Saturday, August 10, 2013

Internal Architecture of 8085 Microprocessor



Internal architecture of 8085


The intel 8085 is complete 8-bit parallel processing unit. The main component of the 8085 A as shown in figure above are the array of registers. The arithmetic logic units, encoder, decoder timing and control circuits. These are linked by an internal data bus. The major components of the block diagram are described as follows.
(1)   Accumulator
The ACC(register) is an 8-bit is an accessible to the program. Actually, the 8085 microprocessor is an accumulator based microprocessor between one of the operation depends on accumulation itself. So it plays key role in the operation of 8085 A.
When data is read from input ports, it is first moved to the accumulator. When data is output to the output ports, it must be 1st placed in the accumulator almost all arithmetic c logic functions are performed on the accumulator.
(2)   Temporary Register 
It is an 8-bit register not accessible to the programmer while executing the instruction. The 8085 A often places the data into temporary register for a period.
(3)   Instruction Register
The instruction register receives the operation codes (op-code) of instruction from internal data bus and passes it to the instruction decodes and machine cycle encoder circuit decodes the op-codes. So that the HP knows which type of operation is to be performed before executing it. Instruction register is not accessible to the programmer.
 
(4)   General Purpose Register
These four 8-bit registers are accessible to programmer and hence can be used to store data during a program executing. These can be used individual as 8-bit register or in pairs BC and DE as 16-bit register. Their content may be incremented or decremented and combined logically with the content of the accumulator.
(5)   Register H,L
Register H and L are 8-bit register that can be used in the same manner of scratch pad register. In addition the register H and L are utilized in indirect addressing mode. In these mode the memory location whose address is specified by the content of the register because a programmer can use them to load or transfer data from the register by using instructions.
(6)   Stack pointer
A stack is an area of memory set assigned for the purpose of storing data by an operation known as stacking. As data is stored and retrieved, its location is specified by a 16-bit register known as the stack pointer.
The beginning of stack is defined by loading 16-bit address in the stack pointer register.
(7)   Program Counter
A computer program consists the sequence of coded instruction. These instructions are stored sequentially in the memory. As the 8085 begins to execute the instruction, the memory address of first instruction to be executed is placed on the register called program counter. So the microprocessor uses this register to sequence the execution of instruction.
Its function is to point to the memory address from which the next byte of the instruction is to be fetched depending upon the instruction type the program counter is incremented by 1,2,3 to point to the next memory location.

(8)   Flags
Register consisting of five flip flops, each holding status of different stage separately is known as flag register and each flip flops are called flags. In the process of performing arithmetic and logic operation, the 8085 can set or reset 1 or more of the flags. These flags are carry flag, syn flag, zero flag, auxiliary flag and parity flag.
Normally, the program is executed sequentially by obtaining instruction and data from memory sequential. However, the microprocessor can be told to branch to a specified address on the condition that anyone of the flag is set or reset.
The states of the flag indicate the reset of arithmetic and logic operation which in term can be used for decision making process. Different flags can be described follows:
. Carry flags: If the last operation generates a carry, the carry flag is set or high.
E.g.    
  1111
            1101                        
1
           11100
S
Z
X
AC
X
P
X
CR



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