INPUT/OUTPUT HANDLING

Transcription

1 Datorarkitektur I Fö 4-1 Datorarkitektur I Fö 4-2 Input/Output Devices INPUT/OUTPUT HANDLING 1. Input/Output Devices and System Buses 2. Bus Arbitration 3. Bus Timing 4. I/O Modules 5. I/O ess Decoding 6. Programmed and Interrupt-driven I/O Input and output devices provide a means for information storage and for people (environment) to interact with the computer. Secondary memory: provides the long-term storage of large amounts of data and program (diskette, hard-disk, magnetic tape, CD-ROM). Sensors and actuators: used for direct interaction with the environment. Human interface: keyboard, monitor, mouse, printer. Communication devices: modem, network. 7. Direct Access Datorarkitektur I Fö 4-3 Datorarkitektur I Fö 4-4 Input/Output Devices (cont d) Input/Output Devices (cont d) Video Keyboard Diskette Hard disk The bus is used - by I/O s for communication to/from CPU or memory - by the CPU for fetching or storing instruction and data from/to memory. Bus Conflicts between CPU and I/O s CPU Main Controller: controls its I/O device and handles bus access for it. Hard disk receives, for example, a read request from the CPU gives corresponding commands to the device in order to execute the request, collects data and organises the incoming bit stream into units to be sent on the bus. A has to interact with the bus, according to a certain protocol, in order to transmit and/or receive. In case of conflicts a device (chip) called bus arbiter ides on access. In general, I/O devices are given preference over the CPU; usually devices cannot be stopped forcing them to wait would result in loss data. - When no I/O is in progress, the CPU has all bus cycles for itself to reference memory. - When some I/O device is also running and requests the bus, it gets it cycle stealing slows down the computer.

2 Datorarkitektur I Fö 4-5 Datorarkitektur I Fö 4-6 Input/Output Devices (cont d) bus Input/Output Devices (cont d) SCSI bus CPU SCSI PCI bridge Video Main Network Different buses are connected through adequate bridges (bridges also perform buffering of information); Sound card Printer ISA bridge PCI Bus Modem Advantages of architectures with multiple buses: avoids bus conflicts; insulates CPU-to-memory traffic from I/O traffic; allows the system to support a variety of I/O devices tailored for different bus standards. ISA Bus ISA (Industry Standard Architecture) bus: relatively slow. PCI (Peripheral Component Interconnect) bus: faster bus. CPU and memory are connected through a dedicated high speed interconnection. SCSI (Small Computer System Interface) is a hard disk interface; it is also a bus standard for high transfer rate peripherals. Up to seven devices can be attached to a SCSI bus; typically: hard discs, CD-ROMs, scanners. Datorarkitektur I Fö 4-7 Datorarkitektur I Fö 4-8 External Interface of the CPU Chip External Interface of the CPU Chip (cont d) essing Bus control Interrupts m n Micro Processor Bus arbitration Coprocessor control pins: - bus control: the CPU uses these pins to control the rest of the system and tell it what it wants to do; control signals are propagated over the system bus. Φ +5V - interrupt pins: on these pins the CPU gets signals from I/O modules; they usually indicate that an I/O operation has been completed; Clock Power signal The CPU pins can be divided into: address pins, data pins, and control pins. address pins: the address is output to the system bus on these pins, for read/write operations. - with m address pins, 2 m locations can be addresses. data pins: data bits are output/received to/from the system bus on these pins. - with n data pins an n-bit word can be read written in a single operation. - bus arbitration: are needed to regulate traffic on the system bus, to prevent devices from trying to use it at the same time; - coprocessor: facilitate communication with coprocessors, such as floating point chips, graphic chips, etc.

3 Datorarkitektur I Fö 4-9 Datorarkitektur I Fö 4-10 System Buses System Buses (cont d) SCSI bus Sound card CPU SCSI Printer PCI bridge bus Video ISA bridge Main Network PCI Bus Modem A system bus typically consists of separate lines. They can be classified into three functional groups: 1. lines: provide a path for moving data between system components. The width of the data bus: 8, 16, 32, 64 lines. 2. ess lines: are used to designate the source or destination of data. The width of the address bus determines the maximum memory size which can be addressed. 3. Control lines: are used to control bus access, synchronize operations, and to propagate commands throughout the system. ISA Bus A bus is a common electrical pathway between multiple devices. One or several buses can be used in a computer system to interconnect CPU, memory, and devices. in addition to such "system buses", there are buses also inside the CPU (internal buses). System buses differ in the number and organization of lines, arbitration, timing, and specific bus operations. In order to connect a device to a bus, the device must fit to the respective bus features. In order to avoid vary large buses multiplexed bus. Instead of having separate address and data lines, we can have 32 lines for address and data together. At the start of the bus operation first the lines are used for address; later on, the same lines are used for data. Bus width can be reduced, but the system becomes slower (for a write, address and data can not be put on the lines simultaneously). Datorarkitektur I Fö 4-11 Datorarkitektur I Fö 4-12 Bus Arbitration Bus Arbitration (cont d) Devices connected to a bus can be of two kinds: 1. Master: is active and can initiate a bus transfer. 2. Slave: is passive and waits for requests. Some devices can act both as master and as slave, depending on the circumstances: CPU is typically a master. A coprocessor, however, can initiate a transfer of a parameter from the CPU CPU acts like a slave. An I/O device usually acts like a slave in interaction with the CPU. Several devices can perform direct access to the memory, in which case they access the bus like a master. The memory acts only like a slave. What happens if two or more devices want to act simultaneously as masters? Since only one unit at a time can transmit over the bus, arbitration is needed. Arbitration mechanisms: Centralized arbitration: there is a single device, the bus arbiter, that determines who goes next. Decentralized (distributed) arbitration: no arbiter is needed. For example: - there are as many request lines on the bus as devices; each device monitors each request line after each bus cycle each device knows if he is the highest priority device which requested the bus if yes, it takes it. With entralized arbitration, no arbiter but more bus lines are needed. Examples: PCI and ISA buses use a centralized arbitration scheme. SCSI buses use a entralized scheme.

