Editing RT-11 (section)

===File system===
RT-11 implemented a simple and fast [[file system]] employing six-character filenames with three-character extensions (6.3) encoded in [[DEC RADIX 50|RADIX-50]], which packed those nine characters into only three 16-bit words (six bytes). This resulted in allowable filename characters being 'A'-'Z', 'a'-'z' forced to upper case, '0'-'9', '$', '%', '*'. Filenames could be prefixed by a device name to specify a device different to the default DK: device, also encoded in RADIX-50.

All files were contiguous, meaning that each file occupied consecutive blocks (the minimally addressable unit of disk storage, 512 bytes) on the disk. This meant that an entire file could be read (or written) very quickly. A side effect of this file system structure was that, as files were created and deleted on a volume over time, the unused disk blocks would likely [[File system fragmentation#Free space fragmentation|not remain contiguous]], which could become the limiting factor in creating large files; the remedy was to periodically “squeeze” (or "squish") a disk to consolidate the unused portions.<ref>{{cite web |website=BitSavers
 |url=http://bitsavers.trailing-edge.com/pdf/dec/pdp11/rt11/v5.6_Aug91/AA-5284H-TC_RT-11_System_Message_Manual_Aug91.pdf
 |title=RT-11 System Message Manual
 |quote=Compress the volume by using the monitor SQUEEZE command}}</ref><ref>{{cite web
|url=https://gordonbell.azurewebsites.net/Digital/RT-11%20System%20Users%20Guide%201977.pdf
|title=RT-11 System Users Guide 1977 |quote=The SQUEEZE command consolidates in a single area all unused ...}}</ref>

Each volume has only one directory which was preallocated at the beginning of the volume. The directory consists of an array of entries, one per file or unallocated space. Each directory entry is 8 (or more) 16-bit words, though a sysgen option allowed extra application-specific storage.<ref>{{cite web
 |url=http://cryptosmith.com/2013/10/19/digitals-rt-11-file-system
 |title=Digital's RT-11 File System |date=20 October 2013 |access-date=January 1, 2015}}</ref>