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Memory Mapping of 8051

  • Memory interfacing is used to provide more memory space to accommodate complex programs for more complicated systems.
  • Types of memories which are most commonly used to interface with 8051 are RAM, ROM, and EEPROM.
  • 8081 can access 64kB of external memory. It can be explained as- total number of address lines in 8051 are 16, therefore it can access 2^16 = 65535 locations i.e. 64kB

    • Note: 2^n=number of memory locations n = number of address lines

    Some of the RAM IC's are given as:
  • 1. IC 2114 -> 1k x 4bits

  • 2. IC 6116 -> 2k x 8bits

  • 3. IC 6264 -> 8k x 8bits



    • Some of the ROM IC's are given as:
  • 1. IC 2708 -> 1k x 8bits

  • 2. IC 2716 -> 2k x 8bits

  • 3. IC 2732 -> 4k x 8bits

  • 4. IC 2764 -> 8k x 8bits

  • 5. IC 27128 -> 16k x 8bits

  • 6. IC 27256 -> 32k x 8bits

  • 7. IC 2708 -> 64k x 8bits

    • Memory mapping of 8051

    Q. Design a minimum system to interface the following specification:
    1. 32kB of RAM using 2 x 16kB RAM IC
    2. 32kB of ROM using 2 x 16kB ROM IC
  • Therefore the spacing for all the 4 ICs are same due to same size of memory ie. 3FFFH.
  • The number of address lines required by each IC's is 14

    • Memory mapping of 8051

    Interfacing Diagram

    There are two type of interfacing:

    1. Partial decoding: In this type of decoding not all the address lines are utilized in the circuit (they are left as unused pins). Ex: In an interface of 4kB memory only A0-A11 address lines are utilized, whereas the remaining A12-A15 address lines are unused.

    2. Complete decoding (exhaustive decoding): In this type of decoding, all the address lines are utilized in circuit for some or the other use (i.e. all pins are exhausted). Ex: In an interface of 4kB memory only A0-A11 address lines are utilized, whereas the remaining A12-A15 address lines are used in Memory selection logic or as any other control signals.


    Memory mapping of 8051






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