XMS Memory Is Not Reported Until Accessed (77219)

SUMMARY

XMS (extended memory specification) memory may not be reported by the MEM command, even if HIMEM.SYS is properly loaded and extended memory is available. XMS memory will not be reported until it has been accessed.

MORE INFORMATION

To reproduce this result, create a CONFIG.SYS file as follows:

   device=c:\dos\himem.sys
   files=30
   buffers=10
   shell=c:\dos\command.com c:\dos\ /p

and AUTOEXEC.BAT as follows:

   echo off
   prompt $p$g
   path=c:\dos;c:\

If you boot your computer with 2 MB of extended memory and the above configuration and immediately run the MEM.EXE command, note that MEM will report the following:

2097152 bytes total contiguous extended memory
2097152 bytes available contiguous extended memory

Because no program has yet accessed XMS memory, no XMS memory will be reported. As soon as an XMS "user" (for example, DOS=HIGH, SMARTDRV.SYS) is loaded, all of the contiguous extended memory will be reported as available XMS memory, less the 64K high memory area (HMA) controlled by HIMEM.SYS itself.