Using DEVICEHIGH in CONFIG.SYS to Load Drivers into Upper Memory

Useful answer

The DEVICEHIGH directive in CONFIG.SYS loads a device driver into the Upper Memory Area (UMA), freeing conventional memory for DOS applications.

Mechanism – worked configuration

Place the XMS driver first, then use DEVICEHIGH for the driver you want to move, and finally enable UMBs with DOS=HIGH,UMB.

; CONFIG.SYS snippet
DEVICE=C:\DOS\HIMEM.SYS          ; loads XMS driver into conventional memory
DEVICEHIGH=C:\DOS\EMM386.EXE NOEMS   ; loads EMM386 into UMA (creates UMBs)
DOS=HIGH,UMB                     ; tells DOS to use high memory and UMBs

After reboot, the MEM command shows how much conventional memory is free and lists loaded drivers in upper memory blocks (UMBs).

Limits

• Requires an 80286 or later CPU and a functional XMS driver (HIMEM.SYS).
• Total UMA size is typically limited to 384 KB (the area between 640 KB and 1 MB).
• The driver must be relocatable and must not rely on interrupts that exist only in low memory.
• If the UMA is fragmented or insufficient, the driver will fall back to conventional memory.

Common mistakes

• Placing DEVICEHIGH before the XMS driver (HIMEM.SYS) – the driver cannot be loaded because UMA support is not yet active.
• Specifying a non‑relocatable driver (e.g., some legacy BIOS extensions) – it will refuse to load into UMA and may cause a system hang.
• Omitting DOS=HIGH,UMB – DOS stays in conventional memory, so DEVICEHIGH has no effect and the driver loads low.
• Mixing DEVICEHIGH with LOADHIGH for TSRs without understanding the memory map – overlapping regions can corrupt memory.

Practical verification

1. Boot the MS-DOS system (or a faithful emulator such as PCem) with the edited CONFIG.SYS.
2. At the C:\> prompt run MEM. Look for a line similar to Total conventional memory: 640K and a list of Upper memory blocks showing the driver’s name.
3. Optionally run MEM /C to see a detailed map of loaded modules and confirm the driver resides in an UMB segment (addresses above 0xA0000).
4. If the driver appears in the conventional memory section, revisit the order of directives or check driver relocatability.