Solution:

The 256K SRAM can be seperate into three parts. 1. System Program used ( while the DOS run, some program should upload to the SRAM) 2. The Region used for the battery backup. usually about 60KB. You can resize it by the following command: "Set_NVRAM_Size" . For example, if you only need 4 KB sectors for NVRAM, then issue Set_NVRAM_Size(1), you'll get around 56 KB free memory. 3. The remained memory will be used as the D: , the setting is on the config.sys DEVICE=VDISK.SYS 64 128 16