6, 0X1E68($a0)
+/* 016501 0x800B5C94 240F0001 */ li	$t7, 1
+/* 016502 0x800B5C98 00AFC004 */ sllv	$t8, $t7, $a1
+/* 016503 0x800B5C9C 01D8C825 */ or	$t9, $t6, $t8
+/* 016504 0x800B5CA0 AC991E68 */ sw	$t9, 0X1E68($a0)
+/* 016505 0x800B5CA4 03E00008 */ jr	$ra
+/* 016506 0x800B5CA8 00000000 */ nop
+```
+
+Let's take a look at the first line.
+
+```
+/* 016500 0x800B5C90 8C8E1E68 */ lw	$t6, 0X1E68($a0)
+```
+
+There are two things that we can take away from this line.
+
+First, `$a0` is in use. `$a0` contains the first argument to the function, and thus we know that this function has at least one argument.
+
+Second, we know that `$a0` is a pointer because it is loading something relative to the address it points to and storing it in `$t6`. Since the `lw` or "load word" instruction is being used, we know its loading a 32-bit integer.
+
+```
+void Actor_SetChestFlag(u32 a0) {
+    u32 t6 = *(u32*)(a0 + 0x1E68);
+
+}
+```
+
+Let's take a look at the next few lines:
+
+```
+/* 016501 0x800B5C94 240F0001 */ li	$t7, 1
+/* 016502 0x800B5C98 00AFC004 */ sllv	$t8, $t7, $a1
+/* 016503 0x800B5C9C 01D8C825 */ or	$t9, $t6, $t8
+```
+
+An integer value 1 is being loaded into register `$t7`. This value is then shifted to the left using `$a1` to determine how many bits it should be shifted, with the result being saved in $t8. Finally, this result is Binary OR'd with the contents of `$t6`. This roughly translates to:
+
+```
+void Actor_SetChestFlag(u32 a0, u32 a1) {
+    u32 t6 = *(u32*)(a0 + 0x1E68);
+    u32 t7 = 1;
+    u32 t8 = t7 << a1;
+    u32 t9 = t6 | t8;
+
+}
+```
+
+Let's take a look at the final three lines:
+
+```
+/* 016504 0x800B5CA0 AC991E68 */ sw	$t9, 0X1E68($a0)
+/* 016505 0x800B5CA4 03E00008 */ jr	$ra
+/* 016506 0x800B5CA8 00000000 */ nop
+```
+
+The result of these calculations are saved back into `$a0 + 0x1E68`, then the function returns. The `nop` after the `jr` is necessary because the instruction in the binary after a jump is executed first due to the use of a delay slot.
+
+This translates to:
+
+```
+void Actor_SetChestFlag(u32 a0, u32 a1) {
+    u32 t6 = *(u32*)(a0 + 0x1E68);
+    u32 t7 = 1;
+    u32 t8 = t7 << a1;
+    u32 t9 = t6 | t8;
+
+    *(u32*)(a0 + 0x1E68) = t9;
+}
+```
+
+This can be simplified down to:
+
+```
+void Actor_SetChestFlag(u32 a0, u32 a1) {
+    *(u32*)(a0 + 0x1E68) |= (1 << a1);
+}
+```
+
+Be sure to comment out the `#pragma GLOBAL_ASM` line with your function. Otherwise, you'll get an error about duplicate functions.
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