diff --git a/src/codegen/x86_64/isel.ts b/src/codegen/x86_64/isel.ts
new file mode 100644
index 0000000..6c305ab
--- /dev/null
+++ b/src/codegen/x86_64/isel.ts
@@ -0,0 +1,4 @@
+import * as mir from "../../mir.ts";
+
+export function selectFnInstructions(fn: mir.Fn) {
+}
diff --git a/src/codegen/x86_64/module.ts b/src/codegen/x86_64/module.ts
new file mode 100644
index 0000000..78b9947
--- /dev/null
+++ b/src/codegen/x86_64/module.ts
@@ -0,0 +1,66 @@
+export class Module {
+    private fns: Fn[] = [];
+}
+
+export class Fn {
+    private blocks: Block[] = [];
+}
+
+export class Block {
+    private insts: Inst[] = [];
+
+    push(inst: Inst) {
+        this.insts.push(inst);
+    }
+}
+
+export class Inst {
+}
+
+export type InstKind =
+    | { tag: "PushR"; src: Reg }
+    | { tag: "PopR"; dst: Reg }
+    | { tag: "MovRR"; dst: Reg; src: Reg }
+    | { tag: "SubRI"; dst: Reg; imm: number }
+    | { tag: "MovRI"; dst: Reg; imm: number }
+    | { tag: "MovMRR"; dst: Reg; offset: number; src: Reg }
+    | { tag: "MovRMR"; dst: Reg; src: Reg; offset: number }
+    | { tag: "AddR"; dst_op1: Reg; op2: Reg }
+    | { tag: "Jmp"; bb: number }
+    | { tag: "Ret" };
+
+export class Reg {
+    static tempId = 1000;
+
+    private constructor(
+        private id: number,
+    ) {}
+
+    static reg(reg: Regs): Reg {
+        return new Reg(reg);
+    }
+
+    static temp(): Reg {
+        return new Reg(this.tempId++);
+    }
+}
+
+export const Regs = {
+    a: 1,
+    b: 2,
+    c: 3,
+    si: 4,
+    di: 5,
+    sp: 6,
+    bp: 7,
+    r8: 8,
+    r9: 9,
+    r10: 10,
+    r11: 11,
+    r12: 12,
+    r13: 13,
+    r14: 14,
+    r15: 15,
+} as const;
+
+export type Regs = typeof Regs[keyof typeof Regs];