1  1998 Morgan Kaufmann Publishers Interfacing Processors and Peripherals

2  1998 Morgan Kaufmann Publishers I/O Devices

3  1998 Morgan Kaufmann Publishers Input/Output Very diverse devices –behaviour (i.e., input vs. output) –partner (who is at the other end?) –data rate Important but neglected –difficulties in assessing and designing I/O systems have often relegated I/O to second class status

4  1998 Morgan Kaufmann Publishers Input/Output I/O design affected by performance, expandability, resilience, etc. I/O System performance depends on: –CPU –memory system (caches, main memory) –buses –I/O controller –I/O device –I/O software (operating system) Performance metrics: –throughput: I/O bandwidth –response time: latency

5  1998 Morgan Kaufmann Publishers I/O Example: Disk Drives To access data: — seek: position head over the proper track (  6 ms) — rotational latency: wait for desired sector (  4 ms) — transfer: one or more sectors (  15 MB/s) Platter Track Platters Sectors Tracks

6  1998 Morgan Kaufmann Publishers Buses Shared communication link (one or more wires) Types of buses: –processor–memory (short, high speed, custom design) –backplane (high speed, often standardised, e.g., PCI) –I/O (lengthy, different devices, standardised, e.g., SCSI) Synchronous –use a clock and a synchronous protocol, fast and small –every device must operate at same rate –wait states possible –clock skew requires the bus to be short Asynchronous –doesn’t use a clock –uses handshaking

7  1998 Morgan Kaufmann Publishers Bus Design Difficult –may be a bottleneck –length of the bus –number of devices –tradeoffs (buffers for higher bandwidth increase latency) –support for many different devices –cost

8  1998 Morgan Kaufmann Publishers Bus Structures

9  1998 Morgan Kaufmann Publishers Bus Structures

10  1998 Morgan Kaufmann Publishers Synchronous Bus Transfer Example: Read T1T2T3 Clock Address lines Memory address Data lines memory Data accepted by processor AS R/W Data from From processor Bi- directional

11  1998 Morgan Kaufmann Publishers Synchronous Bus Transfer Example: Write T1T2T3 Clock Address lines Memory address Data lines AS R/W Data from processor From processor Bi- directional

12  1998 Morgan Kaufmann Publishers Synchronous Write with a Wait State T1T2T3 Clock Address lines Memory address Data lines AS R/W Data from processor From processor Bi- directional WAIT Tw WAIT checked by processor From memory

13  1998 Morgan Kaufmann Publishers Asynchronous Handshaking Example A device requests a word from the memory Three control lines –ReadReq: indicates a read request for memory –DataRdy: indicates the word is ready on the data lines –Ack: acknowledges the Req or Rdy signal of the other party

14  1998 Morgan Kaufmann Publishers Asynchronous Handshaking Waveforms DataRdy Ack Data ReadReq addressdata

15  1998 Morgan Kaufmann Publishers Controlling Bus Access A bus master controls access to the bus –it initiates and controls all bus requests –the slave responds to requests Single bus master –the processor Multiple bus masters –bus arbitration

16  1998 Morgan Kaufmann Publishers Bus Arbitration daisy chain arbitration –bus granted in priority order –a grant line runs through the devices centralized, parallel arbitration –centralised arbiter chooses from among requesting devices –e.g., PCI distributed arbitration by self-selection –each requesting device makes a request on the bus and determines independently who has the highest priority –e.g., NuBus used in Macintosh distributed arbitration by collision detection –each device independently requests the bus, collisions are detected –e.g., Ethernet

17  1998 Morgan Kaufmann Publishers Operating system Responsibilities of the OS arise from –I/O system is shared by multiple programs –I/O systems often use interrupts –low-level control of an I/O device is complex: concurrent events, detailed requirements Functions the OS must provide –protection, by maintaining user rights –abstractions for accessing devices –interrupt handling –equitable access to shared I/O resources Three types of communication –OS gives commands to the devices –devices notify the OS when operations are completed –data transfer between memory and an I/O device

18  1998 Morgan Kaufmann Publishers Giving Commands to I/O Devices Special I/O instructions specifying –device number –command word Memory-mapped I/O –portions of the address space are assigned to I/O devices –those addresses are used by the control, status and data registers of the devices –reads and writes to those addresses are interpreted as commands to the I/O devices –user programs are prevented from issuing I/O operations directly

19  1998 Morgan Kaufmann Publishers Communicating with the Processor The OS needs to know when –the I/O device has completed an operation –the I/O operation has encounted an error Methods –polling –interrupt-driven I/O Polling –the I/O devices put information in status registers –the OS periodically checks the status registers –simple: the processor is totally in control and does all the work –consumes a lot of processor time

20  1998 Morgan Kaufmann Publishers Communicating with the Processor Interrupt-driven I/O –I/O interrupts are just like exceptions except: an I/O interrupt is asynchronous further information needs to be conveyed –interrupts indicate to the processor that an I/O device needs attention –user program progress is only halted but special hardware is needed to send a request (device) detect an interrupt (processor) save the CPU state during the interrupt service routine (processor)

21  1998 Morgan Kaufmann Publishers Direct Memory Access External to the CPU Direct transfer of data to or from the memory without involving the processor The DMA controller becomes the bus master and directs the reads and writes Steps in a DMA transfer –the processor supplies the identity of the device, the operation, the memory address and the number of bytes –the DMA controller completes the operation without bothering the processor –the DMA controller interrupts the processor to inform that the transfer is complete