X PROG 761: Invalid partition ID

What it means

Write Structured Field (WSF) is the 3270 command for anything more complex than painting a screen. Behind the command comes one or more structured fields. Each one starts with a two-byte length, then an ID that says what kind of field it is, then its data. Common outbound structured fields include Read Partition (used to query the terminal), Erase/Reset, Set Reply Mode, Load Programmed Symbols and Outbound 3270DS, which carries an ordinary screen write inside a structured field.

Many structured fields name a partition. A partition is an area of the screen with its own buffer. Every 3270 session has the implicit partition, ID 0, which is the whole screen and is what nearly all applications use. Some terminals also support explicit partitions, which the host creates with the Create Partition structured field and then addresses by their own IDs. A special ID of X'FF' is used with Read Partition to query the terminal rather than read a partition.

If a structured field names a partition that hasn't been created, or uses an ID that isn't allowed for that kind of field, the terminal can't act on it. It shows X PROG 761 and locks the keyboard.

Many emulators support only the implicit partition. An application that relies on explicit partitions will fail on them unless it checks first.

Common causes

• The application writes to an explicit partition that it never created in this session, for example after a reconnect that reset the terminal.
• The application uses explicit partitions, but the emulator supports only the implicit partition.
• A structured field uses X'FF' where a real partition is required, or a real ID where X'FF' is required.
• A program building structured fields put the wrong byte in the partition ID position because of an offset error.

How to find the problem

1. Take a data stream trace and find the WSF command. For each structured field that names a partition, read the partition ID. In most of them it is the byte right after the structured field ID.
2. Check whether a Create Partition structured field for that ID appears earlier in the same session.
3. Check the terminal's Query Reply. It shows whether the terminal supports explicit partitions.
4. If the error started after a reconnect or a Clear, check whether the application recreates its partitions in that case.

How to fix it

• For the user, press Reset and Clear and start the function again.
• Programmers should send Read Partition Query at the start, use explicit partitions only if the reply supports them, and use partition 0 otherwise.
• Create each partition before writing to it, and create it again after anything that resets the terminal.
• Use X'FF' only with Read Partition queries, and real partition IDs everywhere else.
• Correct the offsets in code that builds structured fields by hand.

Technical reviewer: Albert Joseph, WND Software.

Last reviewed 2026-10-18 by the WND Software team. Written in our own words. For the full reference, check IBM’s 3270 or 5250 display documentation.