4 Datorarkitektur I Fö 4-13 Datorarkitektur I Fö 4-14 Bus Timing Timing refers to the way in which events are coordinated on the bus: 1. Synchronous timing: the occurrence of events on the bus is determined by a clock. 2. Asynchronous timing: the occurrence of one event on a bus follows and depends on the occurrence of a previous event. Clock Start Read ess Lines Lines Synchronous Timing Bus cycle Acknowledge Examples: PCI and ISA buses use synchronous timing. SCSI buses use asynchronous timing. The bus includes a clock line; all devices on the bus can read the clock line. All events on the bus start at the beginning of a clock cycle. A bus sequence for a synchronous memory read. - The CPU (master) issues a start signal to mark the presence of address and control information on the bus: the read signal is issued on the respective control line, and the memory address is placed on the address lines. - After a delay of two bus cycles, the memory (slave) places the data on the data lines and issues an acknowledge signal on the respective control line. Datorarkitektur I Fö 4-15 Datorarkitektur I Fö 4-16 Asynchronous Timing Synchronous Timing (cont d) Advantages with synchronous timing: simple to implement and test. Disadvantages with synchronous timing: not flexible. - Execution times have to be multiples of clock cycles (if an operation needs 3.1 clock cycles, it will take 4 cycles). - Bus frequency has to be adapted to slower devices one cannot take full advantage of the faster ones. There is no clock line on the bus. Each event is caused by a prior event, not by the clock pulse. The master will wait exactly as much as is needed for the slave to finish. If a master has to wait long for a certain slow slave, this does not influence how much it will have to wait for another faster slave in a following operation.

5 Datorarkitektur I Fö 4-17 Datorarkitektur I Fö 4-18 MSYN Read ess Lines Lines SSYN Asynchronous Timing (cont d) A bus sequence for an asynchronous memory read. - After the CPU (master) has asserted the address lines and issued the read signal, it waits until this lines are stable and then issues the MSYN signal (Master SYNchronization). - When the memory (slave) sees the MSYN, it performs the work as fast as possible and puts the data values on the data lines and then asserts the SSYN (Slave SYNchronization) signal. - When the master has noticed the SSYN, it knows that data is on the lines and latches it, after which it negates the address lines, MSYN, and read. - When the slave sees the negation of MSYN, it knows that data has been read, and negates data lines and SSYN. I/O Modules I/O modules are usually performing the task of the I/O. Very often they are produced as standard I/O chips. An I/O module has an interface to the device it controls, and another one to the system bus (or directly to the CPU and/or memory). Major functions of an I/O module: control and timing of the operations; bus (possibly direct CPU) communication; device communication; data buffering; error detection. A possible sequence of steps, for a data transfer between a device and the CPU (over the bus): 1. The CPU interrogates (over the bus) the I/O module, over the status of the device. 2. The I/O module returns (over the bus) the device status. 3. If the device is OK and ready, the CPU requests the transfer of data by means of a command to the I/O module. 4. The I/O module issues appropriate commands to the device and obtains a unit of data. 5. The data is transmitted (over the bus) to the CPU. Datorarkitektur I Fö 4-19 Datorarkitektur I Fö 4-20 I/O Modules (cont d) buffering is essential for I/O modules; it allows transfer to/from memory to be performed at memory speed instead of the much lower speed of I/O devices. Structure of an I/O module System bus interface Device interface I/O ess Decoding Problem: How is the external device selected for an I/O operation? lines buffer Status/control reg. Interface Device 1 Status Control There are two possible ways: 1. mapped I/O 2. Isolated I/O ess lines Control lines internal control logic Interface Device n Status Control I/O modules hide several details of handling a device from the CPU the CPU can view the device in terms of simpler operations: Sophisticated modules, I/O channels, take over most of the details and present a high-level interface to the CPU; they are used on mainframes and some minicomputers. Simpler modules, device s, are used on PC s, workstations.

