Operations of 8085 Microprocessor

Operations of 8085 Microprocessor – The 8085 microprocessor can perform various operations on the given data by selecting the necessary instructions from its set. These instructions are stored in memory. The 8085 microprocessor reads (fetches) one instruction at a time, interprets it with the help of the instruction set library, and performs the data manipulation indicated by the instruction.

The result can be stored in memory or sent to the output device. The microprocessor can also respond to externally initiated signals. It can be interrupted, reset, or asked to wait to synchronize with slower peripherals.

Basic Operations of 8085 Microprocessor

All the various functions/operations performed by the 8085 microprocessor can be classified into the following categories:

1. Microprocessor-initiated operations.
2. Internal data operations.
3. Externally initiated operations.

Microprocessor-initiated operations

All those operations in which a microprocessor communicates with its peripheral (memory and I/O device) fall under this category. These operations are

1. Memory Read (data or instruction)
2. Memory Write (data or instruction)
3. Input Read (data)
4. Output Write (data)

All the above-mentioned operations are initiated by the microprocessor itself. The 8085 microprocessor performs these operations using three system buses

1. Address bus
2. Data bus
3. Control bus

Working Process of Microprocessor – The microprocessor has to follow the same steps to perform any microprocessor-initiated operation.

• First, it identifies the address of the memory location or I/O port using the address bus.
• Second, it transfers the data/instruction in binary form using the data bus. The data bus is bidirectional, and data can be transferred from the microprocessor to the peripheral (memory or input/output) or from the peripheral to the microprocessor.
• Third, the timing and control unit of the microprocessor provides appropriate control signals using the control bus.

Figure 1 shows the memory read operation of the microprocessor. In this example, it is assumed that the operation code (op-cod) of the instruction is 4F H(0100 1111) and is stored in memory location XX0AH. To perform this operation, the program counter (PC) places the address (XX0AH) of the memory location on the address bus. The control unit sends the memory read control signal (MEMR) to enable the output buffer of the memory chip. The control signal MEMR is a combination of two control signals IO/M and RD.

As the output buffer of the memory chip is enabled, the opcode stored in memory will be placed on the data bus and copied to the instruction decoder of the microprocessor. The instruction is then decoded and executed according to the binary pattern of the instruction.

Internal Data Operations

The internal architecture of the 8085 microprocessor is responsible for performing internal data operations. The internal data operations of the 8085 microprocessor are as follows:

1. Store 8-bit data in its general-purpose registers (B, C, D, E, H, and L) for the proper execution of data.
2. Perform arithmetic and logic operations on the given operands. The ALU block of the microprocessor is responsible for completing these operations. One of the operands is stored in the accumulator, and the result of the operation is also stored in the accumulator.
3. Test for the conditions of the result by checking the status of five individual bits of the flag register to take further decisions.
4. Sequence the execution of instructions one by one using the program counter (PC).
5. Store data temporarily during execution in the predefined R/W memory locations called the stack, and manage the sequence of the store and read operations of data on the stack using the stack pointer (SP).

Externally initiated operations

Externally, devices can initiate some asynchronous signals abruptly to make the microprocessor perform some specific operations by terminating its normal program execution. These operations are:

1. Reset – When this signal is generated externally, all the internal operations are suspended, and the program counter (PC) is cleared to hold 0000H.
2. Ready– Ready signal is generated by slower devices so that they can synchronize with the faster microprocessor.
3. Interrupt – An interrupt signal is generated by the peripheral to terminate the normal execution of instructions and to execute some other instructions. These special instructions are known as service routines. After completing the service routine of an interrupt, the microprocessor resumes its normal operations.
4. DMA operations – The HOLD pin is used for Direct Memory Access (DMA) operations. Through this pin, the DMA controller requests the microprocessor to relinquish control of buses and allows peripherals to use them.