IO Systems Questions and Answers

Answer: D

The key gives to request a buffer space to receive data as the device driver's first action when data is to be read.

In the usual account of how a blocking read is served, the kernel's I/O subsystem passes the request to the device driver. The driver first obtains a kernel buffer in which the incoming data will be placed and then schedules the I/O. Only after that does it write commands into the device controller's registers, wait for the device, and copy the data out when the interrupt arrives. Lecture notes from several operating-systems courses describe the driver reserving the buffer before the transfer is started.

Checking the device status and sending the read message or a get-ready signal to the controller come later in the sequence, once the destination for the data exists. I could not find a source that words the sequence exactly as this question does, and some texts start the driver's work by checking whether the device is busy, so this follows the official key and the buffer-first description.

Answer: D

The operating system copies 512 bytes.

A disk cannot read or write a single byte. The smallest unit it transfers is a sector, which on a conventional hard disk holds 512 bytes. To change the 34th byte, the operating system reads the whole sector into a buffer in main memory, changes that byte, and writes the whole sector back.

1 and 34 assume byte-level access, which disks do not offer, and 136 (34 × 4) mixes the byte number with the sector number.

Answer: B

The copy of an entire disk is kept by RAID 1.

RAID 1 is mirroring: every write goes to two (or more) disks, so each holds a complete copy of the other, and either can serve the data if one fails.

RAID 0 stripes data across disks with no redundancy. RAID 5 stripes data with distributed parity rather than a full copy. RAID 10 combines mirroring with striping, but the level whose defining feature is a whole-disk copy is RAID 1.

Answer: D

The device that requires a device driver is the Disk.

A device driver is operating system software that controls a peripheral through its controller, translating general requests such as 'read this block' into the device's own commands. A disk is such a peripheral.

Registers, cache and main memory are accessed directly by the processor's instructions and hardware, with no driver involved.