Overview
This lecture covers the architecture and organization of the IAS (von Neumann) machine, explaining its components, registers, and the fetch-execute cycle in modern computers.
Computer Architecture vs. Organization
- Computer architecture describes attributes affecting program execution logic.
- Computer organization refers to how components and their interconnections realize the architecture.
IAS (von Neumann) Architecture
- IAS architecture stores both data and instructions (programs) in memory.
- The main components: main memory, input/output devices, arithmetic logic unit (ALU), and control unit (CPU).
- System bus enables data transfer between memory, CPU, and I/O devices.
Computer Functions and Structure
- Four primary functions: data processing, data storage, data movement, and data control.
- Three levels: input/output devices, main memory, CPU; connected by system buses.
IAS Machine Memory and Instruction Structure
- Memory has 1,000 locations, each storing data or instructions.
- Data word: 39 bits for number, 1 bit for sign.
- Instruction word: divided into left and right instructions, each with opcode (8 bits) and address.
Registers in the IAS Machine
- Registers serve as temporary fast storage in the CPU.
- Memory Buffer Register (MBR): holds data/instructions between memory and CPU.
- Memory Address Register (MAR): holds memory addresses for read/write operations.
- Instruction Register (IR): holds and decodes the opcode of an instruction.
- Instruction Buffer Register (IBR): holds the right instruction from instruction word.
- Program Counter (PC): points to the next instruction to fetch.
- Accumulator (AC) and Multiplier Quotient (MQ): store results from ALU operations.
Fetch and Execute Cycle
- Fetch cycle: PC fetches the instruction from memory to MBR, MAR, and IR/IBR.
- Execute cycle: opcode is interpreted and operation is carried out by the ALU.
Key Features of IAS Machine
- Single read/write memory for both data and instructions (stored program concept).
- Addressable memory locations for precise data and instruction handling.
- Sequential execution of instructions unless explicitly changed.
Programming in Hardware vs. Software
- Hardware programming: circuits are manually wired for each operation (used in ENIAC).
- Software programming: instructions interpreted by control unit, which signals ALU to perform operations based on opcode.
Top-Level Computer Components (IAS View)
- CPU with registers and ALU.
- Main memory (addressable storage for programs and data).
- Input/output modules and ports for devices.
- System bus for communication between components.
Key Terms & Definitions
- Architecture — Logical structure defining computer behavior.
- Organization — Physical arrangement and interconnection of hardware.
- ALU (Arithmetic Logic Unit) — Processes arithmetic and logic operations.
- Register — Fast, small storage area inside CPU.
- Opcode — Portion of an instruction specifying the operation.
- Program Counter (PC) — Register pointing to the next instruction.
- Memory Buffer Register (MBR) — Temporarily stores data/instructions from memory.
- Memory Address Register (MAR) — Holds address of memory to access.
- Instruction Register (IR) — Holds and decodes the current instruction’s opcode.
- Instruction Buffer Register (IBR) — Temporarily holds additional instruction data.
Action Items / Next Steps
- Read Chapter 23 for more details (available in the library).
- Prepare for next week’s deeper dive into the instruction fetch and execute process.