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Functional Block diagram of 8085:
The functional block diagram of 8085 microprocessor is given below:

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

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:
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.
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.
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.
In the 8085 microprocessor, the address bus and data bus are two separate buses that are used
for communication between the microprocessor and external devices.
The Address bus is used to transfer the memory address of the data that needs to be read or
written. The address bus is a 16-bit bus, allowing the 8085 to access up to 65,536 memory
locations.
The Data bus is used to transfer data between the microprocessor and external devices such as
memory and I/O devices. The data bus is an 8-bit bus, allowing the 8085 to transfer 8-bit data
at a time. The data bus can also be used for instruction fetch operations, where the
microprocessor fetches the instruction code from memory and decodes it.
The combination of the address bus and data bus allows the 8085 to communicate with and
control external devices, allowing it to execute its program and perform various operations.
Serial Input/output control:
It controls the serial data communication by using Serial input data and Serial output data.
Serial Input/Output control in the 8085 microprocessor refers to the communication of data
between the microprocessor and external devices in a serial manner, i.e., one bit at a time. The
8085 has a serial I/O port (SID/SOD) for serial communication. The SID pin is used for serial
input and the SOD pin is used for serial output. The timing and control of serial communication
is managed by the 8085’s internal circuitry. The 8085 also has two special purpose registers,
the Serial Control Register (SC) and the Serial Shift Register (SS), which are used to control
and monitor the serial communication.