diff --git a/packages/ash/assembly/__tests__/echo.spec.ts b/packages/ash/assembly/__tests__/echo.spec.ts
index e15896e..533e1a0 100644
--- a/packages/ash/assembly/__tests__/echo.spec.ts
+++ b/packages/ash/assembly/__tests__/echo.spec.ts
@@ -17,12 +17,12 @@ describe("echo", (): void => {
     echo(CommandLine.all())
     let str = Hello + " " + World + "\n";
     let stdoutStr = readString(stdout)
-    expect<u32>(Console.stdout.tell()).toBe(str.lengthUTF8, "Two extra characters for space and \\n")
+    expect<u32>(Console.stdout.tell()).toBe(str.lengthUTF8)
     Console.stdout.reset();
     expect<string>(readString(Console.stdout)).toBe(Hello + " " + World + "\n")
     Console.stdout.reset();
     expect<string>(readString(Console.stdout)).toBe(stdoutStr);
-  })
+  });
 
   it("should print no newline with -n", () => {
     CommandLine.push("-n")
@@ -30,7 +30,7 @@ describe("echo", (): void => {
     CommandLine.push(World)
     echo(CommandLine.all())
     let str = Hello + " " + World;
-    expect<u32>(Console.stdout.tell()).toBe(str.lengthUTF8 + 1, "Two extra characters for space and \\n")
+    expect<u32>(Console.stdout.tell()).toBe(str.lengthUTF8)
     Console.stdout.reset();
     expect<string>(Console.stdout.readString().result).toBe(str)
     Console.stdout.reset();
diff --git a/packages/assemblyscript/assembly/__tests__/wasa.spec.ts b/packages/assemblyscript/assembly/__tests__/wasa.spec.ts
index 303f915..1c2d293 100644
--- a/packages/assemblyscript/assembly/__tests__/wasa.spec.ts
+++ b/packages/assemblyscript/assembly/__tests__/wasa.spec.ts
@@ -52,10 +52,16 @@ describe("readLine", (): void => {
     expect<string>(StringUtils.fromCStringTilNewLine(utfStr, str.lengthUTF8)).toStrictEqual("Hello\n")
   });
 
-  it("should read return null after no more lines", (): void => {
+  it("should read null when reading from a terminating char", (): void => {
     let str = "Hello World";
     let utfStr = str.toUTF8();
-    expect<string>(StringUtils.fromCStringTilNewLine(utfStr, str.lengthUTF8 - 2)).toBeNull();
+    expect<string>(StringUtils.fromCStringTilNewLine(utfStr + str.lengthUTF8, str.lengthUTF8)).toBeNull();
+  });
+
+  it("should read chunk", () => {
+    let hello = "Hello ";
+    let world = hello + "World";
+    expect<string>(StringUtils.fromCString(world.toUTF8(), hello.lengthUTF8)).toStrictEqual(hello);
   });
 });
 
@@ -86,3 +92,35 @@ describe("Open", (): void => {
   });
 
 });
+
+describe("read", (): void => {
+  it("read string should return error if offset is at the eof", () => {
+    let file = _fs.createFile("./tmp");
+    file.result.erase();
+    expect<Wasi.errno>(file.result.readString().error).toBe(Wasi.errno.NOMEM);
+  });
+})
+
+let file: FileDescriptor;
+describe("write", (): void => {
+
+  beforeEach(() => {
+    file = _fs.createFile("./tmp").result;
+    file.erase();
+  });
+
+  it("should update size after a write", () => {
+    expect<usize>(file.size).toBe(0);
+    let str = "hello world";
+    expect<Wasi.errno>(file.writeString("hello world")).toBe(Wasi.errno.SUCCESS)
+    expect<usize>(file.size).toBe(str.lengthUTF8);
+  });
+
+  it("should not update size if offset is less than size", (): void => {
+    let str = "hello world";
+    expect<Wasi.errno>(file.writeString("hello world")).toBe(Wasi.errno.SUCCESS)
+    file.reset();
+    expect<Wasi.errno>(file.writeString("HELLO")).toBe(Wasi.errno.SUCCESS)
+    expect<usize>(file.size).toBe(str.lengthUTF8);
+  })
+})
diff --git a/packages/assemblyscript/assembly/wasa/mock/fs/fs.ts b/packages/assemblyscript/assembly/wasa/mock/fs/fs.ts
index 8206af6..e2bf0dc 100644
--- a/packages/assemblyscript/assembly/wasa/mock/fs/fs.ts
+++ b/packages/assemblyscript/assembly/wasa/mock/fs/fs.ts
@@ -15,25 +15,24 @@ export class FileDescriptor {
     constructor(public fd: fd, public file: File | null, public offset: usize = 0) { }
 
