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
CftS RECORD FORftATS CMS records are BOO-byte blocks containing the data that comprises the
file. For example, the CMS record may contain several card images or
print images, each of which is referred to a record item. Figure 14
shows how chain links are chained together. CftS records can be
variable-length format.
single file.
stored on disk in either fixed-length or
However, the two formats may not be mixed in a
Regardless of their format, the items of a file are stored by CMS in
sequential order in as many BOO-byte records as are required to
accommodate them. Each record (except the last) is completely filled
and items that begin in one record can end on the next record. Figure
15 shows the arrangement of records in files for files containing
fixed-length records and files containing variable-length records.
The location of any item in a file containing fixed-length records is determined by the formula: (Item Number - 1) x Record Length
locations = --------------------------------- 800 where the quotient is the number of the item and the remainder is the
displace.ent of the item into the file.
For variable-length records, each record is preceded by a 2-byte
field specifying the length of the record.
Disk Organization CftS virtual disks (also referred to as minidisks) are blocks
designed to externally parallel the function of real disks.
virtual disks may reside on one real disk.
of data Several A CftS virtual machine may have up to 10 virtual disks accessed during
a terminal session, depending on user specifications. Some disks, such
as the S-disk, are accessed during eMS initialization; however, most are
accessed dynamically as they are needed during a terminal session. PHYSICAL ORGANIZATION OF VIRTUAL DISKS Virtual disks are physically organized in 8CO-byte Records 1
and 2 of each user disk are reserved for IPL. Record 3 contains the disk
label. Record 4 contains the master file directory. The records on the disk contain user file-related information such as the FSTs, chain links, and the individual file records discussed above.
2-88 IBM VM/310 System Logic and Program Determination--Volume 2
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