I'm curious about memory status displayed after typing "mem" at the DOS command prompt.

655360 bytes total conventional memory
655360 bytes available to MS-DOS
598448 largest executable program size

1048576 bytes total contiguous extended memory
0 bytes available contiguous extended memory
941056 bytes available XMS memory
MS-DOS resident in High Memory Area

I see "0 bytes available contiguous extended memory" on 2 computers I have, both with Windows 7.
Is this status ever not "0 bytes available..."? Does this effect PC performance? Thanks.