Architecture of 8085

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Information about Architecture of 8085

Published on April 26, 2014

Author: shilpaitft



Architecture of 8085

Architecture of 8085 .


ALU The ALU performs the arithmetic and logical operations. The operations performed by ALU of 8085 are addition, subtraction, increment, decrement, logical AND, OR, EXCLUSIVE -OR, compare, complement and left / right shift .

Accumulator and temporary register • The accumulator and temporary register are used to hold the data during an arithmetic / logical operation. After an operation the result is stored in the accumulator and the flags are set or reset according to the result of the operation.

TIMING & CONTROL UNIT The timing and control unit synchronizes all the microprocessor operations with the clock and generates the control signals necessary for communication between the microprocessor and peripherals.

INSTRUCTION REGISTER & DECODER • When an instruction is fetched from memory it is placed in instruction register. Then it is decoded and encoded into various machine cycles.

STACK POINTER (SP) • It is also a 16-bit register which is used as a memory pointer. • It maintains the address of the last byte that is entered into stack.

PROGRAM COUNTER (PC) • The program counter (PC) keeps track of program execution. • It is a special purpose register. • A program counter stores the address of the next instruction to be executed.

FLAG REGISTER There are five flags in 8085, which are sign flag (8), zero flag (Z), auxiliary carry flag (AC), parity flag (P) and carry flag (CY). The bit positions reserved for these flags in the flag register are shown in figure below.

The Flags register • There is also a flag register whose bits are affected by the arithmetic & logic operations. • S-sign flag • The sign flag is set if bit D7 of the accumulator is set after an arithmetic or logic operation. • Z-zero flag • Set if the result of the ALU operation is 0. Otherwise is reset. This flag is affected by operations on the accumulator as well as other registers. (DCR B). • AC-Auxiliary Carry • This flag is set when a carry is generated from bit D3 and passed to D4 . This flag is used only internally for BCD operations.

. • P-Parity flag • After an ALU operation, if the result has an even # of 1s, the p-flag is set. Otherwise it is cleared. So, the flag can be used to indicate even parity. • CY-carry flag • This flag is set when a carry is generated from bit D7 after an unsigned operation. • OV-Overflow flag • This flag is set when an overflow occurs after a signed operation. Unspecified 7 6 5 4 3 2 1 0SF ZF X AF X PF X CF

General Purpose Registers • 8085 consists of 6 special types of registers called General Purpose Registers. These General Purpose Registers are used to hold data like any other registers . • The GPR in 8085 processors are B,C,D,E,H,L.Each registers can hold 8-bit data. • They can work in pair such as B-C,D-E,H-L to store 16-bit data.

Temporary Register This register acts as a temporary memory during the arithmetic and logical operations. Unlike other registers, this temporary registers can only be accessed by the microprocessor and it is completely inaccessible to programmers. Temporary register is an 8-bit register.

PSW • The combination of these 8-bits is called Program Status Word(PSW).PSW and the accumulator as a 16-bit unit for the stack operation.

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