ITEC 1011 Introduction to Information Technologies 7. Input / Output Chapt. 8

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ITEC 1011 Introduction to Information Technologies Examples of I/O Devices DeviceInput/OutputDate Rate (Kbytes/s) KeyboardInput0.01 MouseInput0.02 Voice input (microphone)Input0.02 ScannerInput200 Voice output (speaker)Output0.5 Dot-matrix printerOutput1 Laser printerOutput100 Graphics displayOutput30,000 Local area networkInput/output200 – 20,000 Optical diskStorage (I/O)500 Magnetic tapeStorage (I/O)2,000 Magnetic diskStorage (I/O)2,000

ITEC 1011 Introduction to Information Technologies I/O Configurations (1 of 2) CPU I/O module I/O device Keyboard Mouse Voice input (microphone) Scanner Voice output (speaker) Dot-matrix printer Laser printer Graphics display Local area network Optical disk Magnetic tape Magnetic disk Can take many forms, e.g., Device controller Disk controller

ITEC 1011 Introduction to Information Technologies I/O Configurations (2 of 2) I/O device I/O module CPU I/O data I/O address I/O module I/O control

ITEC 1011 Introduction to Information Technologies Does this look familiar? I/O device I/O module CPU I/O data I/O address I/O module I/O control Data bus Address bus Control bus

ITEC 1011 Introduction to Information Technologies A Previous Question How is I/O differentiated from memory? Two possibilities Memory-mapped I/O I/O-mapped I/O

ITEC 1011 Introduction to Information Technologies Memory-Mapped I/O Memory and I/O… reside in the same “space” are accessed in the same manner Differentiated only by their addresses Example: STA address (store the contents of the accumulator at the specified address) STA 2000 might store A in memory STA 8000 might store A in an I/O device p. 236

ITEC 1011 Introduction to Information Technologies Memory Map FFFF 0000 Memory I/O

ITEC 1011 Introduction to Information Technologies A Previous Question How is I/O differentiated from memory? Two possibilities Memory-mapped I/O I/O-mapped I/O

ITEC 1011 Introduction to Information Technologies I/O-Mapped I/O Memory and I/O… Occupy different “spaces” Are accessed by unique instructions Differentiated by instructions Memory instructions Most instructions reference memory I/O instructions (see p. 361) move data to/from a specified I/O address (“port”) and a CPU register (e.g., the accumulator) IN port – inputs data from a device OUT port – outputs data to a device

ITEC 1011 Introduction to Information Technologies Memory Maps FFFF 0000 Memory I/O FFFF 0000

ITEC 1011 Introduction to Information Technologies Implementation of I/O-Mapped I/O Typically, access to memory and I/O uses the same address bus and data bus A dedicated control bus signal differentiates a “memory cycle” from an “I/O cycle” On Intel’s Pentium CPU, this control bus signal is named M/IO M/IO = 0  memory cycle M/IO = 1  input/output cycle

ITEC 1011 Introduction to Information Technologies One of the control bus signals is named M/IO M/IO I/O device CPU I/O data I/O address I/O module I/O control Data bus Address bus Control bus Memory

ITEC 1011 Introduction to Information Technologies Types of I/O Programmed I/O Interrupt-driven I/O Direct memory access (DMA)

ITEC 1011 Introduction to Information Technologies Programmed I/O I/O operations are under direct control of software (program) Software initiates the I/O operation Disadvantage: Slow Uses a lot of CPU resources Advantage: Simple p. 209

ITEC 1011 Introduction to Information Technologies Polling A form of programmed I/O, wherein device “status” is checked to determine if an I/O operation is needed E.g., A keyboard can be polled to determine if a key has been struck and a code is waiting to be read Useful when there are a lot of similar devices connected to one system (e.g., hundreds of terminals)

ITEC 1011 Introduction to Information Technologies Checking Device Status The I/O module has registers for data transfers (of course), plus it also has… Control/command registers To configure and control a device Status registers To check the status of a device These registers are read/written using Memory instructions (memory-mapped I/O) IN/OUT instructions (I/O-mapped I/O)

ITEC 1011 Introduction to Information Technologies Types of I/O Programmed I/O Interrupt-driven I/O Direct memory access (DMA)

ITEC 1011 Introduction to Information Technologies Interrupt-driven I/O I/O operations are initiated by the device The device, or its I/O module, includes a signal to interrupt the CPU These signals are called interrupt lines A typical CPU supports 8 to 16 interrupt inputs Typical names: IRQ1, IRQ2, IRQ3, etc. p. 211

ITEC 1011 Introduction to Information Technologies Servicing an Interrupt When an interrupt occurs (and is accepted), the execution of the current program is suspended A special routine executes to service the interrupt Then, the interrupted program resumes The service routine is called an interrupt handler or interrupt service routine (ISR)

ITEC 1011 Introduction to Information Technologies Saving Registers For the interrupted program to resume, the CPU status and data registers must be saved (because they will change during the ISR) They are saved before the ISR executes They are restored after the ISR executes They are saved either On the stack (a special area of memory to temporarily hold information), or In a process control block (PCB)

ITEC 1011 Introduction to Information Technologies Use of Interrupts As an external event notifier As a completion signal As a means of allocating CPU time As an abnormal event indicator

ITEC 1011 Introduction to Information Technologies Interrupts for External Events An interrupt signal occurs when an “event” occurs in a device – an event that requires the CPU’s attention E.g., Keyboard: a key has been hit (the ISR reads the code for the key) Notebook computer cover: the cover is closed (the ISR puts the computer in standby mode) p. 214

ITEC 1011 Introduction to Information Technologies Use of Interrupts As an external event notifier As a completion signal As a means of allocating CPU time As an abnormal event indicator

