Diagnosing and Fixing Conventional Memory Exhaustion in MS-DOS

Recognizable Condition

The system hangs during boot or displays an 'Out of memory' error when launching a program after several device drivers and TSRs have been loaded from CONFIG.SYS.

Cause and Diagnostic Overview

Cause	Diagnostic Indicator
Large drivers (e.g., ANSI.SYS, RAMDRIVE.SYS) consuming conventional memory	Running MEM /C shows conventional memory close to 0 KiB after the drivers load.
TSRs loaded before upper memory blocks (UMBs) are available	MEM /D reports high conventional usage and little or no UMB usage.

Ordered Checks

1. Boot the system and press F5 (or F8) to step through CONFIG.SYS lines.
2. After each driver loads, run MEM /C from the DOS prompt to record conventional memory free.
3. Identify the point where conventional memory drops sharply; note the driver or TSR loaded at that line.
4. Check the load order: ensure DEVICE=C:\\DOS\\HIMEM.SYS appears before any DEVICEHIGH lines and before DEVICE=C:\\DOS\\EMM386.EXE.

Fixes Tied to Findings

• Move bulky drivers into upper memory blocks using DEVICEHIGH after HIMEM.SYS and EMM386.EXE are loaded.

  DEVICE=C:\\DOS\\HIMEM.SYS
  DEVICE=C:\\DOS\\EMM386.EXE RAM
  DEVICEHIGH=C:\\DOS\\ANSI.SYS
  DEVICEHIGH=C:\\DOS\\RAMDRIVE.SYS 1024 /E
  

• Load TSRs with LOADHIGH (or LH) to keep them out of conventional memory.

  LOADHIGH=C:\\DOS\\MOUSE.COM
  LOADHIGH=C:\\DOS\\KEYB.COM
  

• If EMM386.EXE is used without the RAM switch, add it to enable UMB creation.

  DEVICE=C:\\DOS\\EMM386.EXE RAM
  

Escalation Criteria

If after applying the above changes conventional memory remains below approximately 50 KiB (insufficient for most applications), consider:

• Using a DOS extender (e.g., DOS/4GW) for the target program.
• Migrating to a protected‑mode environment such as Windows 95 or later.

Verification Steps

1. Reboot the system normally.
2. At the C:\\> prompt, run MEM /C and note the 'Free Conventional Memory' value; it should be comfortably above 100 KiB.
3. Run MEM /D to confirm that UMBs are being used (non‑zero 'Upper memory' and 'Reserved' values).
4. Launch the program that previously failed; it should start without an 'Out of memory' message.

Rollback Procedure

1. Boot with step‑by‑step confirmation (F5/F8) and decline the modified CONFIG.SYS lines.
2. Alternatively, rename the edited CONFIG.SYS to CONFIG.SYS.new and restore the original CONFIG.SYS from a backup.
3. Reboot and verify with MEM /C that conventional memory returns to its previous low value, confirming the change was reverted.