IBM 2250 Display Unit Model 4 - All about the IBM 1130 Computing ...

Display program information consists of orders
and data. Orders either initiate a 2250 operation
or establish a mode. Order-initiated operations
include point and vector plotting, branching, and
CPU interrupt generation. Two orders, Set
Graphic Mode and Set Pen Mode, establish a
Graphic mode and a Light Pen mode respectively.
The 2250 is always in one of two Graphic modes
and in one of four Light Pen modes.
Data is defined as information that does not
contain an operation code. Character stroke words
are the only data received by the 2250. Although
a character stroke word may contain one or more
control bits, these bits are used directly to perform
an operation.
SUBROUTINES
Single-level subroutines (linkage from the main
order program to the order subroutine and return
to the main order program) are used frequently in
graphic application. Thus, facilities for a rapid
(unconditional) branch to a subroutine and return
from the subroutine are provided. Since characters
are similar to single-level subroutines, rapid
branching significantly reduces character display
time.
Orders in the display program enable multiplelevel
subroutine linkages to be performed. A
single-level subroutine facility does not allow
characters to be displayed as part of a subroutine,
nor does it permit the organization of an image in
a hierarchy of graphic segments represented by
multiple-level subroutines, as follows:
Main Program 1st Level Subroutine
Image .