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

Disk Address of 11 2nd Chatn Link Disk Address of Disk Address of
3rd Chain Link 1\. Oth Data Block I Disk .Aridress of
1 I 1\. 1st Data Block \ Chain Linkage 1 'I Directory
l • • Disk Address of 40th Chatn Link I
J • 1
Disk Address of
41 st Chatn Link r • T
Disk Address of I I 1st Data Block I I Disk Address of
! I I I 2nd Dala Block -1 ! I I ...l, rL T J Disk Address of h 398 th Data Block I Disk Address of 1\+ 3991h Data Block I D,Sk Address of 1 59th Data Block A" (n-21 • 400' 61 I I wheren = Cha!n Link, Number
Disk Address of 60th Data Block
Figure 14. Format of the First Chain Link and Nth Chain Links
Data block structure for file consisting of fixed-length records Data block structure for file consisting of variable-length records I sao
1st record 1 --------------
2nd record
1st record
sao Sq«l L2J - - - 2nd record T
- ---------'------------ [9
-------[9-------- L3 3rd record L 4
- -------
3rd record SOO sao ----------------1 ____ _
4th record SOO sao
4th record I L Tt 5th record I 800 I Figure 15. Arrangement of Fixed-Length Records
Records in Files
and Variable-Length eMS Method of Operation and Program Organization 2-89