Bit: A bit is a single binary digit. Word: A word refers to the basic data size or bit size that can be processed by the arithmetic and logic unit of the processor. A 16-bit binary number is called a word in a 16-bit processor. Memory Word: The number of bits that can be stored in a register or memory element is called a memory word. Bus: A bus is a group of wires (lines) that carry similar information. System Bus: A system bus is a group of wires used for communication between the microprocessor and peripherals.
- Address Bus: It carries the address, which is a unique binary pattern used to identify a memory location or an I/O port.
- Data Bus: The data bus is used to transfer data between memory and processor or between I/O device and processor.
- Control Bus: The control bus carries control signals, which consists of signals for selection of memory or I/O device from the given address, direction of data transfer and synchronization of data transfer in case of slow devices.
2. 8085 Microprocessor
It is a programmable electronics chip ( Integrated Circuit (IC) ). A single IC has computing and decision-making capabilities similar to the central processing unit of a computer. It is used in almost all types of electronics devices like mobile phones, printers, washing machines, etc. and also used in advanced applications like radars, satellites and flights.
- It has 8-bit data bus and 16-bit address bus, thus it is capable of addressing 64 KB of memory.
- It has 8 bit ALU 8 bit ALU that can perform 8-bit operations.
- Lower order address bus is multiplexed with data bus to minimize the chip size.
- The 8085 microprocessor is an 8-bit processor available as a 40-pin IC package (shown the figure below) and uses +5 V for power. It can run at a maximum frequency of 3 MHz.
- The 8085 has extensions to support new interrupts, with three maskable interrupts (RST 7.5, RST 6.5 and RST 5.5), one non-maskable interrupt (TRAP), and one externally serviced interrupt (INTR).
- Three control signals are available on a chip:
(i) RD: it is an active low signal. Which indicates that the selected IO or Memory device is to be read and data is available on the data bus.
(ii) WR: it is an active low signal which indicates that the data on the data bus is to be written into a selected memory or IO location.
(iii) ALE: it is a +ve going pulse generated every time the 8085 begins an operation (machine cycle), which indicates that the bits on AD7-AD0 are address bits.
- Three status signals are available on a chip:
(i) IO/M: this is a status signal used to differentiate between IO and Memory operations. If it is high then IO operation and If it is low then Memory operation.
(ii) S1 and S0: status signals similar to IO/M, can identify various operations that are rarely used in the systems.
3. Internal Architecture of 8085 Microprocessor
The architecture of 8085 consists of three main sections, ALU (Arithmetic and Logical Unit), timing and control unit and Registers (shown in the following figure).
4. Arithmetic and Logic Unit (ALU)
The ALU performs the actual numerical and logical operations.
- The ALU performs the following arithmetic and logical operations.
- Addition, Subtraction
- Logical AND, Logical OR, Logical Ex - OR
- Complement (logical NOT)
- Increment, Decrement
- Left shift, Right shift
- Clear, etc.
- ALU includes the accumulator, the temporary register, the arithmetic and logic circuits and flags. It always stores the result of operations in Accumulator.
5. Timing & Control Unit
It generates timing and control signals, which are necessary for the execution of instructions.
- It controls data flow between CPU and peripherals (including memory).
- It provides status, control and timing signals, which are required for the operation of memory and I/O devices.
- 8085 System Bus: Microprocessor communicates with memory and other devices (input and output) using three buses: Address Bus, Data Bus and Control Bus.
- Address Bus: The Address bus consists of 16 wires. The size of the address bus determines the size of memory, which can be used. To communicate with memory the microprocessor sends an address on the address bus to the memory. The address bus is unidirectional, i.e., numbers only sent from microprocessor to memory.
- Data Bus: Bus is bi-directional. The size of the data bus determines what arithmetic can be done. Data bus also carries instructions from memory to the microprocessor.
- Memory size = 2A x D where, A denotes the address lines, and D denotes the data lines.
- Control Bus: Control bus are various lines which have specific functions for coordinating and controlling μP operations. The control bus carries control signals partly unidirectional, partly bi-directional. Control signals are things like reading or write.
8085 has six general purpose registers to store 8-bit data, these 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.
- Accumulator: The accumulator is an 8-bit register included as a part of Arithmetic Logic Unit (ALU). This register is used to store 8-bit data and to perform arithmetic and logical operations. The result of an operation is stored in the accumulator.
- Flag Register: The ALU includes five flip-flops. They are called Zero (Z), Carry (CY), Sign (S), Parity (P) and Auxiliary Carry (AC) flags. The microprocessor uses these flags to test data conditions. The conditions (set or reset) of the flags are tested through the software instructions. The combination of the flag register and the accumulator is called Program Status Word (PSW) and PSW is the 16-bit unit for stack operation.
- Program Counter (PC): This 16-bit register deals with sequencing the execution of the instruction. The microprocessor uses this register to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched.
- Stack Pointer (SP): The stack pointer is also a 16-bit register used as a memory pointer. It points to a memory location in read-write memory, called the stack.
- Instruction Register/Decoder: Temporary store for the current instructions of a program. Latest instruction sent here from memory prior to execution. The decoder then takes instruction and decodes or interprets the instruction. Decoded instruction then passed to next stage.
- Memory Address Register: Holds address, received from PC of next program instruction.
- Control Generator: It generates a signal within μP to carry out the instructions which have been decoded.
- Register Selector: This block controls the use of the register stack.
- General Purpose Registers: μP requires extra registers for versatility. It can be used to store additional data during a program.
7. Operations of Microprocessor
The microprocessor performs the following four operations using address bus, data bus and control bus:
- Memory Read: Reads data (or instruction) from memory.
- Memory Write: Writes data (or instruction) into memory.
- I/O Read: Accepts data from the input device.
- I/O Write: Sends data to an output device.
8. The 8085 Instruction Format
An instruction is a command to the microprocessor to perform a given task on a specified data. Each instruction has two parts, one is a task to be performed, called the operation code (opcode), and the second is the data to be operated on called the operand. The 8085 instruction set is classified according to word size.
- One-Byte Instructions: A 1-byte instruction includes the opcode and operand in the same byte. Operands are internal registers and are coded into the instruction.
- Two-Byte Instructions: In a two-byte instruction, the first byte specifies the operation code and the second byte specifies the operand. The source operand is a data byte immediately following the opcode.
- Three-Byte Instructions: In a three-byte instruction, the first byte specifies the opcode and the following two bytes specify the 16-bit address. Note that, the second byte is the low-order address and the third byte is the high-order address.
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