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 file system follows.
How CMS 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
in Ilore detail.
FILE STATUS TABLES files consist of 800-byte records whose attributes are described in
the file status table (FST). The file status table is defined by DSECT
FSTSECT. The FST consists of such information as the filename, 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 into 800-byte blocks called FST Blocks (these
are sometimes referred to in listings as hyperblocks). Each FST block
contains 20 FST entries, each describing the attributes of a separate
file. Figure 13 shows the structure of an PST block and the fields
defined in the FST.
Master File Directory
Address of FSTB File Status Table Block (FSTB) File Status Table Entry
First Chain Link (FCLl CMS Record
Record n I ..... ---800-byte eMS Record Containing Fi!e Data Items _I Figure 12. How File Records Are Chained Together
Nth Chain Link {NCLl 2-86 IBM VM/370 System Logic and program Determination--Volu.e 2
File Status Table Block FST 1
FST 2
8 Fields in a File Status Table Entry FILE TYPE ____ ____ __________ __ __________ -, I FST 5 ': 20 Write Pointer 22 Read Pointer (Number of Item) (Number of Item) FST 6
FST 20 , "'r '.: __ 28 Disk Address 30 Fixed 31 Flag of 1st Chain Link Variable Byte
:: '\\\\l\l: : .. 36 Number of 800-B yte Data Blocks Year
32 Item Length (F)
Max. Item Length (V)
Figure 13. Format of a File Status alack; Format of a File Status Table
CHAIN LINKS Chain links are 200- or SOO-byte blocks of storage that chain the
records of a file in storage. There are two types of chain links: first
chain links and Nth chain links.
The first chain link points to two kinds of data. The first 80 bytes
of the first chain link contain the halfword addresses of the remaining qO chain links used to chain the records of the file. The next 120 bytes of the file are the halfword addresses of the first 60 records of
the file.
The Nth chain links contain only half word addresses of the records
contained in the file.
Because there are 41 chain
addresses for only 60 records), 16,060 SOO-byte records.
links (of which the first
the maximu. size for any CftS contains
file is CftS Method of Operation and Program Organization 2-87
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