8085 Microprocessor Architecture
Architecture Diagram of 8085
8085 Architecture consists of ALU, timing and control unit, instruction register and decoder, register array, interrupt control and serial I/O control.
Let's understand every unit.
It is a 8-bit register which is used to perform airthmetical and logical operations. It can stores the output of any operation and also works as registers for I/O operations.
General Purpose Registers
8085 Microprocessor has 6 general purpose registers of 8-bit each named as B, C, D, E, H and L.
Each register can accomodate 8-bits data.
These registers are used in pairs to carry 16-bits data.Valid pairs are BC, DE and HL
Arithmetic Logic Unit (ALU)
ALU performs arithmetic and logical operations like ANDing, ORing, addition, subtraction etc.
Flag register is a group of flip flops used to give status of different operations result. It consists of 5 Flip-Flops.
It is also known as status registers and is connected to the ALU. The 5 flip-flops are as follows:
Bit position of flip flop in flag register.
If D7 is 1 then sign flag(S) is 1 otherwise RESET.
If D7 is 1, the number is negative else it is positive
Instruction Register (IR)
It is a 8-bit register. The instruction is stored in this register when it is fetched from the memory.
It takes information from the IR and decode the instruction to be performed.
Program Counter (PC)
It is a 16-bit register. It stores the memory address of the next instruction to be executed.
It is also a 16 bit register used as memory pointer. It points to the memory location called stack. Stack pointer maintains the address of the last byte that is entered into stack.
Timing and Control Unit
It provides timing and control signal to microprocessor to perform various operations. It is a very important unit as it synchronizes the registers and flow of data through various registers and other units.
There are three status signals S0, S1, IO/M (M is active low)
|IO/M||S1||S2||Data Bus Status|
This controls the interrupts during a process. For example an Input/output device may send an interrupt signal to notify that the data is ready for input.The microprocessor temporarily stops the execution of main program and transfers control to specific special routine known as "Interrupt Service Routine"(ISR). After ISR, control is transferred back to main program.
There are 6 interrupt pins in this unit. Generally an external hardware is connected to these pins.
Serial Input/Output Control
SID and SOD are used for serial data communication.
Address Bus and Data Bus
Address bus carries the address of data to be stored and Data bus carries the data to be stored.