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D81 (.d81)

.d81 file signature | application/octet-stream

D81 is a disk image format for the Commodore 1581 floppy drive, introduced by Commodore International and preserved by the retrocomputing community. It is used to archive, transfer, and emulate 3.5-inch Commodore disk contents in emulators and preservation tools. As a legacy format, it is generally safe to open, though files from untrusted sources may be malformed or intentionally crafted for emulators.

Safe

Magic Bytes

Offset 0
A0 33 44 A0 A0

Sources: Wikipedia

Extension

.d81

MIME Type

application/octet-stream

Byte Offset

0

Risk Level

Safe

Validation Code

How to validate .d81 files in Python

Python 
def is_d81(file_path: str) -> bool:
    """Check if file is a valid D81 by magic bytes."""
    signature = bytes([0xA0, 0x33, 0x44, 0xA0, 0xA0])
    with open(file_path, "rb") as f:
        return f.read(5) == signature

How to validate .d81 files in Node.js

Node.js 
function isD81(buffer: Buffer): boolean {
  const signature = Buffer.from([0xA0, 0x33, 0x44, 0xA0, 0xA0]);
  return buffer.subarray(0, 5).equals(signature);
}

How to validate .d81 files in Go

Go 
func IsD81(data []byte) bool {
    signature := []byte{0xA0, 0x33, 0x44, 0xA0, 0xA0}
    if len(data) < 5 {
        return false
    }
    return bytes.Equal(data[:5], signature)
}

API Endpoint

GET /api/v1/d81
curl https://filesignature.org/api/v1/d81

See the full API documentation for all endpoints and parameters.

Frequently Asked Questions

What is a .d81 file?

A .d81 file is identified by the magic bytes A0 33 44 A0 A0 at byte offset 0. D81 is a disk image format for the Commodore 1581 floppy drive, introduced by Commodore International and preserved by the retrocomputing community. It is used to archive, transfer, and emulate 3.5-inch Commodore disk contents in emulators and preservation tools. As a legacy format, it is generally safe to open, though files from untrusted sources may be malformed or intentionally crafted for emulators.

What are the magic bytes for .d81 files?

The magic bytes for D81 files are A0 33 44 A0 A0 at byte offset 0. These bytes uniquely identify the file format regardless of the file extension.

How do I validate a .d81 file?

To validate a .d81 file, read the first bytes of the file and compare them against the known magic bytes (A0 33 44 A0 A0) at offset 0. This is more reliable than checking the file extension alone, as extensions can be renamed.

What is the MIME type for .d81 files?

There is no officially registered MIME type for .d81 files. Systems typically use application/octet-stream as a generic fallback when handling this format.

Is it safe to open .d81 files?

D81 (.d81) files are generally safe to open. They are classified as low risk because they primarily contain data rather than executable code. However, always ensure files come from a trusted source.