IBM 3270

hardware A class of terminals made by IBM known as

"Display Devices", normally used to talk to IBM

mainframes. The 3270 attempts to minimise the number of

I/O interrupts required by accepting large blocks of data,

known as datastreams, in which both text and control (or

formatting functions) are interspersed allowing an entire

screen to be "painted" as a single output operation. The

concept of "formatting" in these devices allows the screen to

be divided into clusters of contiguous character cells for

which numerous attributes (color, highlighting, characterset, protection from modification) can be set. Further,

using a technique known as 'Read Modified' the changes from

any number of formatted fields that have been modified can be

read as a single input without transferring any other data,

another technique to enhance the terminal throughput of the

CPU.

The 3270 had twelve, and later twenty-four, special Programmed

Function Keys, or PF keys. When one of these keys was

pressed, it would cause the device to generate an I/O

interrupt and present a special code identifying which key

was pressed. Application program functions such as

termination, page-up, page-down or help could be invoked by a

single key-push, thereby reducing the load on very busy

processors.

A version of the IBM PC called the "3270 PC" was released in

October 1983. It included 3270 terminal emulation.

tn3270 is modified version of Telnet which acts as a 3270

terminal emulator and can be used to connect to an IBM

computer over a network.

See also broken arrow.

(1995-02-07)