Lecture Notes for Operating Systems

9 September 2004 - System Architecture

The von Neumann Architecture

von Neumann Characteristics

• It's the cockroach of computer architectures.

  • You just can't get rid of it.

• Instructions and data are the same.

• Stored-program computer.

• A computer is a giant state machine.

von Neumann Features

• The link between the CPU and storage is the von Neumann Bottleneck.

  • Fast CPUs, slow storage.

    • DRAM has 50-100 ns access times.

    • SRAM has 25 ns access times.

    • Rambus has 0.6-0.8 ns access times.

• Self-modifying code.

Other Architectures

• The Harvard Architecture has separate storages for program and data.

  • Often found in controllers.

• The data-flow architecture treats computations as graphs.

  • The IBM-Toshiba Cell processor.

The Pieces

• The CPU.

• Physical memory.

• Devices

• Other Pieces.

The CPU

• The brain in powerful electronic brain.

• The registers - state.

  • Data registers.

  • Control and status registers.

• The cache - more state; also off-chip cache.

• The Arithmetic-Logic Unit (ALU).

• The control unit.

CPU Structure

Physical Memory

• A big array of units - bytes, words, whatever.

• Memory is actually another device.

• Volatile vs. non-volatile - RAM vs. ROM.

• Memory structure is actually complicated - squeezing out maximum performance is tricky.

• Power-managed memory.

Devices

• General device characteristics.

  • Like special purpose computers - limited instruction set.

  • Many classifications - character or block, synchronous or asynchronous, serial or parallel, storage or communication.

Device controllers

• The interface to the device.

• Generally a bunch of registers - some read, some write, some read-write.

• Device registers may be require special CPU instructions or look like memory.

• Other forms of CPU-device communication for large amounts of data - buffers.

• Device drivers - what the operating system provides.

Other Pieces

• Memory management unit (MMU).

  • On (fast and small) or off (large and slow) the CPU chip.

• Clocks and timers - countdown and running.

Interconnections

• Busses tie the components together.

  • Address, data, io bus.

  • Bus width and speed - less wide busses are slower.

  • Bus arbitration.

  • Many busses, dual-ported memory.

Interrupts

• How devices get the CPU's attention.

• An interrupt is an electric signal on a wire.

• Interrupts stop the current instruction sequence and start another one - what's the state just after the interrupt.

• Other information may be included - device id, interrupt id (called the interrupt level).

• The choice of alternative instruction stream may vary - interrupt vector.

• Interrupts can be disabled, which is dangerous.

Caches

• Caches are fast, small memory.

• On-chip or off-chip caches (CPU).

• Caching is a useful technique for speed-up.

• A cache represents a lot of state.

• Buffers are caches made out of regular memory.

Operation

• Boot up - what happens when the power goes on.

  • Read in a boot program from somewhere (ROM, the boot sector on disk), then execute it.

• Basic operation - fetch, execute, store.

Operating Modes

• Protection and efficient management require access restrictions.

• CPU operating modes - some modes can't execute some instructions.

• User or kernel modes - kernel mode can do anything; user mode can't.

• Traps - user level instructions that switch to kernel mode; an interrupt.

This page last modified on 14 November 2004.