What is 8085 Microprocessor? | Architecture of 8085 Microprocessor | Functional Units of 8085

8085

 8085 Microprocessor 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 is also used in advanced applications like radars, satellites, and flights.

What is 8085 Microprocessor?

8085 is an 8-bit microprocessor as it operates on 8 bits at a time and is created with N-MOS technology. This microprocessor exhibits some unique characteristics and this is the reason it still holds popularity among the microprocessors.

Basically, 8085 was the first commercially successful microprocessor by Intel. As some of the architectural drawbacks associated with 8080 were also eliminated by 8085.

The size of the data bus of 8085 is 8 bits while that of the address bus is 16. Therefore, can address 64 KB (i.e., 216) memory. Also, as it can perform 8-bit operations thus the size of ALU is 8-bit.

It also provides operational advantages, as 8085 needs a single +5V supply with only one clock single of width 320 ns. While 8080 requires 3 power supply lines and 2 clock signals of 500 ns.

   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 consist 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.

Internal Architecture of 8085 Microprocessor:

The architecture of the 8085 microprocessor provides an idea about what are the operations to be executed and how these are performed.

It can perform operations that are given below:

1. Operates on and stores 8-bit data.
2. It executes arithmetic and logic operations.
3. 8085 also sequences the instructions to be executed.
4. Stores data temporarily.

However, to perform all such operations, the processor needs a control unit, arithmetic logic unit, registers, buses,, etc.

We have already discussed how CU, ALU,, and buses function in a microprocessor. Here, in this article, we will discuss the detailed architecture of the 8085 microprocessor and how it operates.

The figure below represents the architectural representation of the 8085 microprocessor:

Functional Units of 8085:

1. Registers: These are nothing but a set of flip flops. These are basically used to hold (store) the data general-purpose pose registers–  8085 microprocessors contain general-purpose  registers that are present inside the microprocessor stories 8-bit dtor to execute a program.

The general-purpose pose registers are B, C, D,E, H and L. These registers can be combined to form pairs – BC, DE andtor to execute the 16-bit operation.

These are programmable registewhichthat means these registers are accessed by the programmer to insert and transfer the data by making use of instructions.

Temporary registers: These registers are used by the ALU to store the data temporarily and these are not accessed by the programmer. These are of 2 types:

1. Temporary data register – It is an 8-bit register that holds the operand and provides it to the ALU for program execution. Also, the immediate results are stored by the ALU in these registers.
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3. W and Z register – These registers are also used to hold the temporary values. It is used by the control section of the microprocessortoo store the data during operations.

• Program Counter (PC): This16-bitt register deals with sequencing the execution ofthe  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.
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• 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.
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• Instruction Register/Decoder: Temporary store for the current instructions of a program. Latest instruction sent here from memorybeforeo execution.The decoderr then takes instruction and decodes or interprets the instruction. Decoded instruction then passed tothe                  next stage.
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• Memory Address Register: Holds address, received from PC of next program instruction.
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• Control Generator: It generates a signal within μP to carry out the instructions which have been decoded.
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• Register Selector: This block controls the use of the register stack.
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• General Purpose Registers: μP requires extra registers for versatility. It can be used to store additional data during a program.

What is 8085 Microprocessor?

• It hasan  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.
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• 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).
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• Three control signals are available on chip: (i) RD : it is a active low signal. Which indicate that the selected IO or Memory device is to be read and data is available on the data bus. (ii) WR : it is a active low signal which indicate that the data on the data bus are 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 indicate that the bits on AD7-AD0 are address bits.
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• Three status signals are available on 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.

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).

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 result of operations in Accumulator.

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. Address bus is unidirectional, i.e., numbers only sent from microprocessor to memory.

• Data Bus: Bus is bidirectional. Size of the data bus determines what arithmetic can be done. Data bus also carries instructions from memory to the microprocessor.
• Memory size = 2^A 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 bidirectional. Control signals are things like reading or writing.

Registers: 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 the 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.

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• Program Counter (PC): This 16-bit register deals with sequencing the execution of an 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 the 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.

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 the output device.

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 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. 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.

The 8085 Addressing Modes:

The various formats for specifying operands are called the addressing modes. For 8085, they are

• Immediate Addressing:
  • Data is provided in the instruction.
  • Load the immediate data to the destination provided.
  • Example: MVI A, 12 H
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• Register Addressing:
  • Data is provided through the registers.
  • Example: MOV B, C
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• Direct Addressing:
  • Used to accept data from outside devices to store in the accumulator or send the data stored in the accumulator to the outside device.

    Applications of 8085 microprocessor:

    8085 finds its major applications in programmable calculators as well as in numerical control and environmental monitoring systems. These are also used in switching, banking, and financial systems.

  • Example: MOV A, [1000]
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• Indirect Addressing:
  • The effective address is calculated by the processor and the contents of the address is used to form a second address. The second address is where the data is stored.
  • Example: MOV A, [[1000]]
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• Implicit addressing:
• In this addressing mode the data itself specifies the data to be operated upon.
• Example: CMA ; Complement the contents of accumulator

8085 Instruction Set:

An instruction is a binary pattern designed inside a microprocessor to perform a specific function. Each instruction is represented by 8 bit binary value. Instruction set can be categorised int0 5 types:

1. Data transfer instructions:
   • These instructions are used to transfer data from one register to another register, from memory to register or register to memory.
   • When an instruction of data transfer group is executed, data is transferred from the source to the destination without altering the contents of the source.
   • Examples: MOV, MVI, LXI, LDA, STA, etc.
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2. Arithmetic instructions:
   • These instructions are used to perform arithmetic operations such as addition, subtraction, increment or decrement of the content of a register or memory.
   • Examples: ADD, ADC, ADI, DAD, SUB, INR, DCR, etc.
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3. Logical instructions:
   • These instructions are used to perform logical operations such as AND, OR, compare, rotate etc.
   • Examples: ANA, ANI, ORA, ORI, XRA, CMA, CMC , STC, CMP, RLC, RAL, RAR, etc.
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4. Branching Instructions:
   • These instructions are used to perform conditional and unconditional jump, subroutine call and return, and restart.
   • Examples: JZ, JNZ, JC, JNC, JP, JM, JPE, JPO, CALL, RET, RST, etc.
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5. Machine Control Instructions:
• These instructions control machine functions such as Halt, Interrupt, or do nothing.
• The microprocessor operations related to data manipulation can be summarized in four functions: copying data, performing arithmetic operations, performing logical operations, testing for a given condition and alerting the program sequence.
• Example: PUSH, POP, HLT, XTHL, NOP, EI, DI, etc.

Applications of 8085 microprocessor:

8085 finds its major applications in programmable calculators as well as in numerical control and environmental monitoring systems. These are also used in switching, banking and financial systems.