From a80600156d5438093fcb72545d94be27fed12dd4 Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Tue, 1 Feb 2022 18:38:31 -0400
Subject: [PATCH] feat(cpu): implement format 18 THUMB instructions

---
 src/cpu.zig                |  5 +++++
 src/cpu/thumb/format18.zig | 15 +++++++++++++++
 2 files changed, 20 insertions(+)
 create mode 100644 src/cpu/thumb/format18.zig

diff --git a/src/cpu.zig b/src/cpu.zig
index ba873a7..05b2c7f 100644
--- a/src/cpu.zig
+++ b/src/cpu.zig
@@ -33,6 +33,7 @@ const format13 = @import("cpu/thumb/format13.zig").format13;
 const format14 = @import("cpu/thumb/format14.zig").format14;
 const format15 = @import("cpu/thumb/format15.zig").format15;
 const format16 = @import("cpu/thumb/format16.zig").format16;
+const format18 = @import("cpu/thumb/format18.zig").format18;
 const format19 = @import("cpu/thumb/format19.zig").format19;
 
 pub const ArmInstrFn = fn (*Arm7tdmi, *Bus, u32) void;
@@ -472,6 +473,10 @@ fn thumbPopulate() [0x400]ThumbInstrFn {
                 lut[i] = format16(cond);
             }
 
+            if (i >> 5 & 0x1F == 0b11100) {
+                lut[i] = format18();
+            }
+
             if (i >> 6 & 0xF == 0b1111) {
                 const is_low = i >> 5 & 1 == 1;
 
diff --git a/src/cpu/thumb/format18.zig b/src/cpu/thumb/format18.zig
new file mode 100644
index 0000000..e8a3139
--- /dev/null
+++ b/src/cpu/thumb/format18.zig
@@ -0,0 +1,15 @@
+const std = @import("std");
+
+const Bus = @import("../../Bus.zig");
+const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
+const InstrFn = @import("../../cpu.zig").ThumbInstrFn;
+const u32SignExtend = @import("../../util.zig").u32SignExtend;
+
+pub fn format18() InstrFn {
+    return struct {
+        fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u16) void {
+            const offset = u32SignExtend(11, opcode & 0x7FF) << 1;
+            cpu.r[15] = (cpu.r[15] + 2) +% offset;
+        }
+    }.inner;
+}