CS251 - Computer Organization and Design - Spring 2008

Lecture 36 - Input/Output

Practical Details

  1. Assignment 9
  2. Optional Assignment

Input/Output

The key concept is the `system bus', which is also known as the

though these two night be separate entities. Combined memory-I/O buses are used in systems with

On the system bus you will find three types of devices

  1. Parts of the User interface, interact with humans

    Note. Data rates that follow are nominal, which does not always match up with useful. E.g. keyboard

    But to get that rate most of us would need to type the same phrase over and over, which is not very useful. That is, I, and probably also you, can't think up worthwhile things to type fast enough to keep up with our typing speed.

    Device Input/Output I/O data rate (Mbit/sec)
    Keyboard Input 0.0001
    Mouse Input/output 0.0038
    Voice Input/output 0.264
    Printer Output 3.2
    Bit-mapped graphics Output 100
  2. Memory
    Device Data rate (Mbit/sec) Access Delay (microsec)
    Magnetic tape 30 1,000,000,000 (human limited)
    Optical disk (CDROM) 80 100,000,000 (human limited)
    Magnetic disk 1,000 10,000
    SDRAM 20,000 0.05

    Built into a `seamless' memory hierarchy, but if you don't know where the seams are your programs won't run very well.

  3. Network interfaces
    Device Data rate (Mbit/sec) Access Delay (microsec)
    Modem 0.06 15,000,000
    Wireless LAN 50 1,000,000
    Wired LAN 1,000 < 1,000

Buses

Processor to Cache

Bandwidth: 1-5 Gwords/sec

Cache to High Bandwidth Devices (North Bridge)

Devices

  1. Main memory
  2. Bit-mapped graphics
  3. Network

Bandwidth: 200-500 Mwords/sec

High Bandwidth Devices to Low Bandwidth Devices (South Bridge)

Devices

  1. Disks
  2. Audio
  3. USB for keyboard, mouse, etc
  4. Slow ethernet
  5. CDROM
  6. Legacy devices

Bus Transactions

Concepts

  1. Master/slave
  2. Bus arbitration
  3. Synchronous/asynchronous
  4. Block transfer
  5. Multiplexed/non-Multiplexed

Typical Bus Transaction (Asynchronous, multiplexed)

  1. Master requests bus
  2. Master receives bus from bus arbitration hardware
  3. Master asserts Address, then Read.
  4. Slave sees Read, latches Address, then asserts Acknowledge
  5. Master sees Acknowledge, releases Read and Address
  6. Slave sees Read released, releases Acknowledge
  7. Slave asserts Data, then DataReady
  8. Master sees DataReady, latches Data, then asserts Acknowledge
  9. Slave sees Acknowledge, releases Data and DataReady
  10. Master sees DatReady released, releases Acknowledge
  11. Master releases bus

On a block transfer steps 7 to 10 are repeated until all data is transferred, the releases bus.

On a synchronous bus, assertion of Read, Acknowledge and DataReady are timed by a bus clock

On a non-Multiplexed bus there are separate address and data lines, and separate acknowledge lines

DMA (direct memory access) is possible if a device can become bus master.

Interrupts

Devices can assert interrupt lines on the bus to request service from the processor.

Interrupt Processing

  1. Device asserts its interrupt output
  2. Interrupt control unit (ICU) receives interrupt signal on its input, asserts its interrupt output
  3. Before each instruction fetch the processor checks its interrupt input.
  4. If it sees the input asserted
    1. It reads a register of the ICU, during which the pipeline drains
    2. The read returns a program counter (called an interrupt vector)
    3. The program counter is used to fetch the first instruction of the interrupt service routine (ISR). We saw this briefly earlier when we were discussing control signals in the processor.

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