The 4GB limit is a result of the fact that Garmin devices use FAT16B
without LFS (only the very oldest) and FAT32 (all the rest) filesystems.
Garmin devices do not support exFAT; though, devices with rewritable
firmware could be made to support exFAT. The reason that FAT file
systems are used are they are freely available and work on Linux,
Windows and Apple operating systems.
Please, see:
https://en.wikipedia.org/wiki/File_Allocation_Table
https://physics.nist.gov/cuu/Units/binary.html
https://en.wikipedia.org/wiki/ExFAT
There are several sizes of concern: cluster size, maximum file size,
maximum number of files, and maximum file system size. Each of these
sizes are powers of two or powers of 2 less one. Because of the powers
of 2, SI binary prefixes will be used. Remember only one file can be
stored in a cluster and a file may have more than one cluster. The space
at the end of the last cluster of a file is wasted.
FAT12 4KiB and 8KiB clusters, file size limited by volume size, 4068
files with 8 KiB clusters, 16MiB with 4KiB and 32MiB with 8 KiB clusters.
FAT16B minimum file system sizes 8 MiB with 128, 32 MiB with 512*, and
256 MiB 4 KiB disk logical sector sizes, 2GiB-1 without LFS and 4GiB-1
with LFS, cluster sizes are powers of 2 from 512 to 32768, 65460 files
with 32 KiB clusters, maximum file systems size is 65525 clusters.
FAT32 32 MiB-4.5 KiB (with 65525 clusters and 512 byte sectors) and 256
MiB-36 KiB (with 65525 clusters and 4 KiB sectors), same filesize limits
as FAT16B, cluster sizes that are powers of 2 from 512 to 2,097,152.
exFAT maximum file size practical 128 PiB-1, structural limit 16 EiB-1,
maximum volume size practical 128 PiB, maximum number of files per
directory 2,796,202.
* almost all non CHR disks are 512 byte sectored.
People will tell you that you can not use a SD card larger than a
certain size dependent on Garmin receiver model. This is not true. The
actual requirement is that a Garmin receiver only looks that the first
primary partition on a SD card. Simply, divide the SD card into two
partitions (in Windows, the disk manager can do this), format that
partition as the appropriate FAT format, and put the map or maps there.
The second partition can be formatted as one desires and used as
desired. This is a handy space in which to store other previously made
maps or even a complete map building system. I have done this on Garmin
receivers as old as Garmin StreetPilot 2610 with SD cards as large as
256 GB (the card uses some of the memory for its management purposes).
On 10/18/2020 2:14 AM, Carlos Dávila wrote:
As far as I know, mkgmap is able to produce unlimited size gmapsupp
files, but SD cards and internal device memories have a limit of 4 GB
in a single file. To overcome that limit, recent versions of Garmin
City Navigator maps are split into several gmapsupp that work as a
single one in devices which include them. Would it be possible to add
an option to mkgmap, so that it automatically splits gmapsupp when you
process a map that is larger than 4 GB?
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