Memory unit consists of 

  • Memory Address Register (MAR)
  • Memory Data Register (MDR)
  • Memory Control logic
  • Memory Cells

For Read Operation, CPU does

  1. Sends the address to MAR
  2. Send READ Signal to Memory Control Unit.

Memory – decodes address bits

- identifies location to be accessed 

- initiates read operation of memory 

- memory takes some amount of time to present contents of location in MDR. 

  1. CPU transfers information from MDR

For write operation, 

  1. Sends address to MAR
  2. Sends data to MDR
  3. Sends WRITE signal to memory control unit
    • Decodes address & identifies location
    • Routes MDR Content to memory & 
    • Initialize write operation 

Memory Access time

Semi conductor Memory

Order of 100ns

Core memory

Order of 800 ns

During Read, 

CPU takes information from MDR after the access time is over. 

Memory Cycle Time:

  • Memory access time + additional recovery time (since, busy with some internal operation)
  • Minimum time interval necessary between two successive memory accesses. 
  • Memory access time & memory cycle time for both read & write.

Auxiliary memory:

Ware house for storing several program & data

  • Floppy disk drive
  • Hard disk drive
  • Magnetic tape drive
  • CD-ROM (Compact Disc – Read only memory)