chore(cpu): refactor ARM functions to make room for THUMB

src/cpu/arm/branch.zig

@@ -0,0 +1,27 @@
+const std = @import("std");
+const util = @import("../../util.zig");
+
+const Bus = @import("../../Bus.zig");
+const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
+const InstrFn = @import("../../cpu.zig").InstrFn;
+
+pub fn branch(comptime L: bool) InstrFn {
+    return struct {
+        fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u32) void {
+            if (L) {
+                // TODO: Debugging beeg.gba w/ MGBA seems to suggest that I don't do anything here
+                cpu.r[14] = cpu.r[15];
+            }
+
+            cpu.r[15] = cpu.fakePC() +% util.u32SignExtend(24, opcode << 2);
+        }
+    }.inner;
+}
+
+pub fn branchAndExchange(cpu: *Arm7tdmi, _: *Bus, opcode: u32) void {
+    const rn = opcode & 0xF;
+    cpu.cpsr.t.write(cpu.r[rn] & 1 == 1);
+
+    // TODO: Is this how I should do it?
+    cpu.r[15] = cpu.r[rn] & 0xFFFF_FFFE;
+}