A control store is the part of a CPU's control unit that stores the CPU's microprogram. It is usually accessed by a microsequencer.
A control store is usually implemented as a diode-array of read-only memory on modern VLSI CPUs. Historically, IBM mainframes loaded their microprograms from floppy disks into a control store consisting of ultra-high speed random-access memory. This permitted IBM to easily repair microprogramming defects in the field.