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Architecture of 8085 microprocessor

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  • Difficulty Level : Medium
  • Last Updated : 27 May, 2022

8085 is an 8-bit, general-purpose microprocessor. It consists of the following functional units:

Architecture of 8085 microprocessor

Arithmetic and Logic Unit (ALU) :

It is used to perform mathematical operations like addition, multiplication, subtraction, division, decrement, increment, etc. Different operations are carried out in ALU: Logical operations, Bit-Shifting Operations, and Arithmetic Operations.

Flag Register:

It is an 8-bit register that stores either 0 or 1 depending upon which value is stored in the accumulator.  Flag Register contains 8-bit out of which 5-bits are important and the rest of 3-bits are Don’t Care conditions. The flag register is a dynamic register because after each operation to check whether the result is zero, positive or negative whether there is any overflow occurred or not, or for comparison of two 8-bit numbers carry flag is checked .so for numerous operations to check the contents of the accumulator and from that contents if we want to check the behavior of given result then we can use Flag register to verify and check. So we can say that the flag register is a status register and it is used to check the status of the current operation which is being carried out by ALU.

Different Fields of Flag Register:

  1. Carry Flag
  2. Parity Flag
  3. Auxiliary Carry Flag
  4. Zero Flag
  5. Sign Flag


Accumulator is used to perform I/O, arithmetic, and logical operations. It is connected to ALU and the internal data bus. The accumulator is the heart of the microprocessor because for all arithmetic operations Accumulator’s 8-bit pin will always there connected with ALU and in most-off times all the operations carried by different instructions will be stored in the accumulator after operation performance.

General Purpose Registers:

There are six general-purpose registers. These registers can hold 8-bit values. These  8-bit registers are B,C,D,E,H,L. These registers work as 16-bit registers when they work in pairs like B-C, D-E, and H-L. Here registers W and Z are reserved registers. We can’t use these registers in arithmetic operations. It is reserved for microprocessors for internal operations like swapping two 16-bit numbers. We know that to swap two numbers we need a third variable hence here W-Z register pair works as temporary registers and we can swap two 16-bit numbers using this pair.

Program Counter : 

Program Counter holds the address value of the memory to the next instruction that is to be executed. It is a 16-bit register.

For Example: Suppose current value of Program Counter : [PC] = 4000H

(It means that next executing instruction is at location 4000H.After fetching,program Counter(PC) always increments 

by +1 for fetching of next instruction.)


Stack Pointer :

It works like a stack. In stack, the content of the register is stored that is later used in the program. It is a 16-bit special register. The stack pointer is part of memory but it is part of Stack operations, unlike random memory access. Stack pointer works in a continuous and contiguous part of the memory. whereas Program Counter(PC) works in random memory locations. This pointer is very useful in stack-related operations like PUSH, POP, and nested CALL requests initiated by Microprocessor. It reserves the address of the most recent stack entry.

Temporary Register:

It is an 8-bit register that holds data values during arithmetic and logical operations.

Instruction register and decoder:

It is an 8-bit register that holds the instruction code that is being decoded. The instruction is fetched from the memory. 

Timing and control unit:

The timing and control unit comes under the CPU section, and it controls the flow of data from the CPU to other devices. It is also used to control the operations performed by the microprocessor and the devices connected to it. There are certain timing and control signals like Control signals, DMA Signals, RESET signals and Status signals. 

Interrupt control:

Whenever a microprocessor is executing the main program and if suddenly an interrupt occurs, the microprocessor shifts the control from the main program to process the incoming request. After the request is completed, the control goes back to the main program. There are 5 interrupt signals in 8085 microprocessors: INTR, TRAP, RST 7.5, RST 6.5, and RST 5.5.

Priorities of Interrupts: TRAP > RST 7.5 > RST 6.5 > RST 5.5 > INTR

Address bus and data bus: 

The data bus is bidirectional and carries the data which is to be stored.  The address bus is unidirectional and carries the location where data is to be stored.

Serial Input/output control:

It controls the serial data communication by using Serial input data and Serial output data.

The flow of an Instruction Cycle in 8085 Architecture :

  1. Execution starts with Program Counter. It starts program execution with the next address field. it fetches an instruction from the memory location pointed by Program Counter.
  2. For address fetching from the memory, multiplexed address/data bus acts as an address bus and after fetching instruction this address bus will now acts as a data bus and extract data from the specified memory location and send this data on an 8-bit internal bus For multiplexed address/data bus Address Latch Enable(ALE) Pin is used. If ALE = 1 (Multiplexed bus is Address Bus otherwise it acts as Data Bus).
  3. After data fetching data will go into the Instruction Register it will store data fetched from memory and now data is ready for decoding so for this Instruction decoder register is used.
  4. After that timing and control signal circuit comes into the picture. It sends control signals all over the microprocessor to tell the microprocessor whether the given instruction is for READ/WRITE and whether it is for MEMORY/I-O Device activity.
  5. Hence according to timing and control signal pins, logical and arithmetic operations are performed and according to that data fetching from the different registers is done by a microprocessor, and mathematical operation is carried out by ALU. And according to operations Flag register changes dynamically.
  6. With the help of Serial I/O data pin(SID or SOD Pins) we can send or receive input/output to external devices .in this way execution cycle is carried out.
  7. While execution is going on if there is any interrupt detected then it will stop execution of the current process and Invoke Interrupt Service Routine (ISR) Function. Which will stop the current execution and do execution of the current occurred interrupt after that normal execution will be performed.
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