MEM /C is not a diagnostic. It is a census, and every census is conducted in preparation for a war.
There is a ritual. Every DOS power user has performed it. It begins with the command MEM /C, which displays a table of every program currently resident in conventional memory, its size in bytes, and its location. The user studies this table with the intensity of a general studying a battlefield map. COMMAND.COM: 3,536 bytes. HIMEM: 1,168 bytes. DOSKEY: 4,144 bytes. MOUSE: 17,552 bytes. The numbers are memorized. The total free conventional memory is noted — and it is never enough. The goal, the holy grail, the number that haunts the dreams of every serious DOS user, is 620K free. Six hundred and twenty thousand bytes of contiguous conventional memory, available for programs. Most machines, after a standard boot, offer somewhere between 580K and 600K. The gap between what you have and what you need is measured in kilobytes, and those kilobytes might as well be continents. The tools of optimization are precise and unforgiving. HIMEM.SYS, loaded in CONFIG.SYS with the line DEVICE=C:\DOS\...
From the lore of Conventional Memory.