To install a program, the Runner pays the program's install cost and places it in his program row. Each program also has a memory cost. The Runner cannot have programs installed that have a combined memory cost greater than his available memory units (MU). The Runner begins the game with four MU, though certain card effects can increase or decrease this value.
If the MU costs of the Runner's installed programs ever exceed his available MU, he must trash his installed programs until he is no longer exceeding his available MU.
The Runner can choose to trash any number of his installed programs at the beginning of an install program action.