Microprocessor 8085 Architecture

Hello friends, today we are going to see the microprocessor 8085 architecture.

The functional block diagram or architecture of 8085 Microprocessor, gives the complete details about the internal Microprocessor. It includes the ALU (Arithmetic and logic unit), timing and control unit, instruction registers and decoder, register array, interrupt control, and serial I/O control etc. To connect all the blocks with each other we need some buses that we have already discussed under the heading Bus Structure Of 8085 Microprocessor.  And we have discussed pin diagram under the heading pin configuration of 8085 microprocessor. Fig. shows the Block diagram of 8085 Microprocessor. 



Architecture of 8085 Microprocessor

The ALU (Arithmetic/Logic unit)

The ALU performs the arithmetic operations and logical operations. this unit includes the accumulator, registers, arithmetic and logic circuits and five flags.

Accumulator (A)

The accumulator is an 8-bit register for storing 8-bit data to perform arithmetic and logical operations. The result of an operation is stored in the accumulator. It is also called register A.

Registers

The 8085 microprocessor has six general-purpose registers to store 8-bit data during program execution. These registers are identified as B, C, D, E, H, and L. They can be combined as register pairs-BC, DE, and HL-to perform some 16-bit operations. These registers are also called programmable registers because a programmer can use them to load or transfer the data from registers by using instructions.Also two additional registers called temporary registers W and Z to hold 8-bit temporary data internally during the program execution. These temporary registers are not available for programmer.The temporary registers and general purpose programmable registers are combinely called as Register Array.

Flags (Flip/Flop)

The ALU having five flip-flops which changes its status according to the result stored in an accumulator and other registers. The conditions of the flags are as follows.·
S- Sign flag: After the execution of ALU operation, if bit D7 of the result is 1 (stored in accumulator), the sign flag is set and the number will be a negative. If D7 is 0, the sign flag is reset and the number will be positive.·
Z- Zero flag: the zero flag is set to 1 if the ALU operation result is zero, and the flag is set to 0, if the ALU operation result is not zero.
AC-Auxiliary carry flag: In an arithmetic operation, when a carry is generated by digit D3 and passed on to digit D4, the AC flag is set to 1. The flag is used only internally for BCD operations (Binary Coded Decimal) and it is not available for the programmer to change the sequence of a program with a jump instruction.
P-Parity flag: if the result has an even number of 1’s, the flag is set to 1. If it has an odd number of 1’s, the flag is reset. I.e. 0000 0011 – even parity and 0001 0011 – odd parity· 
CY- Carry flag; if the results in a carry, the carry flag is set. Otherwise it is reset. The carry flag is also set with borrow in subtractions.The bit positions of these flags in flag registers are as follows.
 

Flag Positions

8085 registers

Program Counter (PC)

This is 16-bit register sequencing the execution of instructions in memory that why it is also called as memory pointer. The function of the program counter is to point out the memory address of the next instruction which is to be executed. When an operation code is being fetched, the program counter is incremented by one to point out the next memory location.

Stack Pointer (SP)

The stack pointer is also a 16-bit register used as a memory pointer. It points to a memory location in R/W memory, called the stack. The beginning of the stack is defined by loading a 16-bit address in the stack pointer (register).

Timing and Control Unit

This unit performs all microprocessor operations with the clock and generates the control signals. These control signals are important for communication between the microprocessor and peripherals.

Instruction Register and Decoder

The instruction register and the decoder are part of the ALU. When an instruction is fetched from memory, it is loaded in the instruction register. The decoder decodes the instruction and establishes the sequence of events to follow.

Well, how you found this article, is this useful? I'm sure this will help you more. If you want more information please let me know through comments in the right below.
Subscribed to the My Computer Tutors for updates. I will keep updating to you with latest tutorials.