6 Datorarkitektur I Fö 4-21 Datorarkitektur I Fö mapped I/O -mapped I/O (cont d) read write lines ess lines Control lines A single read and a single write line are needed on the bus. The particular address is iding if it is going to be a memory read/write or an input/output on a certain device. mod. 1 mod. 2 esses 0.. X 1 esses X X 2 mod. 3 device 1 device 2 esses X X 3 esses X 3 +1,X 3 +2 X esses 3 +3,X 3 +4 Advantage Easy to program: memory reference instructions are used for I/O programming, instead of a quite reduced set of difficult to use specific I/O instructions (see Fö 5). There is a single address space for memory locations and I/O devices. The CPU treats the status and data registers of I/O modules as memory locations the same machine instructions are used to access both memory and I/O devices. Disadvantage Only a part of the address space is available for memory. Another part is dedicated for device addressing. Datorarkitektur I Fö 4-23 Datorarkitektur I Fö 4-24 Isolated I/O lines Isolated I/O (cont d) ess lines mem. read mem. write I/O read I/O write Control lines The CPU treates memory access and I/O differently. I/O operations are performed by specific I/O instructions. When the CPU is executing a memory access instruction, the memory read/write line is asserted. When an I/O instruction is executed, the I/O read/write line is asserted. mod. 1 mod. 2 esses 0.. X 1 esses X X 2 mod. 3 device 1 device 2 esses esses esses X X 3 0, 1 2, 3 Depending on which bus line is asserted, the address on the address lines is interpreted as a memory or as a device address. The address space for I/O is isolated from that for memory the full range of addresses is available for both memory and device addressing. There are separate I/O-read, I/O-write and memoryread, memory-write command lines on the bus.

7 Datorarkitektur I Fö 4-25 Datorarkitektur I Fö 4-26 Input/Output Processing Three techniques are possible for transferring data to/from I/O devices: Programmed I/O The CPU executes a sequence of instructions, being in direct control of the I/O operations (sensing device status, read/write commands, etc.). When the CPU issues a command to the I/O module, it must wait until the I/O operation is complete. 1. Programmed I/O 2. Interrupt-driven I/O 3. Direct memory access A lot of waisted time, because the CPU is much faster then devices. Issue read command not ready read status of I/O module check status ready read word from I/O module write word to memory yes more words? no Datorarkitektur I Fö 4-27 Datorarkitektur I Fö 4-28 Interrupt-driven I/O Interrupt-driven I/O (cont d) After issuing an I/O command, the CPU has not to wait until the operation has finished; instead of waiting, the CPU continues with other useful work. When the I/O operation has been completed, the I/ O module issues an interrupt signal on the bus. After receiving the interrupt, the CPU moves the data to/from memory, and issues a new command if more data has to be read/written. Issue read command read word from I/O module write word to memory Do something useful Interrupt Advantage over programmed I/O Instead of waiting the operation to be finished, the CPU can do some useful work yes more words? no Still a problem The CPU has to take care of each data unit (word), to move it to/from memory, and to issue an I/O command. If large amounts of data have to be moved, this technique is still not efficient, because - the CPU has to take care for each data unit separately; - handling the interrupt also takes some time.

8 Datorarkitektur I Fö 4-29 Datorarkitektur I Fö 4-30 Direct Access (DMA) Direct Access (cont d) An additional module on the system bus, the DMA module, takes care of the I/O transfer for the whole sequence of data. Issue read command for block Do something useful The CPU issues a command to the DMA module and transfers to it all the needed information. continue Interrupt The DMA module performs all the operations (acting similarly to the CPU with programmed I/O): it transfers all the data between I/O module and memory without going through the CPU. A simple structure with DMA module: When the DMA module has finished, it issues an interrupt to the CPU. When starting an operation, the CPU informs the DMA module about: - what operation (read or write); - the address of the I/O device involved; - the starting location in memory where information has to be stored to or read from; - the number of words to be transferred. Video Keyboard Diskette Hard disk Bus CPU DMA Module Main Datorarkitektur I Fö 4-31 Summary Input/Output devices provide a means for information storage and for interaction with the computer. CPU, memory and devices are interconnected by one or several system buses. Having several system buses makes the system faster and allows to connect devices tailored for different bus standards. The CPU chip is connected to the rest of the system through address, data, and control pins. A bus consists of data, address, and control lines; address and data lines can be multiplexed. Bus arbitration avoids simultaneous use of the bus by two masters. Centralized and entralized arbitration schemes can be used. Coordination of events on a bus can be synchronous or asynchronous. Asynchronous timing is more efficient but more difficult to implement. I/O modules are interfacing an I/O device to the system bus. They are hiding the details of I/O devices from the CPU. Selection of an I/O device can be memory-mapped or based on isolated I/O. With memory mapped I/O there is a single address space for memory and I/O devices. With isolated I/O, the two address spaces are separated. Three techniques can be used for I/O data transfer: programmed I/O, interrupt-driven I/O, and direct memory access.