8085 is an 8 bit microprocessor,it has 40 pins fabricated on single LSI(large scale integration) or VLSI(very large scale integration) chip. 8085 operates on +5V DC Supply and its clock speed is 3.125 MHz.
The above diagram shows the Architecture of 8085. From the diagram we can see that the architecture has the following functional units:
[II] Arithmetic and logic unit (ALU):
The ALU carries out the arithmetic and logic operations like addition, subtraction, increment, decrement etc.
and the logical operations are AND, OR, EXOR, Complement, Shift compare etc.
[III] Address and data buffer:
The 8085 contains unidirectional address bus buffer and a bi-directional data bus buffer. these buffers isolate the internal bus system of 8085 from the external system bus and provide enough current gain to run few memory and I/O chips.
[IV] Instruction register and decoder:
Instruction registers :-It is a 8-bit register. When op-code of an instruction is fetched from memory then it is stored in this register. This is not programmable and is not available to he programmer.
Instruction Decoder:- Instruction decoder identifies the instructions. It takes the information from instruction register and decodes the instruction to be performed. It establishes the sequence of events to follow.
[V] Register array:
There are various types of registers in 8085.These registers are used for storage ,manipulating data and instructions.The different types of registers are:
[VII] Interrupt control:-
8085 has five hardware interrupt TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR. It also has INTA for interrupt acknowledgement.
The above diagram shows the Architecture of 8085. From the diagram we can see that the architecture has the following functional units:
- Timing and control unit.
- Arithmetic and logic unit.
- Address and data buffer.
- Instruction register and decoder.
- Register array.
- Serial control.
- Interrupt control.
[I] Timing and control unit :
It provides timing and control signal to the microprocessor to perform the various operation.It has three control signal. It controls all external and internal circuits. It operates with reference to clock signal.It synchronizes all the data transfers.
The three control signals are:-
- ALE-Airthmetic Latch Enable, It provides control signal to synchronize the components of microprocessor.
- WR-This is used for writing operation.This is active low.
- RD- This is used for reading operation. This is active low
There are three status signal used in microprocessor S0, S1 and IO/M. It changes its status according the provided input to these pins. Below is the truth table for various combinations of status signals.
| IO/M (Active Low) | S1 | S2 | Data bus status (Output) |
|---|---|---|---|
| 0 | 0 | 0 | Halt |
| 0 | 0 | 1 | Memory Write |
| 0 | 1 | 0 | Memory read |
| 1 | 0 | 1 | IO write |
| 1 | 1 | 0 | IO read |
| 0 | 1 | 1 | Opcode fetch |
| 1 | 1 | 1 | Interrupt Acknowledge |
[II] Arithmetic and logic unit (ALU):
The ALU carries out the arithmetic and logic operations like addition, subtraction, increment, decrement etc.
and the logical operations are AND, OR, EXOR, Complement, Shift compare etc.
ALU consists of:
1) Accumulator:-It is a 8-bit register which is used to perform arithmetic and logical operation. It stores the output of any operation. It also works as registers for I/O accesses.
2)Temporary register:-It is a 8-bit register which is used to hold the data on which the accumulator is computing operation. It is also called as operand register because it provides operands to ALU. This register is not available to the programmer.
3)Flag register:-It contains 5 single bit flags and three unused bits which changes its status according to the result stored in an accumulator. It is also known as status registers.
There are five flip-flops in the flag register are as follows:
1.Sign flag(S)
2.zero flag(z)
3.Auxiliary carry flag(AC)
4.Parity flag(P)
5.Carry flag(C)
Bit position of flags in flag register
1.Sign (S)- If D7 of the result is 1 then sign flag is set otherwise reset. As we know that a number on the D7 always decides the sign of the number.
if D7 is 1: the number is negative.
if D7 is 0: the number is positive.
2.Zero (Z)-If the result stored in an accumulator is zero then this flip flop is set otherwise it is reset.
3.Auxiliary carry(AC)-If any carry goes from D3 to D4 in the output then it is set otherwise it is reset.
4.Parity (P)- If the no of 1's is even in the output stored in the accumulator then it is set otherwise it is reset for the odd.
5.Carry Flag (CY)- If the result stored in an accumulator generates a carry in its final output then it is set otherwise it is reset.
The 8085 contains unidirectional address bus buffer and a bi-directional data bus buffer. these buffers isolate the internal bus system of 8085 from the external system bus and provide enough current gain to run few memory and I/O chips.[IV] Instruction register and decoder:
Instruction registers :-It is a 8-bit register. When op-code of an instruction is fetched from memory then it is stored in this register. This is not programmable and is not available to he programmer.
Instruction Decoder:- Instruction decoder identifies the instructions. It takes the information from instruction register and decodes the instruction to be performed. It establishes the sequence of events to follow.
[V] Register array:
There are various types of registers in 8085.These registers are used for storage ,manipulating data and instructions.The different types of registers are:- Accumulator.
- Temporary register.
- General purpose register.
- Program Counter.
- Stack Pointer.
- Instruction register.
- Flag register.
Accumulator:-It is a 8-bit register which is used to perform arithmetical and logical operation. It stores the output of any operation. It also works as registers for i/o accesses.
Temporary Register:-It is a 8-bit register which is used to hold the data on which the acumulator is computing operation. It is also called as operand register because it provides operands to ALU.
General purposes registers:-Microprocessor consists 6 general purpose registers of 8-bit each named as B,C,D,E,H and L. Generally theses registers are not used for storing the data permanently. It carries the 8-bits data. These are used only during the execution of the instructions.
These registers can also be used to carry the 16 bits data by making the pair of 2 registers. The valid register pairs available are BC,DE HL. We can not use other pairs except BC,DE and HL . These registers are programmed by user.
Program Counter:-It is a 16 bit register used as memory pointer. It stores the memory address of the next instruction to be executed. So we can say that this register is used to sequencing the program. Generally the memory have 16 bit addresses so that it has 16 bit memory. The program counter is set to 0000H.
Stack Pointer:-It is also a 16 bit register used as memory pointer. It points to the memory location called stack. Generally stack is a reserved portion of memory where information can be stores or taken back together.
Instruction registers :-It is a 8-bit register. When op-code of an instruction is fetched from memory then it is stored in this register. This is not programmable and is not available to he programmer.
Flag register:-It contains 5 single bit flags and three unused bits which changes its status according to the result stored in an accumulator
[VI]Serial Input Output Control:-
There are two pins in this unit. This unit is used for serial data communication. The serial input data enters the 8085 from SID input. and the SOD output transmit serial data from 8085.
[VI]Serial Input Output Control:-
There are two pins in this unit. This unit is used for serial data communication. The serial input data enters the 8085 from SID input. and the SOD output transmit serial data from 8085.
[VII] Interrupt control:-
8085 has five hardware interrupt TRAP, RST 7.5, RST 6.5, RST 5.5 and INTR. It also has INTA for interrupt acknowledgement.
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