Memory data register

The Memory Data Register (MDR) is the register of a computer's control unit that contains the data to be stored in the computer storage (e.g. RAM), or the data after a fetch from the computer storage. It acts like a buffer and holds anything that is copied from the memory ready for the processor to use it.

The MDR is a 'a into memory.

The Memory Data Register is half of a minimal interface between a microprogram and computer storage, the other half is a memory address register.

Far more complex memory interfaces exist, but this is the simplest that can work.