MFS bypass for the 3270 or SLU 2

IMS MFS allows the IMS application program to bypass MFS formatting of input and output messages.

With this option, the IMS application program can load programmed symbol buffers, or send a device-dependent data stream to format and update the 3270 display, or write a message to a 3270 printer. The bypass can be used only on the SLU 2, and 3270 devices. Optionally, the IMS application program can examine an input message with the attention identification (AID) byte, cursor address, SBA orders, and buffer addresses as received from the display. For non-SNA VTAM® transmissions, the data to be sent must be equal to or less than the value specified in the system definition OUTBUF parameter. Data sent to a printer using the MFS bypass is restricted to 4 KB.

MFS recognizes two special message output descriptor (MOD) names: DFS.EDT and DFS.EDTN.

Output messages bypass MFS formatting only if DFS.EDT or DFS.EDTN is supplied as the MOD name parameter of the application program CALL statement IMS system messages, IMS error messages, application program messages with no MOD name, and message switches are always formatted by MFS (using the IMS-supplied formats).

When MFS is bypassed on output, the application program is responsible for constructing the entire 3270 data stream, beginning with the command code and ending with the last data byte. An exception to this could be 3270 output using the MFS bypass and destined to a printer. The hexadecimal EBCDIC command codes for use with the 3271/3274 controllers are:
Command
3271/3274
Erase All Unprotected
6F
Erase/Write
F5
Erase/Write Alternate
7E
Read Buffer
F2
Read Modified
F6
Read Modified All
6E
Write
F1
Write Structured Field
F3

The user-written application program has two ways to send output to printers:

  • By providing the command code and WCC character in the application program and by setting bit 0 to 1 (X'80') in the Z2 field of the message segment to show that the appropriate command is provided.
  • By allowing IMS to provide the command code and other characters. However, to print less than the maximum line length, insert new line (NL) characters at the appropriate places in the data stream. This method is the default.