Operations Initiated Externally:
An external device may initiate the following operations :
While reset is activated from an external device, it suspends all of the internal operations and clears the program counter so that the next program execution can begin at the zero memory address.
While interrupt is activated from an external device, it interrupts the normal operation of a microprocessor and asks to execute some of the emergency functions, after completing those routines only then the microprocessor may run normal operation.
While ready is activated low from an external device, it forces the microprocessor into wait state that enables other slow peripherals to synchronize with the microprocessor.
When even hold is activated from an external device, the microprocessor frees the buses and let it be utilized by the external device.
Internal Data Operations
To perform internal data operations, microprocessor required registers, arithmetic logic unit (ALU), control logic and internal data buses.
This register is a part of ALU indicated by A and utilized for arithmetic and logical operations, the results of an operation are stored in this register.
8085 has six registers to hold 8-bit data viz. B, C, D, E, H and L. They may pair as BC, DE and HL for 16-bit operations. A programmer may use these registers to transfer or load data.
Five Flip Flops are kept for testing data conditions in the accumulator and other registers; they are accordingly set or reset. Five flags are Zero (Z), Sign (S), Carry (CY), Parity (P), and Auxiliary Carry (AC). For instance Zero Flag (Z) is set to one while an operation results in a zero.
Program Counter (PC)
This is a 16-bit register, which does the function of sequencing of programs execution. This basically points to the memory address from which the next machine code is to be collected. While a machine code is obtain PC is incremented by 1.
Stack Pointer (SP)
This is a 16-bit register and points to a memory location in R/W memory.