ASF Files Structure

The Advanced Systems Format (ASF) is the file format used by Windows Media. Audio and/or Video content compressed with a wide variety of codecs can be stored in an ASF file and played back with the Windows Media Player (provided the appropriate codecs are installed), streamed with Windows Media Services or optionally packaged with Windows Media Rights Manager. ASF is an extensible file format designed to store synchronized multimedia data.

The base unit of organization for ASF files is called an object. An ASF file object contains the following data.

Data	Size
A GUID that identifies the object	128 bits
The size of the object	64-bits
Object data. The object data can contain other ASF objects	Varies

Magic Number

Magic numbers are common in programs across many operating systems. Magic numbers implement strongly typed data and are a form of in-band signaling to the controlling program that reads the data type(s) at program run-time. Many files have such constants that identify the contained data. Detecting such constants in files is a simple and effective way of distinguishing between many file format and can yield further run-time information

Unallocated Space

computer operating systems, such as Windows, allocate space on the hard drive as adjacent groups of sectors, known as allocation units, or clusters. When you create a new file, the operating system finds available space and allocates that space to the file. Unallocated space is space that is not allocated to active files within a file system.

The type of object that you can create in unallocated space depends on whether your hard disk is a basic disk, containing primary partitions, extended partitions and logical drives, or a dynamic disk, containing volumes that span multiple disks. In the case of a basic hard disk, you can use unallocated space outside an existing partition, or logical storage unit, to create a primary partition -- typically, the partition used to start the operating system -- or an extended partition. Similarly, you can use unallocated space inside an existing partition to create logical drives, or parts of the same physical disk that are managed as independent units.

Slack Space

Slack space is the unused space between the end of the actual file and the end of the the defined data unit (cluster).cluster is the smallest unit of storage that the operating system can deal with.

For example, assume that the OS uses a 4k cluster and 512 byte sector, meaning it writes data in 4k increments made up of eight 512 byte sectors, regardless of the actual size of the file being written. This means that if a 2000 byte file were written to this cluster the remaining 2096 bytes would be slack. Within this slack space there are two areas to consider – the first is that between the end of the actual file and the sector in which the file ends, and the second is the remaining sectors in the cluster that contain no data as depicted below.

File System Structured

FAT (File Allocation Table) system is a file system that uses a file allocation table structure as the way he operates. 

FAT 16 is a file system that use allocation unit have limit until 16-bit. so can save unit allocation until 65536. This file system has a capacity limit of up to 4 Gigabyte sizes only. Allocation unit size used by the FAT16 partition depends on the capacity that was about to be formatted: if the partition size is less than 16 megabytes, then Windows will use the FAT12 file system, and if the partition size larger than 16 megabytes, then Windows will use the FAT16 file system.  below is layout of FAT16