Header, FAT & Size Constraints¶

This page covers the IMG file header, FAT (File Allocation Table) structure, MPS metadata subfile, size constraints, and date encoding. The header and FAT form the outer container; the GMP subfiles inside are described in GMP Container.

1. File Header Structure¶

The IMG file begins with a 512-byte header containing metadata and file system information.

1.1 Header Field Reference¶

1.2 Creation Date Encoding¶

Offset: 0x39-0x3E — 6 bytes, little-endian (confirmed by Mechalas spec):

byte 0-1: year   (uint16 LE)
byte 2:   month  (0-11, NOT 1-12 as in some references)
byte 3:   day    (1-31)
byte 4:   hour   (0-23)
byte 5:   second (0-59)

Note: offset 0x3E stores seconds, not minutes. The header does not include minutes. The Mechalas spec confirms: year(2) + month(1) + day(1) + hour(1) + minute(1) + second(1) at 0x39-0x3F but some references show only 6 bytes (0x39-0x3E).

1.3 Block Size Calculation¶

BLOCK_SIZE = 512 × 2^E2 = 512 × 2^6 = 32768 bytes

The FAT block size is always 512 bytes. The data block size is 32768 bytes.

1.4 Partition Table¶

At offset 0x1BE, a standard MBR partition table entry:

• 0x1BE: Boot indicator (0x00 = not bootable)
• 0x1BF-0x1C1: Start CHS
• 0x1C2: System type (0xFF = auto-detect)
• 0x1C3-0x1C5: End CHS
• 0x1C6-0x1C9: Relative sectors (LBA start, uint32 LE)
• 0x1CA-0x1CD: Total sectors (uint32 LE)

2. FAT (File Allocation Table) Structure¶

2.1 FAT Layout¶

GMT reports format: fat: <start> - <directory> - <extent>

• FAT start offset: 0x1000 (4096 bytes from file start)
• Physical block number: 8 (stored in header at 0x40)
• FAT entry size: 512 bytes each

2.2 FAT Entry Format (512 bytes)¶

2.3 Special Directory FAT Entry¶

The first FAT entry is a special directory entry that covers the blocks from offset 0 through the start of the data region:

• Name: 8 spaces
• Type: 3 spaces
• Flag2: 0x03 (special)
• Block table: sequential block numbers 0..N (header + FAT blocks)

2.4 Subfile FAT Entries¶

Each subfile gets one or more FAT entries:

• Name: For GMP subfiles, this is the map ID as 8-char uppercase hex (e.g., 09C102B0). For MPS, it's MAPSOURC.
• Large subfiles span multiple FAT entries (part 0, 1, 2...) each holding up to 240 block pointers.
• Block pointers are physical block numbers (offset / BLOCK_SIZE), not FAT indices.

3.9 MPS Subfile (98 bytes)¶

8. Size Constraints and Limits¶

8.1 File Size Limits¶

8.2 FAT Block Capacity¶

Each FAT entry holds 240 block pointers (240 × 32KB = 7.5MB per FAT entry). For large files:

• 1.4 GB GMP ≈ 45,623 data blocks ≈ 191 FAT entries
• Single-map FAT extent: 0x20000 (131,072 bytes = 256 FAT entries)

8.3 Map Splitting¶

When approaching 4 GB, split into multiple .img files by geographic region (e.g., large maps are split by region). Each file is self-contained with no cross-file references.

9. Garmin Date Format¶

9.1 6-byte Header Date (at offset 0x39)¶

bytes 0-1: year   (uint16 LE)
byte 2:    month  (1-12)
byte 3:    day    (1-31)
byte 4:    hour   (0-23)
byte 5:    second (0-59)

9.2 7-byte Sub-Header Date (in common headers)¶

Same as 6-byte but with an additional byte for day-of-week (or padding):

bytes 0-1: year   (uint16 LE)
byte 2:    month  (1-12)
byte 3:    day    (1-31)
byte 4:    hour   (0-23)
byte 5:    minute (0-59)
byte 6:    second (0-59)
byte 7:    dow    (0, padding)