Microprocessor elements

We have already discussed the general purpose registers, the Accumulator, and the flags.

The Internal Architectureo

The Program Counter (PC)

This is a register that is used to control the sequencing of the execution of instructions.

This register always holds the address of the next instruction.

Since it holds an address, it must be 16 bits wide.

The Stack pointer

The stack pointer is also a 16-bit register that is used to point into memory. 

The memory this register points to is a special area called the stack.

The stack is an area of memory used to hold data that will be retreived soon.

The stack is usually accessed in a Last In First Out (LIFO) fashion.

Externally Initiated Operations

External devices can initiate (start) one of the 4 following operations:

Reset 

All operations are stopped and the program counter is reset to 0000.

Interrupt

The microprocessor’s operations are interrupted and the microprocessor executes what is called a “service routine”.

This routine “handles” the interrupt, (perform the necessary operations). Then the microprocessor returns to its previous operations and continues.

Ready

The 8085 has a pin called RDY. This pin is used by external devices to stop the 8085 until they catch up.

As long as the RDY pin is low, the 8085 will be in a wait state.

Hold

The 8085 has a pin called HOLD. This pin is used by external devices to gain control of the busses.

When the HOLD signal is activated by an external device, the 8085 stops executing instructions and stops using the busses.

This would allow external devices to control the information on the busses. Example DMA.

The Design and Operation of Memory

Memory in a microprocessor system is where information (data and instructions) is kept. It can be classified into two main types:

Main memory (RAM and ROM) 

Storage memory (Disks , CD ROMs, etc.)

The simple view of RAM is that it is made up of registers that are made up of flip-flops (or memory elements). 

The number of flip-flops in a “memory register” determines the size of the memory word.

ROM on the other hand uses diodes instead of the flip-flops to permanently hold the information.

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).

Tri-State Buffers

An important circuit element that is used extensively in memory.

This buffer is a logic circuit that has three states:

Logic 0, logic1, and high impedance.

When this circuit is in high impedance mode it looks as if it is disconnected from the output completely.

The Tri-State Buffer

This circuit has two inputs and one output.

The first input behaves like the normal input for the circuit.

The second input is an “enable”.

If it is set high, the output follows the proper circuit behavior.

If it is set low, the output looks like a wire connected to nothing.

The Basic Memory Element

The basic memory element is similar to a D latch.

This latch has an input where the data comes in. It has an enable input and an output on which data comes out.

                                     

the remaining part of the topic will be post in the new article very soon , be in tough with this blog to read more .............