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York University
Information Technology
ITEC 1000

Introduction to Information Technology ITEC 1000 – Winter 2010 – Peter Khaiter Lecture 8 – Input/Output – Mar 1 I/O Considerations - Speed Issues o CPU operates at speeds much faster than the fastest I/O device o Devices operate at different speeds o Bursts of data o Block data transfer required for some devices - Coordination o I/O devices completely operate under CPU program control o Several devices perform I/O simultaneously o Unexpected input (Ctrl-C, Ctrl-S, Ctrl-Alt-Del) o Various input formats o Status information needed for each device I/O Device Interface Issues - Different formats o Parallel interface o Serial interface - Buffering of data - Burst vs. Stream - Different control requirements o Electromechanical I/O Module Functions - Recognizes messages from device(s) addressed to it and accepts commands from the CPU - Provides a buffer where the data from memory can be held until it can be transferred to the disk - Provides the necessary registers and controls to perform a direct memory transfer - Physically controls the device - Copies data from its buffer to the device/from the CPU to its buffer - Notifies with interrupts Programmed I/O - I/o module connected to a pair of i/o registers in cpu via bus - Transfer: one word at a time - Input: i/o device i/o module i/o register ar under program control - Output: ar i/o register i/o module i/o device - In practice: multiple i/o devices connected to cpu - I/o data and address registers in cpu: work similarly to mar and mdr - Address information must be sent with the i/o instruction in the address field - Requires full instruction fetch/execute cycle for each word data transfer - Very slow - Primary use: - Keyboards - Communication with i/o modules to control i/o operations (see dma) Polling - A form of programmed I/O, wherein device “status” is checked to determine if an I/O operation is needed o 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) Interrupts - Signal that causes the CPU to alter its normal flow of a program/instruction execution o Frees CPU from waiting for events o Provides control for external input - Examples o Unexpected input o Abnormal situation o Illegal instructions o Multitasking, multiprocessing - Provided by special control lines to CPU o Called interrupt lines o In modern pcs: up to 32 labeled IRQ0..IRQ31 o Transmits message called interrupt o Causes computer to suspend program execution and jump to interrupt processing program Interrupting Terminology - Interrupt lines (hardware) - Interrupt request - Interrupt handlers o Program that services the interrupt o Also known as an interrupt routine o Determines appropriate course of actions when interrupt occurs - Process Control Block (PCB) o Located in a part of memory known as the stack area o All registers of a program (last instruction executed, data value in registers) are saved here before control is transferred to the interrupt
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