From 2f9933ae49b68123edd3caa8b0fca856668390bf Mon Sep 17 00:00:00 2001
From: negativeExponent <negativeExponent@users.noreply.github.com>
Date: Wed, 12 Aug 2020 21:06:57 +0800
Subject: [PATCH] GBA BIOS: ArcTan/ArcTan2 fixes for HLE bios

Fix ArcTan and ArcTan2 based on mgba's hle bios fixes
- References:
- https://github.com/mgba-emu/mgba/commit/14dc01409c9e971ea0697f5017b45d0db6a7faf5#diff-8f06a143a9fd912c83209f935d3aca25
- https://github.com/mgba-emu/mgba/commit/b154457857d3367a4c0196a4abadeeb6c850ffdf#diff-8f06a143a9fd912c83209f935d3aca25
---
 src/gba/bios.cpp | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/gba/bios.cpp b/src/gba/bios.cpp
index 48b0c3ed..8831f690 100644
--- a/src/gba/bios.cpp
+++ b/src/gba/bios.cpp
@@ -42,6 +42,11 @@ int16_t sineTable[256] = {
     (int16_t)0xF384, (int16_t)0xF50F, (int16_t)0xF69C, (int16_t)0xF82B, (int16_t)0xF9BB, (int16_t)0xFB4B, (int16_t)0xFCDD, (int16_t)0xFE6E
 };
 
+// 2020-08-12 - negativeExponent
+// Fix ArcTan and ArcTan2 based on mgba's hle bios fixes
+// https://github.com/mgba-emu/mgba/commit/14dc01409c9e971ea0697f5017b45d0db6a7faf5#diff-8f06a143a9fd912c83209f935d3aca25
+// https://github.com/mgba-emu/mgba/commit/b154457857d3367a4c0196a4abadeeb6c850ffdf#diff-8f06a143a9fd912c83209f935d3aca25
+
 void BIOS_ArcTan()
 {
 #ifdef GBA_LOGGING
@@ -52,7 +57,8 @@ void BIOS_ArcTan()
     }
 #endif
 
-    int32_t a = -(((int32_t)(reg[0].I * reg[0].I)) >> 14);
+    int32_t i = reg[0].I;
+    int32_t a = -((i * i) >> 14);
     int32_t b = ((0xA9 * a) >> 14) + 0x390;
     b = ((b * a) >> 14) + 0x91C;
     b = ((b * a) >> 14) + 0xFB6;
@@ -60,8 +66,9 @@ void BIOS_ArcTan()
     b = ((b * a) >> 14) + 0x2081;
     b = ((b * a) >> 14) + 0x3651;
     b = ((b * a) >> 14) + 0xA2F9;
-    a = ((int32_t)reg[0].I * b) >> 16;
-    reg[0].I = a;
+    reg[0].I = (i * b) >> 16;
+    reg[1].I = a;
+    reg[3].I = b;
 
 #ifdef GBA_LOGGING
     if (systemVerbose & VERBOSE_SWI) {
@@ -84,7 +91,7 @@ void BIOS_ArcTan2()
 
     int32_t x = reg[0].I;
     int32_t y = reg[1].I;
-    uint32_t res = 0;
+    int32_t res = 0;
     if (y == 0) {
         res = ((x >> 16) & 0x8000);
     } else {
@@ -109,6 +116,7 @@ void BIOS_ArcTan2()
         }
     }
     reg[0].I = res;
+    reg[3].I = 0x170;
 
 #ifdef GBA_LOGGING
     if (systemVerbose & VERBOSE_SWI) {