"free" buffer field

[BH]

What does it mean when the field marked XXXXX is negative?

             total       used       free     shared    buffers     cached
Mem:       1028936      42792     986144          0       1072      24512
-/+ buffers/cache:      XXXXX    1011728
Swap:       787176          0     787176

(This was not the output at the time)

System is 2.4.12 with 1GB ram, P3 550 BX chipset.  I had a mounted ro NTFS
playing a 100mb mp3 with mpg123.  The Mem free field was rapidly declining
during this, and XXXXX was becoming more and more negative.  It all
stopped when I killed mpg123 and umounted the NTFS.

Thanks.