depends upon the type of the ESD item, as does the IBPO field. ESD Item VALUE Field SD (CSECT or START) lbsolue address
Absolue address
Absolue address
Assigned value
(starting fro. 0) LD (ENTRY) CM (COftMON) PR (Pseudo register)
FLAG2 Byte 0 Unused i Unused 2 Unused 3 Unused It!! l1§ni]!g 4 Unused 5 Name was located in a TITLIB
6 Section definition entry
7 Name specifically loaded from co •• and line.
Entries may be created in the loader reference table prior to tbe
actual defining of the symbol. For example, an entry is created for a
symbol if it is referenced by means of an EITRN (ER) even if the symbol
has not yet been defined or its type known. Furthermore, coa.on (CM) is
not assigned absolute addresses until prior to the start of execution by
the START command. These circumstances are determined by the setting of the flag byte;
if the symbol's value has not yet been defined, the value field
specifies the address of a patch control block (PCB). PATCH CONTROL BLOCK (PCB) These are allocated from free storage and pointed at from REFTBL entries
or other PCBs. Mea.!!i.!!.9 Address of next PCB 5-7 Location of ADCON in storage
4 Flag byte
All address constant locations in loaded program for undefined syabols are placed on PCB chains. LOADER INPUT RESTRICTIONS All restrictions which apply to object files for the as linkage editor
apply to CMS loader input files. Processing Commands that Manipulate the File System
Figure 9 lists the CMS modules that perfora either general file systea support functions or that perform data manipulation. CftS Method of Operation and Program Organization 2-85

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