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Pictures for the 3279 have a maximum data stream of approximately
50K bytes. This can be compressed to approximately 25K for remote
operation. There are pictures which become too complex to display
fully on the 3279, because they cause PS overflow. PS overflow does
not occur on a 5279 or 5379, and there is no limit to the complexity of
the picture that can be displayed.
Specialized application areas, such as technical publications,
cartographies, and civil engineering, produce pictures of widely varying
complexity and data stream. These pictures could be less than 64K bytes
when displayed on a 5279 or 5379, but can range up to 200K bytes and
more.
GDDM application programs may use characters from fonts other than
the standard font for the work station.
If the work station session has been configured with sufficient
segment memory, the symbol set containing the font is sent to the
session's segment memory. GDDM simply references characters in
the font. The work of expanding these is carried out by the work
station, as are any transformations such as, scale, rotation, or
translation. The symbol set will remain there for the duration of
the GDDM session and can be used in subsequent picture displays.
Symbol sets cannot be shared between sessions in the work station.
GDDM-supplied symbol sets containing fonts are less than llK
bytes each.
If there is insufficient memory in the work station, the character
definitions are expanded in the host. For vector symbol sets, this
adds 50-100 bytes to the data stream for each character from the
symbol set.
•
The ability to manipulate the picture within the work station will bring
about performance gains for host application programs designed to take
advantage of this.
These gains can only be made if there is sufficient segment memory in
the work station for the GDDM application program to operate in
retained mode. In retained mode:
1.
If the host application program makes a change to one segment,
only that segment (rather than the whole picture) need be
retransmitted to the work station. In some cases, such as segment
deletion or repositioning, only control information (rather than the
segment itself) need be transmitted.
2.
Operators can perform functions such as moving and picking
segments in the work station. The work station will relay
information about what the operator has done back to the host
application program.
108
Introducing the IBM 3270 Personal Computer/G and /GX Work Stations
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