13-04-2012, 11:20 AM
Microprocessor Architecture
8085 Microprocessor Architecture PPT.ppt (Size: 303 KB / Downloads: 52)
The Address Bus
16 bits wide (A0 A1…A15)
Therefore, the 8085 can access locations with numbers from 0 to 65,536. Or, the 8085 can access a total of 64K addresses.
“Unidirectional”.
Information flows out of the microprocessor and into the memory or peripherals.
When the 8085 wants to access a peripheral or a memory location, it places the 16-bit address on the address bus and then sends the appropriate control signals.
The Data Bus
8 bits wide (D0 D1…D7)
“Bi-directional”.
Information flows both ways between the microprocessor and memory or I/O.
The 8085 uses the data bus to transfer the binary information.
Since the data bus has 8-bits only, then the 8085 can manipulate data 8 bits at-a-time only.
Microprocessor Initiated Operations
These are operations that the microprocessor itself starts.
These are usually one of 4 operations:
Memory Read
Memory Write
I/O Read (Get data from an input device)
I/O write (Send data to an output device)
The Read Operation
To read the contents of a memory location, the following steps take place:
The microprocessor places the 16-bit address of the memory location on the address bus.
The microprocessor activates a control signal called “memory read” which enables the memory chip.
The memory decodes the address and identifies the right location.
The memory places the contents on the data bus.
The microprocessor reads the value of the data bus after a certain amount of time.
Accessing Information in Memory
For the microprocessor to access (Read or Write) information in memory (RAM or ROM), it needs to do the following:
Select the right memory chip (using part of the address bus).
Identify the memory location (using the rest of the address bus).
Access the data (using the data bus).
Data Lines
All of the above discussion has been regarding memory length. Lets look at memory width.
We said that the width is the number of bits in each memory word.
We have been assuming so far that our memory chips have the right width.
What if they don’t?
It is very common to find memory chips that have only 4 bits per location. How would you design a byte wide memory system using these chips?
We use two chips for the same address range. One chip will supply 4 of the data bits per address and the other chip supply the other 4 data bits for the same address.