[
LINE=nJ LINE=O is the number of the line, 0 to 23, on the display terminal that is to be
written. Line number 0 is the default.
[
BYTES=bbbb ] BYTES=1760 is the number of bytes (0 to 1760) to be written on the display termi­
nal. 1760 bytes is the default. [ERASE=YES] specifies that the display screen is to be erased before the current
data is written. The screen is erased regardless of the line or number
of bytes to be displayed. Specifying ERASE= YES causes the screen
to go into "MORE" status.
[CANCEL=YES] causes the CANCEL operation to be performed; the output area
is erased.
Note: It is advisable for the user to save registers before issuing the DISPW macro
and to restore them after the macro, because the modules called by DISPW macro
do not save the user's registers. The DISPW macro saves and restores register 13.
Functional Information 351

Using the DASD Block I/O System Service from eMS The DASD Block I/O System Service provides a virtual machine with
device-independent access to its virtual DASD devices. Programs using the DASD Block I/O System Service bypass the CMS file system, and they read or write
directly from CP. Before using the DASD Block I/O System Service, you should issue the CMS RESERVE command and the CMS DISKID function.
The CMS RESERVE command allocates all available blocks of a 512-, 1K-, 2K-,
or 4K-byte block formatted minidisk to a unique CMS file. The file created has the
following format:
filename, filetype, and filemode letter the user specified
filemode number 6, if the filemode number was not specified in the command
logical record length equal to the CMS disk block size
fixed (F) record format
the number of records is the total number of blocks available on the disk minus
the number of blocks used by CMS. The number of blocks used by CMS is
referred to as the offset. This CMS overhead varies with the size of the mini­
disk. The data blocks physically follow the blocks used by CMS. The file created can be read or written via the DASD Block I/O System Service or
the CMS file system. Because a CMS file structure has been created on the disk,
the file may be accessed using the CMS file system. Let's consider the following
example:
Suppose you have a 3330 device with one cylinder formatted with 1024-byte block
size. There will be 209 blocks available. After you issue the RESERVE command,
the file created has the following format:
1 I 2 I 3 I 4 I 5 I 6 I 7 I 8 I 9 110 III I ... 120712081209 where:
Physical block
number(s)
1 and 2
3
4 or 5
6
7
8
9 through 209 Description
Contain the IPL records
Contains the volume label
Contain the CMS directory file
Contains the allocation map
Contains the alternate allocation map CMS level 1 pointer block
Data blocks
Physical blocks 1 through 8 are the blocks used by the CMS file system. Physical
blocks 9 through 209 are the data blocks. Next, issue the following format of the
FILEDEF command:
352 VM/SP System Programmer's Guide