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Dialog Edit Boot Sector

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Digitoxin edited this page Dec 22, 2025 · 3 revisions

Edit Boot Sector

Edit Boot Sector Dialog

The Boot Sector dialog allows inspection and modification of the boot record for the active image.

It provides structured access to the BIOS Parameter Block (BPB), extended boot parameters, and the bootstrap code, making it possible to review or adjust disk layout information and boot-related metadata in a controlled way.

How to Open

  • Menu: Edit → Boot Sector

Boot Record

The Boot Record section contains the fields that define disk layout, filesystem structure, and boot behavior.

Most of the fields shown here are part of the BIOS Parameter Block (BPB), which describes how the operating system interprets the disk.

Disk Format

Selects a predefined disk layout.

Changing the disk format updates related BPB fields to match the selected format.

This provides a convenient way to align BPB values with known disk layouts while still allowing manual adjustment when needed.

BIOS Parameter Block (BPB)

The following fields are part of the BIOS Parameter Block and describe the logical structure of the disk.

  • Bytes per Sector
    Number of bytes in each physical sector

    Allowed Values: 512, 1024, 2048, 4096

    Note: This value should be 512 for all floppy disks.

  • Sectors per Cluster
    Number of sectors per cluster

    Allowed Values: 1, 2, 4, 8, 16, 32, 64, 128

  • Reserved Sectors
    Number of reserved sectors in the reserved region of the volume starting at the first sector of the volume

    Allowed Values: Any non-zero value

    Note: This value should typically be set to 1.

  • Number of FATs
    Number of File Allocation Table (FAT) copies on the volume

    Allowed Values: Any non-zero value

    Note: This value should typically be set to 2.

  • Root Directory Entries Number of entries in the root directory

    Allowed Values: Value multiplied by 32 should be an even multiple of Bytes Per Sector.

  • Total Sector Count
    Total number of sectors in the volume

  • Media Descriptor
    Identifier used by the filesystem to describe the media type

  • Sectors per FAT
    Number of sectors allocated to each copy of the File Allocation Table (FAT)

  • Sectors per Track
    Number of sectors per track on the disk

  • Number of Heads
    Number of physical heads (sides) on the disk

    Typical Values: 1 or 2

  • Hidden Sectors
    Number of sectors proceeding the first sector of a partitioned volume

    Note: This value should be 0 for all floppy disks.

  • Boot Sector Signature
    Indicates to the BIOS that the sector is executable.

    Allowed Values:

    • AA 55
    • 00 00 (PC DOS 1.x)
    • 54 42 (Tandy 2000)

Other Boot Record Fields

The following fields are part of the boot record but are not part of the BPB itself.

  • Jump Instruction
    This instruction indicates where the bootstrap code starts

    Allowed Values: EB xx 90, E9 xx xx

  • OEM Name
    Can be set by a FAT implementation to any desired value

    Typically this is some indication of what system formatted the volume.

Extended Parameter Block

The Extended Parameter Block contains additional fields that may be present in newer FAT boot sectors. These fields are primarily used for volume identification and boot metadata.

Some images may not include all extended fields, depending on format and boot sector type.

Fields

  • Total Sector Count (Large)
    Total number of sectors in a FAT16 volume larger than 65,535 sectors

    Note: This value should be 0 for all floppy disks.

  • Drive Number
    Interrupt 13h drive number

    Allowed Values: 0, 128

  • Extended Boot Signature
    A marker that indicates extended volume fields are present (such as serial number and volume label). If this signature is missing, the related fields may be ignored by some systems.

    Typical Values:

    • 28h - Volume Serial Number is present
    • 29h - Volume Serial Number, Volume Label, and File System ID are present

  • Volume Serial Number The Volume Serial Number is a 32-bit random number used in conjunction with the Volume Label for removable media tracking

    Note: This is typically generated by converting the current date and time into a 32-bit value.

    You can click the calendar icon to generate a new volume serial number based on a user-supplied date and time.

  • Volume Label
    This field typically matches the 11-byte volume label in the root directory of the disk or has the value "NO NAME " if the volume label does not exist.

  • File System ID
    The File System ID is informational only

    Typical Values: FAT12, FAT16, FAT

Additional Data

The Additional Data section displays the raw boot sector bytes starting after the Boot Record and Extended Parameter Block, including the bootstrap code and any remaining data not shown above.

This view is presented in both hexadecimal and ASCII form.

What You Can Do Here

  • Inspect the actual boot loader and embedded boot sector data
  • Make direct edits to the raw bytes when needed (advanced use)

Edits made here affect the executable bootstrap portion of the boot sector and can change boot behavior.

Note: This section is intended for advanced users. Incorrect changes may prevent an image from booting.

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