Instructions:
An instruction is a binary pattern designed to perform a specific function.
Thank you for reading this post, don't forget to subscribe!• The list of entire instructions is called the instruction set. The instruction set determines what function the microprocessor can perform.
• The execution of instructions and data flow within the Intel 8085 microprocessor involves several stages.
Let’s explore the basic steps involved in the instruction execution cycle:
1.)Instruction Fetch (IF):
- Program Counter (PC):
- Holds the address of the next instruction to be fetched.
- Memory Read (MAR, MDR):
- PC content is placed on the Memory Address Register (MAR).
- Memory is read, and the instruction is stored in the Memory Data Register (MDR).
- Increment PC:
- PC is incremented to point to the next instruction.
2.)Instruction Decode (ID):
- Instruction Register (IR):
- The fetched instruction is moved to the Instruction Register (IR).
- Decode:
- The control unit decodes the opcode to determine the operation to be performed.
3.) Execute (EX):
- ALU Operations:
- Arithmetic and logical operations are performed by the Arithmetic Logic Unit (ALU) based on the decoded instruction.
- Data Movement:
- Data is moved between registers or between registers and memory based on the instruction.
- Branching:
- Conditional or unconditional branches may occur based on the instruction.
4.) Memory Access (MA):
- Effective Address Calculation:
- For instructions involving memory, the effective address is calculated based on addressing modes.
- Memory Read/Write:
- Data is read from or written to the memory location specified by the effective address.
5.) Write Back (WB):
- Registers and Flags:
- The result of the ALU operation or the data read from memory is written back to the appropriate registers.
- Update Flags:
- Status flags are updated based on the result of the operation.
- Data Flow:
- Registers:
- Data flows between various registers like Accumulator (A), General Purpose Registers (B, C, D, E, H, L), and Special Purpose Registers.
- Buses:
- Data buses (address bus and data bus) facilitate communication between the processor, memory, and peripherals.
- Flags:
- Flags (Sign, Zero, Auxiliary Carry, Parity, Carry) are set or reset based on the result of arithmetic or logical operations.
- Control Signals:
- Control signals are generated by the control unit to coordinate the flow of data and manage the execution of instructions.
Example:
- Let’s consider a simple MOV instruction:
- The instruction fetches the opcode for MOV from memory into the IR.
- The control unit decodes the opcode and determines that it is a data movement instruction.
- The contents of register A are transferred to register B.
- Flags are updated based on the operation.