Microprocessor architecture: to know 8085 architecture

Microprocessor architecture

8085 is pronounced  as "eighty-eighty-five" "eighty-eighty-five" microprocessor. It is an 8-bit microprocessor designed by Intel in 1977 using NMOS technology.

It has the following configuration −

• 8-bit data bus
• 16-bit address bus, which can address up to 64KB
• A 16-bit program counter
• A 16-bit stack pointer
• Six 8-bit registers arranged in pairs: BC, DE, HL
• Requires +5V supply to operate at 3.2 MHZ single-phase clock

It is used in washing machines, microwave ovens, mobile phones, etc.

8085 Microprocessor – Functional Units

8085 consists of the following functional units −

• Accumulator

It is an 8-bit register used to perform arithmetic, arithmetic, logical, logical, I/O & LOAD/ST LOAD/STORE operations. operations. It is connected to internal data bus & ALU.

• Arithmetic and logic unit

As the name suggests, suggests, it performs arithmetic and logical logical operations like Addition, Addition, Subtraction, Subtraction, AND, OR, etc. on 8-bit data.

• General-purpose register

There are 6 general-purpose registers in the 8085 processor, i.e. B, C, D, E, H & L. Each register can

hold 8-bit data. These registers can work in pairs to hold 16-bit data and their pairing combination combination is like B-C, D-E & H-L.

• Program counter

It is a 16-bit register used to store the memory address location of the next instruction to be

executed. executed. Microprocessor increments the program whenever an instruction is being executed, executed so that the program counterpoints to the memory address of the next instruction that is going to be executed.

• Stack pointer

It is also a 16-bit register that works like a stack, which is always incremented/decremented incremented/decremented by 2 during push & pop operations.

• Temporary register

It is an 8-bit register, which holds the temporary data of arithmetic and logical operations.

• Flag register

It is an 8-bit register having five 1-bit flip-flops, flip-flops, which holds either 0 or 1 depending upon the result stored in the accumulator.

These are the set of 5 flip-flops −

1. Sign (S)
2. Zero (Z)
3. Auxiliary Carry (AC)
4. Parity (P)
5. Carry (C)

Its bit position is shown in the following table −

• Instruction register and decoder

It is an 8-bit register. When an instruction is fetched from memory then it is stored in the Instruction register. The instruction decoder decodes the information present in the Instruction register.

• Timing and control unit

It provides timing and control signals to the microprocessor to perform operations. operations. Following are the timing and control signals, which control external and internal circuits −

• Control Signals: READY, RD’, WR’, ALE
• Status Signals: S0, S1, IO/M’
• DMA Signals: HOLD, HLDA
• RESET Signals: RESET IN, RESET OUT 

• Interrupt control

As the name suggests it controls the interrupts during a process. process. When a microprocessor is

executing the main program and whenever an interrupt occurs, occurs, the microprocessor shifts the control from the main program to process the incoming incoming request. request. After the request is completed, completed, the control goes back to the main program.

There are 5 interrupt signals in the 8085 microprocessor: INTR, RST 7.5, RST 6.5, RST 5.5, TRAP.

• Serial Input/output control

It controls the serial data communication by using these two instructions: instructions: SID (Serial (Serial input data) and SOD (Serial output data).

• Address buffer and address-data buffer

The content stored in the stack pointer and program counter is loaded into the address buffer and

address-data address-data buffer to communicate with the CPU. The memory and I/O chips are connected to these buses; the CPU can exchange the desired data with the memory and I/O chips.

• Address bus and data bus

The data bus carries the data to be stored. stored. It is bidirectional, whereas the address bus carries the location to where it should be stored and it is unidirectional.It is used to transfer the data & Address Address I/O devices