-:Architecture of 8085 microprocessor:-
Figure shows the internal architecture of 8085 then the programmable registers. It includes the ALU(arithmetic logic unit), Timing and control, instruction registers and decoder, register array, Intex control and serial I/O control.
It is general purpose microprocessor that is ideally suited to some application these are-
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2-Arithmetic and logic unit
3-Instruction decoder
4-Address buffer
5-Address/data bus
6-Interrupt control
7-Serial I/O control
8-Timing and control circuitry
1-Registers:-
The register are used to store the temporary data for execution the program. It is also known as temporary storage device.
The 8085 microprocessor registers are classified into some categories-
(I) General purpose registers
(II) Temporary registers
(III) Special purpose registers
(IV)16 bit registers
(I) General purpose register-
B, C, D,H and L are 8 bit general purpose registers which can be used as a separate 8 bit registers or it is also used in a 16 bit register pair with BC, DE, HL.
(II) Temporary register-
Temporary register are of following types
(a)- Temporary Data Register
(b)- W and Z Resisters
(a) Temporary Data Register-
It is provide input to the earlier the programmer cannot access temporary data it is internally used for execution of most of arithmetic and logical instructions.
(b)-W and Z registers-
These registers are used to hold 8 bit data during execution of some instructions. These registers are not available for the programmer since 8085 use them internally.
(III) Special purpose register-
Special purpose registers are of following types -
(a) Accumulator(register A)
(b)Flag register
(c)Instruction register
(a) Accumulator(register A)-
It is an 8 bit register it is used to extensively in arithmetic, logic, load and store operations. Most of the x result of arithmetic and logical operation is stored in accumulator (register A ).
(b) Flag register -
It is also an 8 bit registers in which 5 of the bits carries significant information in the form of flag: S (sign flag) Z(zero flag) AC(auxiliary carry flag) P(parity flag) and CY(carry flag). Flag registers are classified into five categories these are-
(i)S (sign flag)-
After the automatic and logical operation is the result of bit D7 is 1, the flag is set.
If the D7 is 1, the number will be viewed as negative number.
If D7 is 0, the number will be considered as positive.
(ii) Z (zero flag)-
If the result of ALU operation is zero, the zero flag is set and Pan flag is reset when the operation result is non zero.
the zero flag is also set if our certain register content become zero following and increment and decrement operation of that register.
(iii) AC (Auxiliary carry flag)-
This is set if there is overflow out of bit 3 i.e Carry from lower nibble to higher nibble (D3 to D4).
This flag register is only use for BCD operation not for programmer.
(iv) P (parity flag) -
If the result of arithmetic or logical operation is even number then parity flag is set.
If the result is odd the parity flag is reset.
(v) CY (carry flag)-
If the bit is an overflow out of 7 then the flag is set.
The carry flag also serves as a borrow flat for subtraction.
(c) Instruction Registers-
The process of first Fetches the opcode of instruction from memory. The CPU stores this code in a register called the instruction register.
(IV) 16 bit register-
It is are of two types -
(a)Program Counter (PC)
(b)Stock Pointer(SP)
(a) Program Counter (PC)-
The PC is a special purpose register which store the address of next instruction to be fetched. This acts as a pointer to next instruction
(b) Stack Pointer (SP)-
The stack pointer is a reserved area of the memory in the RAM where temporary information may be stored. A 16 bits stack pointer is used to hold the address of the most stack entry.
2-Arithmetic logic unit (ALU)-
It performs arithmetic and logical function on 8 bit variable. The arithmetic unit performs bitwise operations such as addition and subtraction and the logic unit performs logical operation such as complement, AND, OR and EXOR.
3-Instruction decoder-
The code of instructions which is fetched by the processor is stored in the instruction register, then it is sent to the instruction decoder. The instruction decoder decodes it and accordingly gives the timing and control signals which controls the register and the data buffers, ALU and external periferal signal INTA
4-Address buffer-
This is 8 bit unidirectional buffer it is used to drive external high order address bus (A15-A8). it is also used to tri state the higher order address bus under certain conditions such as rest, hold, halt and when address line are not in use.
5-Address/Data buffer -
This is also 8 bit but it is bidirectional buffer. It is used to drive multiplexed address/data bus i.e lower order address bus (A7-A0) and data bus (D7-D0). The address and data offer are used to drive external address and data bus respectively.
6-Interrupt control-
8085 microprocessor have five interrupt input RST5.5 ,RST 6.5,RST7.5 and INTR.
Sometime it is necessary to execute the collection of special routine whenever special condition exist within a program. The occurrence of this special condition is referred as interrupt.
7-Serial I/O control-
It is the 1-bit input/output port of 8085 microprocessor. The 8085 serial input/output controls provides two lines, SOD and SID for serial communication.
The serial output data (SOD) line used to send data serially.
The serial input data (SID) line used to to receive the data serially.
8-Timing and control circuitry-
It provides timing and control signal to the microprocessor to perform a various operation. It controls all external and internal circuits, it operates with reference to clock signal, it synchronize all the data transfer.
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