How Does the 8086 Execution Unit Work? | Architecture of 8086 | Microprocessor & Peripherals Interfacing

TL;DR

The 8086 execution unit executes instructions by sending queued opcodes to its control system, which decodes them and generates the required control signals. The bus interface unit fetches instructions from memory and transfers them into a 6-byte queue when two bytes are free. Physical addresses are calculated as segment address × 10H + offset address. Read on to understand how fetching, queuing, decoding, and execution connect.

Transcript

hi students today we will discuss about execution unit in the last class i explained a calculation of physical address in the before that class i explained bus interface unit please if you didn't see the video please go through the all these videos this is very very helpful to you if you know these basics you can very very easily understand further... Read More

Key Insights

• 🎭 The execution unit performs arithmetic and logical operations on data fetched from memory.
• 🎮 The control system in the execution unit decodes opcodes and generates control signals.
• 🏪 General purpose registers store data, while segment registers store offset addresses.
• 👨‍💼 The bus interface unit fetches instructions from memory for execution.
• 🇦🇪 The execution unit stores results in registers and updates flags to indicate status.
• 🫦 Physical addresses are calculated to access the address bus using 16-bit registers.
• 💁 An assembler converts assembly language instructions into binary format.

Questions & Answers

Q: How does the execution unit work in the 8086 architecture?

The execution unit executes instructions whose data is available in the instruction queue. The first instruction goes to the control system, which decodes its opcode and generates control signals specifying the operation to perform.

Q: What does the control system in the 8086 execution unit do?

The control system decodes, or understands, the opcode of an instruction. Because each instruction has a separate opcode, the control system uses it to generate the signals needed for operations such as addition or arithmetic processing.

Q: What is the purpose of the bus interface unit in the 8086?

The bus interface unit fetches instructions that are available in memory. It transfers the fetched instruction data through the C-bus to the instruction queue.

Q: What is the size of the 8086 instruction queue?

The instruction queue is six bytes in size. The bus interface unit waits until two bytes, or 16 bits, are free before transferring data into it.

Q: Why does the 8086 calculate a physical address?

The address bus is 20 bits wide, while the registers are 16 bits. A physical address therefore must be calculated to access the 20-bit address bus.

Q: How is the physical address calculated in the 8086?

The stated formula is segment address × 10H + offset address. For the code segment, the dedicated offset address is held in the instruction pointer.

Q: How is an assembly-language program stored in memory?

Memory stores the program in binary, or machine-language, format. An assembler converts an assembly-language instruction such as “MOV AL, BL” into that binary representation or object file.

Q: What is an opcode in an 8086 instruction?

Each instruction has a separate opcode, also called an operational code. The opcode is the binary instruction information that the control system decodes to determine which control signals to generate.

Summary & Key Takeaways

• The execution unit executes instructions and performs arithmetic and logical operations.

• It fetches data from the bus interface unit and stores results in registers.

• General purpose registers hold data, while segment registers store offset addresses.