feat(cpu): implement ARM multiply instructions

src/cpu/arm/multiply.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").ArmInstrFn;
+
+pub fn multiply(comptime A: bool, comptime S: bool) InstrFn {
+    return struct {
+        fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u32) void {
+            const rd = opcode >> 16 & 0xF;
+            const rn = opcode >> 12 & 0xF;
+            const rs = opcode >> 8 & 0xF;
+            const rm = opcode & 0xF;
+
+            const result = cpu.r[rm] * cpu.r[rs] + if (A) cpu.r[rn] else 0;
+            cpu.r[rd] = result;
+
+            if (S) {
+                cpu.cpsr.n.write(result >> 31 & 1 == 1);
+                cpu.cpsr.z.write(result == 0);
+                // V is unaffected, C is *actually* undefined in ARMv4
+            }
+        }
+    }.inner;
+}