ITEC 1011 Introduction to Information Technologies Interrupts for Completion Signals An interrupt signal occurs when a device has completed an operation – and the CPU should know about it E.g., Printer: the output buffer is empty (the CPU can send more data) Scanner: a data transfer is complete (the CPU/application can proceed to process the image data) p. 215

ITEC 1011 Introduction to Information Technologies Use of Interrupts As an external event notifier As a completion signal As a means of allocating CPU time As an abnormal event indicator

ITEC 1011 Introduction to Information Technologies Interrupts for Allocating CPU Time Useful on multi-tasking systems – systems that can execute more than one program at a time E.g., A timer is programmed to interrupt the CPU every 100 µs (for example) The ISR is a “dispatcher program” Execution switches to another program (for 100 µs), etc. p. 216

ITEC 1011 Introduction to Information Technologies Use of Interrupts As an external event notifier As a completion signal As a means of allocating CPU time As an abnormal event indicator

ITEC 1011 Introduction to Information Technologies Interrupts for Abnormal Events An interrupt signal occurs when an abnormal event occurs that needs immediate system attention E.g., A heat sensor near the CPU chip – if the temperature is too high, an interrupt is generated, the ISR activates the fan near the CPU chip p. 216

ITEC 1011 Introduction to Information Technologies Types of I/O Programmed I/O Interrupt-driven I/O Direct memory access (DMA)

ITEC 1011 Introduction to Information Technologies Why DMA? Used for high-speed block transfers between a device and memory During the transfer, the CPU is not involved Typical DMA devices: Disk drives, tape drives Remember (1 st slide) Keyboard data rate  0.01 KB/s (1 byte every 100 ms) Disk drive data rate  2,000 KB/s (1 byte every 0.5 µs) Transfer rate is too high to be controlled by software executing on the CPU p. 223

ITEC 1011 Introduction to Information Technologies How The CPU “prepares” the DMA operation by transferring information to a DMA controller (DMAC): Location of the data on the device Location of the data in memory Size of the block to transfer Direction of the transfer Mode of transfer (burst, cycle steal) When the device is ready to transfer data, the DMAC takes control of the system buses (next few slides)

ITEC 1011 Introduction to Information Technologies “Taking Control” (1 of 2) CPUDMAC BR BG BGACK BR BG BGACK BR = Bus request (DMAC: May I take control of the system buses?) BG = Bus grant (CPU: Yes, here you go.) BGACK = BG acknowledge (DMAC: Thanks, I’ve got control.) Control Bus signals

ITEC 1011 Introduction to Information Technologies DMAC issues a BR (“bus request”) signal CPU halts (perhaps in the middle of an instruction!) and issues a BG (“bus grant”) signal DMAC issues BGACK (“bus grant acknowledge”) and releases BR DMAC has control of the system buses DMAC “acts like the CPU” and generates the bus signals (e.g., address, control) for one transfer to take place Then… “Taking Control” (2 of 2)

ITEC 1011 Introduction to Information Technologies DMA Transfers (2 of 2) Burst mode This transfer is repeated until complete DMAC relinquishes control of the system buses by releasing BGACK Cycle steal mode DMAC relinquishes control of the system buses by releasing BGACK A BR-BG-BGACK sequence occurs for every transfer, until the block is completely transferred DMAC interrupts the CPU when the transfer is complete This is an example of a “completion signal” interrupt

ITEC 1011 Introduction to Information Technologies BR-BG-BGACK Timing BR BG BGACK CPU cycles CPU cycles DMA cycles time

ITEC 1011 Introduction to Information Technologies Burst Mode vs. Cycle Steal Mode Burst mode: Cycle steal mode: Legend: CPU cycle DMA cycle BR/BG/BGACK sequence time

ITEC 1011 Introduction to Information Technologies DMA includes all three types of I/O. Let’s see… Types of I/O Programmed I/O Interrupt-driven I/O Direct memory access (DMA)

ITEC 1011 Introduction to Information Technologies Program-Controlled I/O (in DMA) Data bus Address bus Control bus Memory DMAC CPU Disk The CPU “prepares” the DMAC

ITEC 1011 Introduction to Information Technologies DMA Data bus Address bus Control bus Memory DMAC CPU Disk The transfer takes place

ITEC 1011 Introduction to Information Technologies Interrupt-driven I/O (in DMA) Data bus Address bus Control bus Memory DMAC CPU Disk The DMAC interrupts the CPU when the transfer is complete IRQ

ITEC 1011 Introduction to Information Technologies I/O System Architectures Bus architecture Channel architecture

ITEC 1011 Introduction to Information Technologies Bus Architecture Used in (pretty well all) PCs, workstations, and some mainframe computers We have already met the data, address, and control buses that connect a CPU to memory and I/O modules Collectively, these are the “CPU bus”or “system bus” Between the I/O modules and I/O devices, an “I/O bus” is required A “bus interface” connects one bus to another Let’s have another look… p. 228

ITEC 1011 Introduction to Information Technologies “I/O bus” “Bus interface” “CPU bus” or “System bus” CPU-Memory-I/O Architecture CPU I/O module Memory I/O device

ITEC 1011 Introduction to Information Technologies I/O System Architectures Bus architecture Channel architecture

ITEC 1011 Introduction to Information Technologies Channel Architecture An alternative I/O architecture Used by IBM in their 370/XA and ESA/370 mainframe computers I/O occurs through an “I/O processor” – the “channel subsystem” Frees the CPU for other tasks Has its own instruction set – “channel control words” Channel control words stored as “programs”, just like other CPU instructions Channel programs transfer data between I/O devices and memory via DMA p. 236

ITEC 1011 Introduction to Information Technologies CPUMemory Channel subsystem Control unit Device Channel paths Device I/O Channel Architecture

ITEC 1011 Introduction to Information Technologies Thank you