Inside the computer system, data are stored on different storage devices de- pending on their usage requirements. Permanent data are stored on the hard magnetic disks. Temporary data are stored on the random access memory (RAM) or memory. Frequently accessed data are temporarily stored on the cache memory for quick accesses. Data are either read or written to these stor- age devices by the running user programs or operating system programs. Traditional computer systems run programs when they are invoked by users or timer events. A job task is a fragment of codes belonging to a running pro- gram and it is executed by the CPU. A program may invoke one or more job tasks. Many tasks belonging to different programs are concurrently executed by the CPU. Since the CPU can serve only one job task at any one particular moment, the tasks are served on a time-slice manner. After the CPU serves a task for one unit of time, it switches to another task. The order of service is determined by the job scheduling policy.
When a task arrives at a code to receive input from the keyboard, output to the screen, read from hard disk, or other input/output operations, the running task will be suspended and put into the waiting queue until the I/O instruction
is finished. The CPU then resumes the suspended task and continues the task
after the I/O operation.
Inside the computer system, the memory bus connects all the main compo- nents, including the CPU and memory (Figure 3.4). Other peripheral devices
Figure.3.4..I/O.processor
Figure .. I/O Processor
CPU I/O Processor Memory Disk 1 I/O bus System bus Disk 2 … Disk 3
and the hard disks are connected to the I/O bus. An input/output processor (IOP) connects the I/O bus to the memory bus. Since the input and output devices are very slow devices when compared to the memory and CPU. The memory bus would be very slow if the I/O devices are directly connected. With the help of the I/O processor, the I/O devices can communicate with the CPU and memory without slowing them down.
When the CPU executes a line of code that performs an I/O instruction, it works with the I/O processor to execute the I/O instruction in four steps:
1. The CPU issues an I/O instruction to the I/O processor.
2. The I/O processor reads a command from memory.
3. The I/O processor transfers data to/from memory directly.
4. The I/O processor sends an interrupt to CPU when done.
In the first step, the CPU issues an I/O instruction to the I/O processor as
shown in Figure 3.5. The I/O instruction is composed of the operation code (OP), the target device number (device), and the command address (address).
The operation code specifies which command to execute. The device speci-
fies the target device number. The address contains the address location of
the I/O command inside the memory.
Figure.3.5..Step.1:.CPU.issues.I/O.instruction.to.I/O.processorFigure .. Step : CPU issues I/O instruction to I/O Processor.
command
I/O instruction
OP Device Address
target device
where commands are
looks in memory for commands I/O Instruction CPU I/O Processor Memory Disk 1 I/O bus System bus Disk 2 … Disk 3
In the second step, the I/O processor looks in the memory for the command
as shown in Figure 3.6. The command is composed of four fields: the OP field, the Addr field, the Cnt field, and Other field. The OP field specifies what to do. The Addr field specifies where to put data. The Cnt field specifies
the count of how much data can be accessed by the command. The Other
field only specifies details of the command. The I/O processor then reads the
command from memory and executes the command.
In the third step, the I/O processor executes the command as shown in Figure 3.7. Most I/O commands need to access memory. When data are transferred, the I/O processor directly transfers data to and from the memory without in- terfering with the CPU. When a sector is read from the disk, a sector of data (512 bytes) is read from the disk and directly transferred to the memory.
When the I/O command has finished, the I/O processor executes the last step.
It sends an interrupt to the CPU (Figure 3.8). When the CPU receives this interrupt, it executes the interrupt in a preemptive manner. The CPU suspends the currently running task even though the task has not been executed for one time unit. It then performs the O/S command for the I/O interrupt. The job task that issues the I/O instruction is resumed. The task is removed from the list of suspended tasks and placed in the list waiting for CPU. The CPU then resumes the previously suspended task and continues to serve it.
what to do command I/O instruction OP Addr Cnt Other where to put data how much special requests Command CPU I/O Processor Memory Disk 1 I/O bus System bus Disk 2 … Disk 3 Figure.3.6..Step.2:.IOP.reads.command.from.memory
Storage.Devices
Magnetic disks are inexpensive disks. The storage device is inexpensive be- cause it stores data using two-dimensional circular disk platter and the disk platters are stacked up on the third dimension. Magnetic disks are composed of disk platters and read/write heads as shown in Figure 3.9. The disk platters are connected together at the centre on a spindle. When the spindle rotates, all the disk platters move at the same speed.
