Partitions are the first of several
special hardware features that BMS supports. Standard
BMS is required for partitions.
Some IBM® displays allow you to divide the screen into areas which you can
write to and read from separately, as if they were independent screens. The
areas are called partitions, and features of BMS that allow you to take advantage
of the special hardware are collectively called "partition support".
The IBM 3290 display, which is a member of the 3270 family, and the IBM 8775 are the primary examples of devices
that support partitioning. You should consult the device manuals9
to understand the full capabilities of a partitioned device, but the essential
features are these:
- You can divide the physical screen into any arrangement of one to eight
non-overlapping rectangular areas. The areas are independent from one other,
in the sense that the operator can clear them separately, the state of the
keyboard (locked or unlocked) is maintained separately for each, and you write
to and read from them one at a time.
- Only one partition is active at any given time.
This is the one containing the cursor. The operator is restricted to keying
into this partition, and the cursor wraps at partition boundaries. When a
key that transmits data is depressed (the ENTER key or one of the program
function keys), data is transmitted only from the active partition.
- The operator can change the active partition at any time by using the "jump"
key; your program can also, as explained in Determining the active partition.
- BMS also writes to only one partition on a given SEND, but you can issue
multiple SENDs and you do not have to write to the active partition.
- The partition configuration is sent to the device as a data stream, so
that you can change the partitions for each new task or even within a task.
The BMS construct that defines the partitions is called a partition set and is described in Partition definition.
- You also can use the terminal in base state (without
partitions) and you can switch from partitioned to base state with the same
command that you use to change partition arrangements.
- When you specify how to partition the screen area, you also divide up
the hardware buffer space from which the screen is driven. In partitioned
devices, the capacity of the buffer is generally greater than that of the
screen, so that some partitions can be assigned extra buffer space. The screen
area allocated to a partition is called its viewport
and the buffer storage is called its presentation space.
BMS uses the presentation
space as its page size for the partition, so that you can send as much data
as fits there, even though not all of it can be on display at once. Keys on
the device allow the operator to scroll the viewport of the partition vertically
to view the entire presentation space. Scrolling occurs without any intervention
from the host.
- Some partitioned devices allow you to choose among character sets of different
sizes. We talk about this in 3290 character size.
In spite of the independence of the partitions, the display is still a
single terminal to CICS®. You cannot have more than one task at a time with
the terminal as its principal facility, although you can use the screen space
cooperatively among several pseudoconversational transaction sequences if
they use the same partition set (see Terminal sharing).
Note:
The 3290 can be configured internally so that it behaves as
more than one logical unit (to CICS or any other system); this definition
is separate from the partitioning that may occur at any one of those logical
terminals.
This chapter describes:
IBM 3290 Information Display Panel Description and Reference for the
3290 and IBM 8775 Display Terminal Component Description for the 8775.
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