     write(bytes: Array<u8>): Wasi.errno {
-        if (bytes.length + this.offset > this.length) {
-            return Wasi.errno.NOMEM
+        let res = this.file!.writeBytes(this.offset, bytes.buffer_.data, bytes.length);
+        if (res == Wasi.errno.SUCCESS) {
+            this.offset += bytes.length;
         }
-        memory.copy(this.data + this.offset, bytes.buffer_.data, bytes.length)
-        this.offset += bytes.length;
-        return Wasi.errno.SUCCESS;
+        return res;
     }
 
     writeString(str: string, newline: boolean = false): Wasi.errno {
         // TODO: Add error checking
         let _str = str + (newline ? "\n" : "");
-        if (_str.lengthUTF8 as usize + this.offset > this.length) {
-            return Wasi.errno.NOMEM
+        let res = this.file!.writeBytes(this.offset, _str.toUTF8(), _str.lengthUTF8);
+        if (res == Wasi.errno.SUCCESS) {
+            this.offset += _str.lengthUTF8;
         }
-        memory.copy(this.data + this.offset, _str.toUTF8(), _str.lengthUTF8);
-        this.offset += str.lengthUTF8 + 1;
-        return Wasi.errno.SUCCESS;
+        return res;
     }
 
+
     read(bytes: Array<u8>): Wasi.errno {
         if (!this.hasSpace(bytes)) {
             return Wasi.errno.NOMEM;
@@ -57,16 +56,19 @@ export class FileDescriptor {
     }
 
     readString(max: usize = 4096): WasiResult<string> {
-        let str = StringUtils.fromCString(this.data + this.offset, this.length - this.offset);
+        let _max = <usize>Math.min(max, this.length - this.offset);
+        let str = StringUtils.fromCString(this.data + this.offset, _max);
+        log<string>(str);
         if (str == null) {
             return WasiResult.fail<string>(Wasi.errno.NOMEM)
         }
-        this.offset += str.lengthUTF8 + 1; //For null character
+        this.offset += str.lengthUTF8; //For null character
         return WasiResult.resolve<string>(str);
     }
 
-    readLine(): WasiResult<string> {
-        let str = StringUtils.fromCStringTilNewLine(this.data + this.offset, this.length - this.offset);
+    readLine(max: usize = 4096): WasiResult<string> {
+        let _max = <usize>Math.min(max, this.length - this.offset);
+        let str = StringUtils.fromCStringTilNewLine(this.data + this.offset, _max);
         if (str == null) {
             return WasiResult.fail<string>(Wasi.errno.NOMEM)
         }
@@ -92,7 +94,7 @@ export class FileDescriptor {
                 break;
             }
             case Wasi.whence.END: {
-                newOffset = <usize>(this.length - <u64>Math.abs(<f64>offset));
+                newOffset = <usize>(this.size - <u64>Math.abs(<f64>offset));
                 break;
             }
             case Wasi.whence.SET: {
@@ -147,6 +149,11 @@ export class FileDescriptor {
         }
         return Wasi.errno.NOMEM;
     }
+
+    get size(): usize {
+        return this.file!.size;
+    }
+
 }
 
 export class DirectoryDescriptor extends FileDescriptor {
@@ -162,7 +169,8 @@ export class DirectoryDescriptor extends FileDescriptor {
     listDir(): DirectoryEntry[] {
         let files = new Array<DirectoryEntry>();
         for (let i: i32 = 0; i < this.children.length; i++) {
-            files.push(new DirectoryEntry(path.basename(this.children[i].path), this.children[i].type))
+            let child = this.children[i];
+            files.push(new DirectoryEntry(path.basename(child.path), child.type, child.size))
         }
         return files;
     }
@@ -175,8 +183,11 @@ export class DirectoryDescriptor extends FileDescriptor {
 export class File {
     private _data: ArrayBuffer;
     static DefaultSize: u32 = 1024;
+    size: usize = 0;
+
     constructor(public path: string, public type: Wasi.filetype = Wasi.filetype.REGULAR_FILE) {
         this._data = new ArrayBuffer(File.DefaultSize);
+        this.writeEOT(0);
     }
     /**
      * 
@@ -206,6 +217,8 @@ export class File {
 
     erase(): Wasi.errno {
         this._data = new ArrayBuffer(File.DefaultSize);
+        this.size = 0;
+        this.writeEOT(0);
         return Wasi.errno.SUCCESS;
     }
 
@@ -213,6 +226,31 @@ export class File {
         return this._data.byteLength;
     }
 
