block devices

block devices - Block Devices Computer Engineering...

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Computer Engineering Block Devices
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Computer Engineering Introduction • So far, looked at protocols, hardware for connecting relatively slow I/O devices to CPU. • Tried to optimize usage of each component – avoid CPU being slowed down to speed of slowest device. • To understand communication, looked at electrical/mechanical structures, operations of devices.
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Computer Engineering Introduction • Now, look at other I/O devices: – Different mechanical, electrical structures – Different constraints on design of computer system • Peripheral devices capable of operating at relatively high speeds (e.g. graphics display) • Any device interfaced to main computer via large memory buffer - “high speed device”
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Computer Engineering Introduction • Some of communications protocols designed for low speed devices (e.g. serial line) not suitable if device data transfer speed increases. • Higher speed devices still have mechanical parts – new communication problems! • E.g. magnetic tapes, disks • Need new architectural developments to maximize performance of system.
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Computer Engineering Introduction • Focus: devices that transfer data in large chunks – blocks – rather than 1 character at a time.
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Computer Engineering Buffering and Direct Access • Character transmission, coupled with interrupts can slow down both the CPU and the device • Buffered terminals – Rather than sending one character at a time, include some RAM in the device…. – Helps fast device communicate with slow device without need for fast device to slow down.
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Computer Engineering
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Computer Engineering Buffered Terminals • Data can be sent to the printer until the buffer is full. • Thus one interrupt can allow many characters to be printed. • The character transmission technique is known as BURST FEEDING
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Computer Engineering Buffered Terminals • In much the same way buffers can be added to the device interface and the device driven via a spooler in the operating system: – piece of code whose job is to burst feed interface from area of memory that contains data waiting to go to output device
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Computer Engineering Buffered Terminals • The printer mechanism itself may be speeded up by allowing BIDIRECTIONAL PRINTING • On alternate lines the printer prints from right to left, thus avoiding slow carriage returns – this requires yet more logic to reverse the RAM addressing whenever a carriage return is found.
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Computer Engineering Direct Memory Access (DMA) • If a buffer full of data can be transferred in one ‘burst’, it can slow the transfer if the CPU is expected to move characters one at a time from memory to the device. • DMA: move CPU out of data transfer loop – More efficient communications, – more effective use of each component – while still permitting it overall control over system.
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Direct Memory Access (DMA) Data transferred via CPU. . cost = 2 bus transfers (i.e. 2 CPU move instructions)
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block devices - Block Devices Computer Engineering...

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