ABCs of z/OS System Programming Volume 3 - IBM Redbooks

4.42 VSAM: Mapping a linear data set
WINDOW
(Address space)
(Dataspace)
or
(Hiperspace)
Figure 4-52 Mapping a linear data set
LDS
BLOCK3
BLOCK4
BLOCK5
OFFSET
SPAN
Mapping a linear data set
To establish a map from a linear data set to a window (a program-provided area in multiples of
4 KB on a 4 KB boundary), the program issues:
► DIV IDENTIFY to introduce (allocate) a linear data set to data-in-virtual services.
► DIV ACCESS to cause a VSAM open for the data set and indicate access mode
(read/update).
► GETMAIN to allocate a window in virtual storage where the LDS will be mapped totally or
in pieces.
► DIV MAP to enable the viewing of the data object by establishing an association between
a program-provided area (window) and the data object. The area may be in an address
space, data space, or hiperspace.
No actual I/O is done until the program references the data in the window. The reference will
result in a page fault which causes data-in-virtual services to read the data from the linear
data set into the window.
DIV SAVE can be used to write out changes to the data object. DIV RESET can be used to
discard changes made in the window since the last SAVE operation.
Chapter 4. Storage management software 175