Fixing ‘Out of Memory’ in MS‑DOS: A Step‑by‑Step Diagnostic Guide

Recognizing the Problem

When a DOS program refuses to start and the console flashes Out of memory even though the machine has 640 KB of RAM, the cause is almost always a lack of free conventional memory. Conventional memory is the first 640 KB of the address space that DOS and all resident programs use. If that area is exhausted, DOS can’t load anything new.

Why Conventional Memory Gets Exhausted

• DOS kernel (IBMDOS.COM + IBMBIO.COM) occupies ~64 KB.
• Device drivers and TSRs that are loaded with DEVICE or LOAD consume conventional memory.
• Extended memory (above 1 MB) is inaccessible to 16‑bit DOS programs unless it is first mapped into upper memory blocks (UMB) via HIMEM.SYS or a memory manager.

Diagnostic Checklist

Check	What to Look For
1. MEM output	Free conventional memory < 40 KB?
2. HIMEM.SYS loaded?	"HIMEM.SYS" in MEM or boot messages
3. CONFIG.SYS lines	Presence of DOS=HIGH,UMB before any DEVICE
4. Driver placement	Drivers listed with DEVICE vs DEVICEHIGH
5. TSR invocation	TSRs loaded via LOADHIGH?
6. EMM386.EXE status	Is it loaded? With or without NOEMS?

Step 1 – Run MEM

At the DOS prompt, type:

C:\> MEM

You’ll see a table listing total, used, and free conventional memory. If the free value is below ~40 KB, the system is likely the problem.

Step 2 – Verify HIMEM.SYS

Check the boot messages or the MEM output for a line ending in HIMEM.SYS. If it’s missing, extended memory can’t be mapped into UMB.

Step 3 – Inspect CONFIG.SYS

Open CONFIG.SYS with a text editor and look for:

DEVICE=C:\DOS\HIMEM.SYS
DOS=HIGH,UMB
DEVICE=C:\DOS\SOME.DRV
DEVICEHIGH=C:\DOS\ANOTHER.DRV

• DOS=HIGH,UMB tells DOS to load its kernel into the UMB, freeing conventional memory.
• Place DOS=HIGH,UMB **before** any DEVICE lines that load drivers into conventional memory.

Step 4 – Move Drivers to UMB

Replace low‑loaded drivers with DEVICEHIGH where the driver supports it. Example:

DEVICE=C:\DOS\KEYBOARD.DRV   ; loads into conventional memory
DEVICEHIGH=C:\DOS\KEYBOARD.DRV ; loads into UMB

Not all drivers support DEVICEHIGH; check the driver’s documentation.

Step 5 – Load TSRs with LOADHIGH

In AUTOEXEC.BAT, replace plain LOAD or CALL with LOADHIGH if the TSR supports it:

LOADHIGH C:\DOS\MEMMAKER.EXE

Step 6 – Use EMM386.EXE for UMBs (Optional)

If you need more than the default 64 KB of UMB, load EMM386.EXE in CONFIG.SYS:

DEVICE=C:\DOS\EMM386.EXE NOEMS

The NOEMS switch disables EMS support, which is unnecessary for most DOS applications and reduces the risk of memory corruption.

Fix Summary

• Add DEVICE=C:\DOS\HIMEM.SYS and DOS=HIGH,UMB early in CONFIG.SYS.
• Replace low‑loaded drivers with DEVICEHIGH where possible.
• Load TSRs via LOADHIGH in AUTOEXEC.BAT.
• Optionally add EMM386.EXE NOEMS for extra UMBs.

Verification

1. Reboot and run MEM again. Free conventional memory should increase by ~46 KB (the size of the DOS kernel) plus any driver/TSR savings.
2. Launch the previously failing application; it should start without the memory error.
3. Optional: run MEMMAKER /S to let the optimizer confirm the configuration is near‑optimal.

Escalation Criteria

If, after applying the above fixes, conventional memory remains insufficient:

• Consider installing a third‑party memory manager such as QEMM or 386MAX.
• Upgrade to a newer DOS release (e.g., DOS 5.0 or DOS 6.22) that has improved memory handling.
• Move to a protected‑mode environment (Windows 95/98, OS/2, or a Linux console) where 32‑bit memory addressing is available.

Potential Pitfalls

• Loading too many drivers into UMB can destabilize older chipsets; test after each change.
• Using a HIMEM.SYS version that does not match your CPU (e.g., HIMEM.SYS 2.xx on an 80386) may fail to enable the A20 line, blocking high‑memory access.
• Adding EMM386.EXE with the RAM option on systems lacking compatible extended memory can hang the machine or corrupt memory.