Chapter 7

What is the sum of rotational delay and seek time called?

Access time

What is zoned bit recording?

All the sectors on a disk in a rigid disk drive are the same size and more sectors are placed on the outer tracks than the inner tracks, to enable more bytes to be stored on the disk.

How is channel I/O similar to DMA?

An IOP must also cycle steal to function. Both are block oriented and interrupt the CPU when their tasks are completed and the blocks of data are ready to be sent. Both process I/O, to different degrees, independent of the CPU.

Explain the relationship among disk platters, tracks, sectors and clusters.

Disk platters are one or more metal or glass disks that have a thin film of magnetizable material bonded to it. These platters contain the sectors and tracks. Tracks are concentric circles on the disk platters. Sectors are divisions of tracks. Clusters are groups of sectors that are addressed by the operating system.

How does direct memory access (DMA) work?

DMA executes simple I/O instructions, so the CPU does not get bogged down. The DMA is provided with the location of the bytes to be transferred, the number of bytes to be transferred and the destination device or memory address. The DMA then takes this information, and requests the bus. If the bus is not being used by the CPU, it then steals the cycle, activates address lines, places blocks of bytes on bus data lines and releases the bus. The DMA repeats this cycle until the total bytes transferred equals the total requested. Finally, the DMA notifies the CPU of completion.

What does it mean when someone refers to I/O as bursty?

Data is sent in blocks or clusters.

Why are magnetic disks called direct access devices?

Each unit of storage on a disk, the sector, has a unique address that can be accessed independently of the sectors around it.

What are 3 types of durable storage?

Magnetic Disks - A non volatile memory where data is stored on a magnetized medium. Optical Disks - The data is stored on an optical medium. Magnetic Tapes - The data is stored as digital information on the magnetic tapes as a digital recording.

What is polling?

Generally, polling is the activity of actively examining an external device's status by a client program. Specifically in programmed I/O, the CPU continuously monitors the control register associated with each I/O port.

How are address vectors used in interrupt-driven I/O?

Service routines can accommodate hardware changes by modifying the address vectors to point to vendor specific code because vectors for different hardware are kept in the same locations in systems with the operating system of the same type and level.

What distinguishes an asynchronous bus from a synchronous bus?

Synchronous transfer requires both the sender and the receiver to share a common clock for timing. Synchronous bus contains the control lines with clock in it and has a fixed protocol for the communication relative to the clock. Asynchronous bus protocols require a clock bit timing to delineate the signal transitions. It can connect many device of varying clock speeds as it isn't clocked.

What is speedup?

1) The ratio of the time an older component takes to complete a task, divided by the time the newer component takes to complete the same task. 2) When working with percentages, speedup is one plus the percent increase divided by one hundred.

How is channel I/O different from interrupt driven I/O?

Channel I/O has one or more dedicated I/O processors that control various I/O pathways called channel paths. I/O channels are driven by small CPUs called IOPs. The entire data transfer is controlled by the IOPs with only minimal data required from the CPU. An interrupt is sent to the CPU only after the IOPs have completed the data transfer. In interrupt driven I/O the CPU must process I/O itself, whenever an interrupt is raised by an I/O device.

What is multiplexing?

In channel I/O, it is the term for the grouping of slow I/O device signals onto one channel, managed by one controller.

What are the physical components of a rigid disk drive?

Platters Spindle Motor Read/Write Heads Actuator Arm Disk Packs

Explain how programmed I/O is different from interrupt driven I/O.

Programmed I/O constantly checks (polls) a control register associated with each I/O port. Interrupt driven I/O is the opposite, I/O devices send requests for service via interrupts to an interrupt controller, that sends an interrupt signal to the CPU. Programmed I/O is less efficient than interrupt driven I/O. Programmed I/O is relatively changeable as far as polling time. Interrupt driven I/O has I/O device priority hardwired into the system.

Why are protocols important in I/O bus technology?

Protocols work like traffic directors. They specify the rate of transmission of data between the receiver and the sender. Protocols include various techniques for detecting and recovering from transmission errors and for encoding and decoding data.

Why does DMA require cycle stealing?

The DMA and the CPU both share a bus, so the DMA cannot do its job at the same time the CPU is executing its fetch-decode-execute cycle. I/O takes priority over the CPU because I/O operates within tight parameters. If no activity is detected by I/O devices within a specified period of time, they timeout and abort the I/O process. Cycle stealing is done to avoid these timeouts.

What is settle time and what can be done about it?

The amount of time it takes for a signal level to stabilize, in the I/O transfers. Nothing can be done.

What is a bus master?

The device in a computer that drives the address bus and the bus that controls signals.

What is a protocol ?

The exact form and meaning of signals exchanged between a sender and a receiver. The rules involved in data exchange between a sender and a receiver.

State Amdahls Law in words.

The overall speedup of a computer system depends on both the speed up of a particular component and how much that component is used by the system. The overall speedup equals the inverse of the fraction of work performed by the faster component subtracted from one, added to the fraction of work performed by the faster component divided by the speedup of a new component.

What is seek time?

The time it takes for a disk arm to position itself over a required track.

What is cycle stealing?

When the DMA uses memory cycles that would otherwise be used by the CPU.