[mkgmap-dev] Split gmapsupp.img

Sun Oct 18 16:06:57 BST 2020

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?
>
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
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