diff --git a/main.ts b/main.ts
index 39b335a..901517f 100644
--- a/main.ts
+++ b/main.ts
@@ -2,19 +2,32 @@ import { basename, join } from "jsr:@std/path";
 import { Mutex } from "npm:async-mutex";
 import { downloadChunk } from "./workerpool.ts";
 
-export async function download(
-  url: string,
-  dirpath: string,
-  threads: number,
+export async function download({
+  url,
+  dirpath,
+  threads,
   chunkSize = 5 * 1024 * 1024,
-  headers: HeadersInit = {}
-) {
+  headers = {},
+  startChunk = 0,
+  endChunk = Infinity,
+}: {
+  url: string;
+  dirpath: string;
+  threads: number;
+  chunkSize?: number;
+  headers?: HeadersInit;
+  startChunk?: number;
+  endChunk?: number;
+}) {
   async function getURLInfo() {
     const r = await fetch(url, {
       method: "HEAD",
       headers: headers,
     });
-    const contentLength = +r.headers.get("content-length")!;
+    const contentLength = Math.min(
+      +r.headers.get("content-length")!,
+      endChunk - startChunk + 1
+    );
     const disposition = r.headers.get("content-disposition");
     const filename = (
       disposition
@@ -38,8 +51,8 @@ export async function download(
   try {
     const chunkCount = Math.ceil(contentLength / chunkSize);
     const chunks: Chunk[] = Array.from({ length: chunkCount }, (_, i) => ({
-      start: i * chunkSize,
-      end: Math.min((i + 1) * chunkSize, contentLength) - 1,
+      start: startChunk + i * chunkSize,
+      end: startChunk + Math.min((i + 1) * chunkSize, contentLength) - 1,
     }));
     await downloadChunk<Chunk, ArrayBuffer>(
       threads,
@@ -79,5 +92,13 @@ if (import.meta.main) {
   const url = Deno.args[0];
   const threads = +Deno.args[1] || 32;
   const dirpath = Deno.args[2] || ".";
-  await download(url, join(Deno.cwd(), dirpath), threads);
+  const startChunk = +Deno.args[3] || 0;
+  const endChunk = +Deno.args[4] || Infinity;
+  await download({
+    url,
+    dirpath: join(Deno.cwd(), dirpath),
+    threads,
+    startChunk,
+    endChunk,
+  });
 }
diff --git a/main_test.ts b/main_test.ts
index 85fe2e5..d99cfcc 100644
--- a/main_test.ts
+++ b/main_test.ts
@@ -10,7 +10,42 @@ Deno.test("Download ISO file test", async () => {
   const threads = 32;
 
   console.time("Download ISO file test");
-  const outputPath = await download(url, dirpath, threads);
+  const outputPath = await download({ url, dirpath, threads });
+  console.timeEnd("Download ISO file test");
+
+  console.time("Calculate file hash test");
+  const file = await Deno.open(outputPath, { read: true });
+  const readableStream = file.readable;
+  const fileHashBuffer = await crypto.subtle.digest("SHA-256", readableStream);
+  const fileHash = encodeHex(fileHashBuffer);
+  console.log(fileHash);
+  console.timeEnd("Calculate file hash test");
+  assertEquals(
+    fileHash,
+    "6f6a087a4a8326fdb55971d95855f1185f5b547dfe92cf770da4ec555b763d3f"
+  );
+});
+
+Deno.test("Download ISO file test", async () => {
+  const url =
+    "https://mirrors.tuna.tsinghua.edu.cn/debian-cd/12.9.0-live/amd64/iso-hybrid/debian-live-12.9.0-amd64-kde.iso";
+  const dirpath = ".";
+  const threads = 32;
+
+  console.time("Download ISO file test");
+  const outputPath = await download({
+    url,
+    dirpath,
+    threads,
+    startChunk: 0,
+    endChunk: 700,
+  });
+  await download({
+    url,
+    dirpath,
+    threads,
+    startChunk: 700,
+  });
   console.timeEnd("Download ISO file test");
 
   console.time("Calculate file hash test");