Determining The Number Of System Buffers; Receive Queues; Device Buffers - IBM 3708 Description

Determining the Number of System Buffers
You can determine how many system butTers have been defined for your
configuration and also how many are "free" at any given time by doing the
following procedure:
Using the 3708 control terminal, display memory at location IBOA for 2 bytes.
The data displayed is the count (in hex) of the number of system butTers that
exist with the active configuration. Then display memory at location IBI0 for 2
bytes. The data displayed is the count (in hex) of free butTers; This number
changes dynamically as data is transferred through the 3708, and as sessions
become active or inactive.
Receive Queues
Device Buffers
The receive queues can be configured as small (79 characters), medium (519
characters), and large (1079 characters). Small is the default. The larger sizes are
provided primarily for protocol enveloping file transfer or data streaming
applications of data from the terminal to the host having large block transfer sizes
up to 540 or 1080 characters. Data blocks of up to these sizes might be sent by the
terminal (or an IBM Personal Computer) in protocol enveloping mode to the host
without any pause or acknowledgement. Medium or large receive queue sizes should
be configured as necessary to satisfy the requirements of large block transfers from
the terminal to the host. However, use of larger than necessary receive queue sizes
provides no advantage, and unnecessarily uses up RAM space that would otherwise
be available for system butTers.
Device butTers are used with each port except SNA host ports. The butTers are 2126
bytes long, unless the port is configured to allow model 3, 4, or 5 emulation. The
following table shows the butTer size for these ports:
Model
Number
3
4
5
Buffer
Size
2808
3746
3860
When a port is being used in protocol enveloping mode, the device butTer is merely
used as a work area into which host data is copied during the translation step before
being transmitted to the terminal.
When a port is being used in protocol conversion mode for an LV type 2 (keyboard
display) or LU type 3 (DSC printer), the device butTer is used to contain the ASCII
screen image as the image is formed during the translation step. Data is represented
by 7-bit ASCII codes. The 3270 field attribute bytes are also imbedded in the screen
image and represent the start of fields.
When a port is being used in protocol conversion mode for an LV type I (SNA
character set printer), the device butTer is used to contain the expanded ASCII data
stream formed as the host data is translated and the SCS printer orders are
encountered. Data is represented by 7- or 8-bit ASCII codes.
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IBM 3708 Description