feat(cpu): implement format19 THUMB instructions

src/cpu/thumb/format19.zig

@@ -0,0 +1,25 @@
+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 format19(comptime is_low: bool) InstrFn {
+    return struct {
+        fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u16) void {
+            const offset = opcode & 0x3FF;
+
+            if (is_low) {
+                // Instruction 2
+                const old_pc = cpu.r[15];
+
+                cpu.r[15] = cpu.r[14] + (offset << 1);
+                cpu.r[14] = old_pc | 1;
+            } else {
+                // Instruction 1
+                cpu.r[14] = (cpu.fakePC() & 0xFFFF_FFFC) + (u32SignExtend(11, @as(u32, offset)) << 12);
+            }
+        }
+    }.inner;
+}