Microprocessor and Computer Architecture

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Registers, Data and Address Bus

Register is a small amount of storage within the CPU that is used to quickly store and retrieve data.

• Registers are the fastest form of memory in a computer system, and they play a crucial role in the execution of instructions and management of data within the CPU.

• The Intel 8085 is an 8-bit microprocessor that was widely used in early computer systems. It has a set of registers that play crucial roles in its operation.

• The accumulator is an 8-bit register used for arithmetic and logic operations.

• It stores one of the operands for these operations, and the result is often stored back in the accumulator.

• These are six 8-bit general-purpose registers that can be used for various purposes.

• They can be combined as register pairs (BC, DE, and HL) to store 16-bit data.

→ B and C form the BC register pair.

→ D and E form the DE register pair.

→ H and L form the HL register pair.

• The stack pointer is a 16-bit register used to point to the top of the stack.

• The stack is a region of memory used for storing addresses and data during subroutine calls and interrupts.

• The program counter is a 16-bit register that holds the address of the next instruction to be fetched and executed.

• It gets incremented automatically after each instruction fetch.

• The status register contains various flags that indicate the status of the CPU after arithmetic and logic operations.

Sign (S) flag: Indicates the sign of the result.

Zero (Z) flag: Set if the result is zero.

Auxiliary Carry (AC) flag: Used for binary-coded decimal (BCD) arithmetic.

Parity (P) flag: Indicates whether the number of set bits in the result is even or odd.

Carry (CY) flag: Set if there is a carry out of the most significant bit during addition or a borrow during subtraction.

The data bus in the Intel 8085 microprocessor is a set of electrical connections used to transfer data between the microprocessor and other external devices, such as memory and input/output (I/O) devices.

• In the case of the Intel 8085, the data bus is 8 bits wide, meaning it can transfer 8 bits (or 1 byte) of data at a time.

• The data bus is 8 bits wide, allowing it to transfer 8 bits of data simultaneously.

• The data bus can transfer data in both directions: from the microprocessor to external devices (output) and from external devices to the microprocessor (input).

• The data bus is connected to various components of the microprocessor, including the CPU, memory, and I/O devices.

• During the execution of instructions, data is transferred between the microprocessor’s registers (such as the accumulator and general-purpose registers) and memory or I/O devices through the data bus.

• The data bus is controlled by various control signals generated by the microprocessor to indicate the direction of data transfer, the timing of data transfer, and other aspects of the data transfer process.

The address bus in the Intel 8085 microprocessor is a set of electrical connections used to specify memory locations during the execution of instructions.

• It is responsible for transmitting the address of the memory location or I/O port that the microprocessor wants to access.


• The address bus in the Intel 8085 is 16 bits wide. This allows the microprocessor to address a maximum of 2^16 (64K) memory locations.

With a 16-bit address bus, the Intel 8085 can address memory locations from 0000H to FFFFH (in hexadecimal), covering a total of 64 kilobytes of memory.

• The address bus is connected to the address buffer inside the microprocessor, which helps in transmitting the address to the external memory or I/O devices.

• The address bus is unidirectional, meaning it only carries information from the microprocessor to external devices, specifying the address where data needs to be read from or written to.

• The address bus works in conjunction with various control signals generated by the microprocessor.

• These control signals indicate the type of operation (read or write), the status of the bus (whether it is carrying a valid address), and the timing of the address bus signals.

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