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# Memory Mapping of 8085

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 8085 are RAM, ROM, and EEPROM. 8085 can access 64kB of external memory. It can be explained as- total number of address lines in 8085 are 16, therefore it can access 2^16 = 65535 locations i.e. 64kB.

Note: 2^n=number of memory locations. Where, 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

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.

•  A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0 spacing IC details 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0000H ROM IC -1 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3FFFH 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4000H ROM IC- 2 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 7FFFH 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 8000H RAM IC- 3 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 BFFFH 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 C000H RAM IC- 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 FFFFH

## Interfacing Memories with 8085

Memories can be interfaced in two ways:
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.

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