diff --git a/src/gba/gba-bios.c b/src/gba/gba-bios.c
index 2a870e2bb..1059f5fa4 100644
--- a/src/gba/gba-bios.c
+++ b/src/gba/gba-bios.c
@@ -201,6 +201,9 @@ void GBASwi16(struct ARMBoard* board, int immediate) {
 	case 0x8:
 		gba->cpu.gprs[0] = sqrt(gba->cpu.gprs[0]);
 		break;
+	case 0xA:
+		gba->cpu.gprs[0] = (atan2f(gba->cpu.gprs[0] / 16384.f, gba->cpu.gprs[1] / 16384.f) / 2 * M_PI) * 0x10000;
+		break;
 	case 0xB:
 		_CpuSet(gba);
 		break;