Total Memory available for system use - Mem Usage is the amount of RAM available for any new process to instantly use. If more than that is needed, then lower priority tasks start getting moved down into Virtual Memory to free up RAM. Just think of the 'committed' memory as the total amount of RAM that all the running processes would take up if there were no Virtual Memory to store them in. And remember that scientific programs consist of megabytes of code, kilobytes of data, and hundreds of megabytes of scratchpad arrays (initially all 0) to hold temporary intermediate calculations.