Edit CONFIG.SYS and update/insert these lines:
Note the HIMEM.SYS is a driver that forfills the eXtended Memory Specification (XMS), which specifies how to allocate memory beyond the first 1024 KByte and therefore also the HMA. The memory beyond the HMA is called extended memory or just XMS.
More Info MS KB91088
More Info MS KB72302
More Info MS KB96711
More Info MS KB105557