X PROG 752: Invalid buffer address

What it means

A 3270 screen is a buffer of character positions numbered from zero. A 24 by 80 screen has 1,920 positions, numbered 0 to 1919. A 27 by 132 screen has 3,564. The host places data on the screen with orders that carry a buffer address. Set Buffer Address (SBA) moves the write position. Repeat to Address (RA) fills up to an address with one character. Erase Unprotected to Address (EUA) clears input fields up to an address.

If any of those addresses is beyond the last position of the screen in use, the terminal can't follow it. It rejects the write, shows X PROG 752 and locks the keyboard.

Addresses are coded in a compact form. The common 12-bit form can reach position 4095, and a 14-bit form exists for larger screens. A wrong form, or a byte pair that has been damaged, also decodes to a position that isn't on the screen.

Common causes

• The application formats screens for a larger model than the session. A map built for 27 by 132 (Model 5) or 43 by 80 (Model 4) sent to a 24 by 80 (Model 2) session is the classic case.
• The terminal definition on the host says the terminal has an alternate screen size it doesn't really have, so the application uses Erase/Write Alternate and addresses beyond the default size.
• The emulator session was changed to a smaller model, but the host still has the old size from an earlier sign-on or a fixed definition.
• A program that builds the data stream by hand computed a row or column wrong, for example counting rows from 1 instead of 0, or forgot to encode the address.

How to find the problem

1. Check the model or screen size set in the emulator session. Compare it with what the application expects. Many applications state the screen size they need.
2. Ask the CICS team to look at the terminal's definition (TYPETERM DEFSCREEN and ALTSCREEN, or what autoinstall chose). In IMS, check the device type and size the MFS formats were built for. In TSO and ISPF, check what screen size the session reports.
3. Take a data stream trace and find the SBA (X'11'), RA (X'3C') and EUA (X'12') orders. Decode the two address bytes that follow each one and see which one points past the end of the screen.
4. Note whether the failing write uses Erase/Write or Erase/Write Alternate. Erase/Write Alternate switches to the alternate size, so its addresses are checked against that size.

How to fix it

• For the user, press Reset, then Clear. If the error repeats, change the emulator session to the screen model the application needs, or ask which model to use.
• Make the host definition match the emulator. In CICS, set DEFSCREEN and ALTSCREEN on the TYPETERM to the real sizes, and check which autoinstall model and logmode the session uses.
• Programmers should compute addresses from the actual screen width and height, with rows and columns counted from zero, and use the 14-bit address form for positions above 4095.
• If the application needs a large screen, check the size at the start and give the user a clear message instead of sending a screen that can't fit.

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.