Choosing HIMEM.SYS vs EMM386.EXE for Conventional Memory Optimization in MS‑DOS

Problem Statement

Running memory‑hungry DOS applications (e.g., Turbo C, large spreadsheets, or games) often fails with the classic Out of memory error. The culprit is the limited 640 KB of conventional memory that DOS can address directly. Extended memory managers (EMMs) can expose additional memory, but the way they do so matters. Two common managers are HIMEM.SYS and EMM386.EXE. Choosing the right combination can free up tens of kilobytes of conventional memory for your programs.

Decision to Make

Should you load only HIMEM.SYS, or should you add EMM386.EXE to create Upper Memory Blocks (UMB) for drivers and TSRs? The answer depends on your hardware, software stack, and how much conventional memory you need.

Constraints

• Processor: 80286 or newer (required for HIMEM.SYS)
• RAM: Minimum 1 MB (necessary for HMA, XMS, and UMB)
• BIOS: Must support 286+ memory mapping; buggy chipsets may stall when UMBs are enabled
• CONFIG.SYS order: Device drivers that need UMB must be listed before any TSRs that rely on them

Options Comparison

Option	Memory Provided	Typical Conventional Memory Saved	Complexity
HIMEM.SYS only	HMA (64 KB) + XMS	~0 KB (HMA used by DOS itself)	Low
HIMEM.SYS + EMM386.EXE (no UMB)	HMA + XMS + EMS	~0 KB	Medium
HIMEM.SYS + EMM386.EXE with UMB	HMA + XMS + UMB (up to 64 KB)	Up to ~64 KB	Medium‑High
No extended manager	None	0 KB	None

Trade‑offs

• HIMEM.SYS only keeps the configuration simple and guarantees that all drivers stay in low memory. However, it offers no UMBs, so drivers that could otherwise be moved to upper memory remain in conventional memory, limiting the amount available to applications.
• Adding EMM386.EXE with UMB allows you to relocate drivers and TSRs into the 64 KB of upper memory, freeing that space for your programs. The trade‑off is a more complex CONFIG.SYS and a slight risk of incompatibility on older 286 boards that cannot handle UMB mapping.
• Using EMM386.EXE without UMB gives you EMS (Expanded Memory Specification) support, useful for applications that rely on EMS, but it does not free conventional memory.
• Skipping both managers is the simplest path but offers no memory optimization.

Concrete Implementation

Step 1 – Edit CONFIG.SYS

Place the following lines in CONFIG.SYS. Replace C:\\DOS with the actual path to your MS‑DOS installation if different.

DEVICE=C:\\DOS\\HIMEM.SYS
DEVICE=C:\\DOS\\EMM386.EXE NOEMS
DOS=HIGH,UMB

• DEVICE=C:\\DOS\\HIMEM.SYS loads the High Memory Area (HMA) and XMS support.
• DEVICE=C:\\DOS\\EMM386.EXE NOEMS loads the 386 EMM, disables EMS if you do not need it, and prepares UMB support.
• DOS=HIGH,UMB tells DOS to move the DOS kernel into the HMA and to create UMBs for drivers and TSRs.

Ensure that any DEVICE= entries that need UMB come before the DOS=HIGH,UMB line. If you have TSRs that require UMB, list them after the DOS=HIGH,UMB line.

Step 2 – Reboot and Verify

After rebooting, run the DOS memory manager to confirm that UMBs are allocated:

MEM /C

Look for a line such as Upper memory block size: 64 KB. This indicates that 64 KB of conventional memory has been moved to upper memory. The MEM /D command will display a detailed distribution of conventional, high, and upper memory usage.

Step 3 – Test with a Memory‑Intensive Application

As a practical check, start a program that historically runs out of memory, such as Turbo C 3.0, and confirm it compiles large projects without error. Successful compilation implies that conventional memory is now sufficient.

Common Pitfalls and Mitigation

• NOEMS Switch: Use NOEMS only if your applications do not require EMS. If you need EMS, omit the switch to provide EMS compatibility.
• Hardware Compatibility: Some early 80286 boards with buggy memory controllers may hang when UMBs are enabled. Test stability with a simple MEM /C before loading large applications.
• Driver Order: Incorrect ordering of DEVICE= lines can prevent drivers from loading into UMB, causing them to occupy conventional memory. Follow the order guidelines above.

Practical Example

Assume a system with 1 MB of RAM and a 286+ processor. The following CONFIG.SYS frees 64 KB of conventional memory:

# Sample CONFIG.SYS
DEVICE=C:\\DOS\\HIMEM.SYS
DEVICE=C:\\DOS\\EMM386.EXE NOEMS
DOS=HIGH,UMB

After reboot, a MEM /C output might look like:

Conventional memory:  640 KB   ( 640 KB available)
Upper memory block size: 64 KB
High memory area:  64 KB

Running Turbo C now compiles a 200 KB source file without hitting the memory limit.

Conclusion

For most DOS users needing a modest increase in conventional memory, loading HIMEM.SYS alone is sufficient. If you regularly run applications that push the 640 KB boundary, adding EMM386.EXE with the UMB switch is the optimal choice, provided your hardware supports it. Always verify with MEM /C and test with a memory‑intensive program before relying on the configuration for production use.