Figure.3.7..Step.3:.IOP.transfers.data.to/from.memory.directlydirectly command I/O instruction CPU I/O Processor Memory Disk 1 I/O bus System bus Disk 3 … Figure.3.8..Step.4:.IOP.sends.interrupt.to.CPU.when.done
Figure .. Step : IOP sends interrupt to CPU when done
command I/O instruction
I/O interrupt CPU I/O Processor Memory Disk 1 I/O bus System bus Disk 2 … Disk 3
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The read/write heads are supported by disk arms. The disks look like a hair comb structure in which each read/write head is a tip of the comb. Each read/ write head is placed above a surface top of a disk platter. When the disk platters rotate, the heads hover at a very thin layer of air above the disk surface. While the read/write heads are fixed and the disk platters are rotating, each head forms a circle on the corresponding disk platter surface. These circles are the tracks when data are written onto the disk surface. These tracks are circular in shape. The shorter tracks that are closer to the centre of the disks are called inner tracks. The longer tracks that are near the circumference of the disks are called outer tracks. All the tracks on different surfaces with the same radius together form a cylinder.
When data are accessed, the disk takes the following steps:
1. All read/write heads move together at a direction perpendicular to the
circumference of the circular tracks until the heads reach the required cylinder.
2. The control servo waits for the read/write heads to settle above the required cylinder after the movement.
3. The head above the required tracks within the cylinder is chosen.
4. The heads then wait for the rotation of the disk until the beginning of
the required data on the track come under the head.
5. The I/O path from the disk controller to the memory is established.
6. When the beginning of the required data comes under the head, data
are immediately transferred between the disk and the memory.
Data are written in units of 512 bytes. Each unit of 512 bytes is called a sector. When the read/write head is above a track, it can access all the data on this track
Figure.3.9..Magnetic.disks Sectors Track Platters Platter Tracks Read/write heads
by waiting for the disk to rotate. At any moment, only one of the read/write
heads can transfer data. When the read/write head is fixed, it can access all the
data on the cylinder by choosing the appropriate read/write heads.
Traditionally, the magnetic disks rotate at a fixed angular speed and the
read/write heads transfer data at a fixed speed. All the tracks store the same number of bytes. When the heads are close to the disk centre, the length of the circular tracks is short and data bits on the tracks are densely written. When the heads are far from the disk centre, the tracks are longer in length and data bits on the tracks are sparsely written. Thus, the recording density varies when the heads are close to or far from the centre of disks. Thus, the traditional disk recording format is called variable density recording. In these traditional magnetic disks, the disk platters simply rotate at fixed speed. However, it does not fully utilize the storage capacity of the long outer tracks. In order to store more data on the outer tracks, the constant recod- ing density method is widely accepted in recent years. The constant density recording format stores more data on the longer outer tracks and less data on the shorter inner tracks. This constant density recording is applied in two layouts: the zoned disk layout and the spiral track layout. These two layouts are described in the paragraphs below.
After that, we shall describe the millipede disks and the nanodisks. For mobile devices, the storage devices need to be small, compact, and light. The millipede disks and the nanodisks are products that address these requirements.
Zoned.Disks
Magnetic disks use the zoned disk format to increase their storage capacities. The disk surface of magnetic zoned disks is divided into zones as shown in Figure 3.10. Each zone is a group of neighbouring tracks within a range of radii. Thus, each zone is a ring-shaped region on the disk surface.
Within a zone, the disks operate like a variable density recording disk. The
disks rotate at a fixed angular speed. Thus, all the tracks within a zone store
the same number of sectors and the number of sectors per track is fixed
within a zone. To store the maximum number of sectors within a zone, the innermost track within the zone should store the most sectors. Other tracks in the same zone then store the same number of sectors.
