1.1 Systems Architecture
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What is the purpose of the control unit (CU)?
• Co-ordinates all activities of the CPU • Directs the flow of data between the CPU and other devices • Accepts, decodes, executes and stores results back in memory
What is the purpose of the program counter (PC)?
• Holds the address of the next instruction to be executed • e.g. the next instruction in a sequence of instructions or an address to jump to if the current instruction is a command to branch
What is the purpose of the memory address register (MAR)?
• Holds the address of the memory location from which data or an instruction is to be fetched or to which data is to be written • Sends the address to memory down the address bus
What is the purpose of the memory data register (MDR)?
• Used to temporarily store the data which is read from and written to memory • All data to and from memory must travel down the data bus and pass through the MDR
What is the purpose of the current instruction register (CIR)?
• Holds the current instruction being executed • Contents of the MDR are copied if it’s an instruction • Contains the opcode and operand of the current instruction
What is the purpose of the arithmetic logic unit (ALU)?
• Performs logical and arithmetic operations on data
What is the purpose of the accumulator?
• Holds the results of calculations made in the ALU
What are buses?
• A physical set of parallel wires connecting and carrying groups of bits between several components of a computer
What is an address bus?
• Carries memory address which identify where data is read from or written to
What is a data bus?
• Carries that binary that make up the actual information being transmitted around the CPU
What is a control bus?
• Controls command and control signals to and from every other component of the CPU
What are the factors that affect the performance of the CPU?
• Clock speed • Cache size • Number of cores
Factors that affect the performance of the CPU: clock speed
• The number of instruction cycles it can execute per second • Measured in hertz • The faster the clock ticks, the more FDE cycles can be executed per second
Factors that affect the performance of the CPU: cache size
• Holds data and instructions that are frequently accessed and used by the CPU • Much quicker to access than RAM
Factors that affect the performance of the CPU: number of cores
• A complete copy of the CPU • CPUs with multiple cores have the power to run multiple programs at the same time
Why does doubling the number of cores not necessarily mean it will double the performance?
• Extra communication is needed between the cores = extra time • Many programs are not designed to make use of multiple cores
What is pipelining?
• The process of completing the FDE cycles of three separate instructions simultaneously
What is the instruction pipeline?
• Consists of various stages an instruction must move through the processor
What is the arithmetic pipeline?
• Parts of the arithmetic operation that can be broken down and overlapped as they are carried out
Von Neumann architecture (4)
• Shared memory space for data and instructions (in the same format) • A single CU/processor follow a linear FDE cycle • One instruction at a time • Register used as fast access to data
Harvard architecture (4)
• Instructions and data stored in separate memory units • Each has its own bus • Reading and writing can be done at the same time as fetching an instruction • Used by RISC processors
Contemporary Processor architecture
• A modern computer architecture combining elements of both Von Neumann and Harvard architecture
What is the the purpose of Single Instruction Multiple Data (SIMD)?
• Parallel processing is where a processor carries out a single instruction on multiple data items at the same time
What is the the purpose of Multiple Instruction Multiple Data (MIMD)?
• Multiple instructions are carried out on multiple data across several cores