Maximizing Conventional Memory in MS-DOS 5.0–6.22 with HIMEM.SYS and EMM386.EXE

The Problem: Conventional Memory Ceiling

MS-DOS applications can only address the first 640 KB of RAM, known as conventional memory. Device drivers, terminate-and-stay-resident programs (TSRs), and the DOS kernel itself consume this space, often leaving too little for large programs such as spreadsheets, databases, or games. The solution is to relocate as much of this overhead as possible into two regions above 640 KB: the High Memory Area (HMA) — the first 64 KB of extended memory — and Upper Memory Blocks (UMBs) — unused gaps in the 640 KB–1 MB address range reserved for hardware ROMs.

Desired Outcome

After configuration, MEM /C /P should report Total conventional memory free above 600 KB, the HMA showing MS-DOS resident, and multiple drivers/TSRs listed under Modules using upper memory. The target application must launch without an "Out of memory" error.

Prerequisites

• MS-DOS 5.0, 6.0, 6.2, or 6.22 installed on the boot drive (C:).
• HIMEM.SYS (XMS driver) and EMM386.EXE (EMS/UMB provider) present in the DOS directory (typically C:\DOS).
• An Intel 80386 or later CPU — EMM386.EXE requires 386 protected-mode features.
• Backups of the original CONFIG.SYS and AUTOEXEC.BAT on a separate floppy or partition.

Procedure: CONFIG.SYS Configuration

Edit C:\CONFIG.SYS with a text editor (e.g., EDIT). Order is critical: HIMEM.SYS must load first, then EMM386.EXE, then the DOS=HIGH,UMB directive.

DEVICE=C:\DOS\HIMEM.SYS /TESTMEM:OFF
DEVICE=C:\DOS\EMM386.EXE NOEMS I=B000-B7FF
DOS=HIGH,UMB
DEVICEHIGH=C:\DOS\ANSI.SYS
DEVICEHIGH=C:\DOS\SETVER.EXE
FILES=30
BUFFERS=20

Parameter notes:

• /TESTMEM:OFF skips the extended memory test at boot, saving several seconds.
• NOEMS disables Expanded Memory Specification (EMS) page framing, freeing more UMB space for drivers. Use RAM instead if any application requires EMS.
• I=B000-B7FF includes the monochrome video ROM region as usable UMBs. Adjust or add X= (exclude) ranges for network adapter ROMs, SCSI BIOS, or VGA BIOS at C000-C7FF. Run MEM /D after first reboot to see actual UMB layout.
• DEVICEHIGH= loads the specified driver into a UMB instead of conventional memory.

Procedure: AUTOEXEC.BAT Configuration

Edit C:\AUTOEXEC.BAT. Prefix each TSR with LOADHIGH (or its alias LH) to request UMB placement.

@ECHO OFF
PROMPT $P$G
PATH C:\DOS;C:\WINDOWS
LH C:\DOS\MOUSE.COM
LH C:\DOS\MSCDEX.EXE /D:MSCD001
LH C:\DOS\SMARTDRV.EXE 2048 1024
SET TEMP=C:\TEMP

SMARTDRV.EXE parameters shown are cache size (2 MB) and minimum free space (1 MB); adjust for your disk size.

Expected Checks After Reboot

1. At the command prompt, run MEM /C /P. Verify:
   • Total conventional memory free ≥ 600 KB.
   • High memory area line reads MS-DOS resident.
   • Upper memory blocks section lists ANSI.SYS, MOUSE.COM, MSCDEX.EXE, SMARTDRV.EXE (or your specific drivers/TSRs).
2. Run MEM /D to inspect UMB fragmentation. Each block shows Free or a module name. If large gaps remain but drivers failed to load high, UMBs may be fragmented — see Cautions.
3. Launch the target application (e.g., 123.EXE for Lotus 1-2-3, DOOM.EXE). Confirm it starts without memory errors.

Recovery Options

If the system hangs, displays "Device driver not found," or reports "UMB not available":

• F5 during "Starting MS-DOS..." bypasses both CONFIG.SYS and AUTOEXEC.BAT entirely, booting to a minimal environment.
• F8 enables step-by-step confirmation for each line in both files; press N on the problematic line.
• Boot from a clean floppy containing only DEVICE=A:\HIMEM.SYS in CONFIG.SYS, then copy your backed-up CONFIG.SYS and AUTOEXEC.BAT back to C:\.

Cautions and Common Pitfalls

• Hardware conflicts: Some network cards (e.g., 3Com 3C509) or SCSI host adapters reserve UMB ranges that EMM386 must exclude. Add X=C800-CFFF (or the adapter's documented range) to the EMM386.EXE line.
• UMB fragmentation: If MEM /D shows many small free blocks but DEVICEHIGH/LOADHIGH fails, try FRAME=E000 on the EMM386.EXE line to force a 64 KB page frame at a different address, or switch to RAM mode (enables EMS) which reorganizes UMBs differently.
• Third-party memory managers: QEMM, 386MAX, or Helix Netroom replace Microsoft's drivers entirely. Do not mix them with HIMEM.SYS/EMM386.EXE — instability will result.
• TSR refusal: Certain TSRs (e.g., older SHARE.EXE, some copy-protection drivers) cannot relocate. They remain in conventional memory, limiting gains. Check vendor documentation for /H or /LH switches.
• Line ordering: DOS=HIGH,UMB must appear after both HIMEM.SYS and EMM386.EXE. Placing it earlier prevents UMB creation.

Verification Checklist

Check	Command	Pass Criteria
Conventional memory free	MEM /C	≥ 600 KB
Largest executable program size	MEM /C	Approaches 600 KB
HMA status	MEM /C	Shows "MS-DOS resident"
UMB module list	MEM /D	All intended drivers/TSRs appear under "Modules using upper memory"
Application launch	Run target app	No "Out of memory" error
Stability	Multiple warm/cold boots + typical workload	No hangs, crashes, or data corruption

Limitations

This approach applies only to real-mode MS-DOS 5.0–6.22 on 386+ hardware. It does not apply to Windows 3.1x running in 386 Enhanced Mode (which uses its own virtual memory manager), nor to DOS extenders (DPMI) that bypass conventional memory entirely. Maximum conventional memory free is bounded by the number and size of UMBs available on a given machine — typically 600–620 KB is the practical ceiling.