Managing the CMS File System A description of the structure of the CftS file system and the flow of
routines that access and update the filesystem follows.
HowCMS Files Are Organized in Storage CMS files are organized in storage by three types of data blocks: the
file status table (FST), chain links, and file records. Figure 12 shows
how these types of data blocks relate to each other; the following text
and figures describe these relationships and the individual data blocks
inIlore detail.
FILESTATUS TABLES files consist of 800-byte records whose attributes are described in
the file status table (FST). The file status table is definedby DSECT
FSTSECT. The FST consists of such information as thefilename, filetype, and filemode of the file, the date on which the file was last
written, and whether the file is in fixed-length or variable for.at.
Also, the FST contains a pointer to the first chain link. The first
chain link is a block that contains addresses of the data blocks that
contain the actual data for the file.
The FSTs are grouped into800-byte blocks called FST Blocks (these
aresometimes referred to in listings as hyperblocks). Each FST block
contains20 FST entries, each describing the attributes of a separate
file. Figure 13 shows the structure of anPST block and the fields
defined in the FST.
MasterFile Directory
Address ofFSTB File Status Table Block (FSTB) File Status Table Entry
FirstChain Link (FCLl CMS Record
Record nI ..... ---800-byte eMS Record Containing Fi!e Data Items _I Figure 12. How File Records Are Chained Together
NthChain Link {NCLl 2-86 IBM VM/370 System Logic and program Determination--Volu.e 2
routines that access and update the file
How
file status table (FST), chain links, and file records. Figure 12 shows
how these types of data blocks relate to each other; the following text
and figures describe these relationships and the individual data blocks
in
FILE
the file status table (FST). The file status table is defined
FSTSECT. The FST consists of such information as the
written, and whether the file is in fixed-length or variable for.at.
Also, the FST contains a pointer to the first chain link. The first
chain link is a block that contains addresses of the data blocks that
contain the actual data for the file.
The FSTs are grouped into
are
contains
file. Figure 13 shows the structure of an
defined in the FST.
Master
Address of
First
Record n
Nth