+    writeEOT(offset: usize): Wasi.errno {
+        return this.writeByte(offset, StringUtils.EOT);
+    }
+
+    writeByte(offset: usize, byte: u8): Wasi.errno {
+        if (offset >= this.length) {
+            return Wasi.errno.NOMEM;
+        }
+        store<u8>(this.data + offset, byte);
+        return Wasi.errno.SUCCESS;
+    }
+
+    writeBytes(offset: usize, ptr: usize, size: usize): Wasi.errno {
+        if (offset + size >= this.length) {
+            return Wasi.errno.NOMEM;
+        }
+        let dst = offset + this.data;
+        memory.copy(dst, ptr, size);
+        if (offset + size > this.size) {
+            this.size = offset + size;
+            this.writeEOT(this.size);
+        }
+        return Wasi.errno.SUCCESS;
+    }
+
 
     stat: Wasi.filestat
 }
@@ -229,7 +267,7 @@ class Directory extends File {
 }
 
 class DirectoryEntry {
-    constructor(public path: string, public type: Wasi.filetype) { }
+    constructor(public path: string, public type: Wasi.filetype, public size: usize) { }
 }
 
 
@@ -374,12 +412,12 @@ export class FileSystem {
         return res.result.read(data);
     }
 
-    readString(fd: fd, offset: usize = 0): WasiResult<string> {
+    readString(fd: fd, max?: usize): WasiResult<string> {
         let res = this.get(fd);
         if (res.failed) {
             return WasiResult.fail<string>(res.error);
         }
-        return res.result.readString();
+        return res.result.readString(max);
     }
 
     readline(fd: fd, max?: usize): WasiResult<string> {
@@ -387,7 +425,7 @@ export class FileSystem {
         if (res.failed) {
             return WasiResult.fail<string>(res.error);
         }
-        return res.result.readLine();
+        return res.result.readLine(max);
     }
 
     writeString(fd: fd, data: string, newline: boolean): Wasi.errno {
diff --git a/packages/assemblyscript/assembly/wasa/mock/utils/index.ts b/packages/assemblyscript/assembly/wasa/mock/utils/index.ts
index d06f287..cde6f02 100644
--- a/packages/assemblyscript/assembly/wasa/mock/utils/index.ts
+++ b/packages/assemblyscript/assembly/wasa/mock/utils/index.ts
@@ -1,33 +1,41 @@
 
-const newLine: u8 = 10;
 
 export class StringUtils {
-  static isNewLine(ptr: usize): boolean {
-    return load<u8>(ptr) == newLine;
-  }
+    static readonly NUL: u8 = 0;
+    static readonly EOT: u8 = 4;
+    static readonly LF: u8 = 10;
 
-  static fromCString(cstring: usize, max: usize = 4096): string | null {
-    let size: usize = 0;
-    while (load<u8>(cstring + size) != 0 && size < max) {
-      size++;
+    static isNewLine(ptr: usize): boolean {
+        return load<u8>(ptr) == this.LF;
     }
-    if (size >= max && load<u8>(cstring + size) != 0) {
-      return null;
+
+    private static terminates(ptr: usize): bool {
+        let char: u8 = load<u8>(ptr);
+        return char == this.NUL || char == this.EOT
     }
-    return String.fromUTF8(cstring, size);
-  }
 
-  static fromCStringTilNewLine(cstring: usize, max: usize): string | null {
-    let size: usize = 0;
-    while (load<u8>(cstring + size) != 0 && size < max) {
-      size++;
-      if (this.isNewLine(cstring + size - 1)) {
-        break;
-      }
+    static fromCString(cstring: usize, max: usize = 4096): string | null {
+        let size: usize = 0;
+        while (!this.terminates(cstring + size) && size < max - 1) {
+            size++;
+        }
+        if (size == 0) {
+            return null
+        }
+        return String.fromUTF8(cstring, size);
     }
-    if (size >= max && load<u8>(cstring + size) != 0) {
-      return null;
+
+    static fromCStringTilNewLine(cstring: usize, max: usize): string | null {
+        let size: usize = 0;
+        while (!this.terminates(cstring + size) && size < max - 1) {
+            size++;
+            if (this.isNewLine(cstring + size - 1)) {
+                break;
+            }
+        }
+        if (size == 0) {
+            return null
+        }
+        return String.fromUTF8(cstring, size);
     }
-    return String.fromUTF8(cstring, size);
-  }
 }
\ No newline at end of file
diff --git a/types/wasa/index.d.ts b/types/wasa/index.d.ts
index 833c1dd..a65d5e2 100644
--- a/types/wasa/index.d.ts
+++ b/types/wasa/index.d.ts
@@ -84,6 +84,10 @@ declare class FileDescriptor {
   file: File | null;
   fd: u32;
   offset: u32;
+  /**
+   * Number of bytes written to the file.
+   */
+  size: usize;
 
   /**
    * Write an array of bytes to a file;
@@ -152,7 +156,7 @@ declare class DirectoryDescriptor extends FileDescriptor {
   /**
    * Returns list of names of entries in the directory.
    */
-  listDir(): string[];
+  listDir(): DirectoryEntry[];
 
   /**Path of parent directory */
   parent: string;
@@ -342,6 +346,7 @@ declare class fs {
 declare class DirectoryEntry {
   path: string;
   type: Wasi.filetype;
+  size: usize;
 }
 
 /**