RD`is an active low output signal. RD`is pin number 32 of 8085. When RD`= 0, AD7-0 pins are input pins. When RD`= 1, it means the 8085 is not reading information.

Ready: This is an active high input control signal. It is used by microprocessor to detect whether a peripheral has completed (or is Ready for) the data transfer or not. The main function of this pin is to synchronize slower peripheral to faster microprocessor.

AD0-AD7 constitutes the Address/Data bus. They are time multiplexed. These pins are used for least significant bits of address bus in the first machine clock cycle and used as data bus for second and third clock cycle.

The decoder circuit generates output after every clock(-ve edge triggered) pulse. The output i.e S will be act as Control signal for other circuitry. The generation of a control signal takes place after each negative edge clock and timing diagram for such control signal may also be drawn as shown below.

The outputs of the state machine are the control signals. The sequence of the operation carried out by this machine is determined by the wiring of the logic elements and hence named as “hardwired”. Fixed logic circuits that correspond directly to the Boolean expressions are used to generate the control signals.

There are three main types of control signals namely;

• Those that activate an ALU function.
• Those that activate a data path.
• Those that are signals on the external system bus or other external interface.

The ALU is designed to combine two source operands to produce a result. The source operand fetch activity fetches the two source operands. One source operand is always the register specified by the rs instruction field.

ALU Control. The ALU control unit decides which type of result will be output from the ALU. ALU. The ALU is the arithmetic/logic unit. It is used to perform all operations for R-type instructions, performs addition for loads and stores, and performs a subtraction for branches.

1-0 Mux with Read data and ALU result as inputs. This multiplexor selects whether the Read data or the ALU result will be sent to the register file. The MemtoReg is asserted on a load (selects the Read data input) and is deasserted on an R-type instruction (selects the ALU result).

CU – Control Unit. ALU– Arithmetic Logical Unit. MU- Memory Unit.

There are basically two kinds of internal memory: ROM and RAM. ROM stands for read-only memory. It is non-volatile, which means it can retain data even without power.

An arithmetic logic unit (ALU) is a digital circuit used to perform arithmetic and logic operations. It represents the fundamental building block of the central processing unit (CPU) of a computer. Modern CPUs contain very powerful and complex ALUs. In addition to ALUs, modern CPUs contain a control unit (CU).

The difference between ALU and CPU is that ALU is an electronic circuit, which is a subsystem of CPU that performs arithmetic and logical operations while a CPU is an electronic circuit that handles instructions to operate the computer.