Since the innermost track of the inner zones are shorter than the innermost track of the outer zones, tracks of the inner zones store less data than the
tracks of the outer zones. Although the number of sectors per track is fixed
within a zone, each zone may have a different number of tracks. The storage capacity of a zone is found as the product of the storage capacity of a track and the number of tracks within the zone.
In addition, the I/O path transfers data at a fixed number of bits per second and the disks rotate at a fixed speed. All the data on one track can be accessed by
one disk revolution. Thus, the data transfer rate within a zone is fixed. Since the track capacity of outer zones is larger than the track capacity of the inner zones, data are transferred faster when the heads are above the outer zones. Thus, the outer zones have higher data transfer rate than the inner zones. Magnetic zoned disks have two main advantages over traditional magnetic disks. First, they have higher storage capacity than traditional magnetic disks of the same size. Second, data on the outer tracks of zoned disks can be ac- cessed more quickly. In traditional magnetic disks, the motor speed is fixed. Whereas in zoned disks, the motor speed changes when the heads change from one zone to another. Since changing the motor speed is very simple, it
is not difficult to be implemented.
Spiral.Track.Layout
Optical disks, such as compact disk (CD) and digital versatile disks (DVD) use the spiral track to increase their storage capacities. The optical disks can
Figure.3.10..Zoned.disk.format 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1 2 3 4 5 6 1 2 3 4 5 6 7 8 9 10 11 12 7 8 9 Inner Zone Outer Zone
record data at a fixed speed continuously for a very long time.
On the surface of the optical disks, data are recorded on a long spiral track in sectors. The spiral track runs continuously from the inside near the centre of the disk to outside near the rim. Dual layer DVD may have a second spiral track at the second layer that runs in the same or opposite direction.
The motor changes the disk rotation speed according to the position of the optical read/write head. The servo controls the motor speed and changes it automatically. While the optical head is near the centre of the disk, the optical disk speeds up. While the optical head is near the rim of the disk, the disk slows down. The motor speed is maintained so that the data on the track pass
the optical head at a fixed linear speed.
Millipede.Project
The millipede project creates a new type of disk (Paulson, 2002). The size and shape of the millipede disk looks like a postage stamp. The disk is com- posed of silicon tips above a polymer. Data are written on the polymer by punching holes on the polymer with a silicon tip. The holes are separated at a distance of around 10 nanometres or 50 atoms. The disk can record data at a density of 1 trillion bits per square inch. It records data at 20 times denser than the magnetic disks.
Figure.3.11..CD.and.DVD.layout
Spiral Track
The disk is rewritable. Data on the polymer can be read or written by changing the temperature of the silicon tips. Data on the polymer are written with hot tips at 400°C. Data are read from the polymer with warm tips at 300°C. In addition, data on the polymer can be erased using hot tips. Since the time to conduct heat to the polymer is rather long, the data recording speed is 1,000 times slower than hard disks. In order to compensate for the long access latency, the disk uses 1024 silicon tips working in parallel.
Nano.RAM
Another new disk is the Nano-RAM disk in Paulson (2003). Nano random access memory (NRAM) is one of the first storage devices that use the nano- technology. It is small and compact. The NRAM is small and compact. The NRAM is composed of carbon nanotubes that are a billionth of a metre in size. The disk head sends differing electrical charges into the nanotube and swings the tubes into one of the two positions. One of the two positions represents a binary digit 0 while the other position represents a binary digit 1.
Inside the NRAM, the nanotubes only move a very short distance, and it takes
a very short time to finish this movement. Thus, the read/write operations can be finished very quickly. This short latency feature makes the NRAM suitable for high performance systems.
The position of the nanotubes is nonvolatile. The nanotubes do not need power to maintain their current positions as in random access memory. Thus, the NRAM is suitable for permanent storage of information. In addition, the NRAM does not need to maintain continuous rotations like magnetic disks and optical disks. It saves power, and the NRAM can be used in mobile devices.
The NRAM is 50 times stronger than steel. The nanotubes can swing into positions many times in order to support a large number of write cycles. Recent developments on quality control help to select only nanotubes that are growing properly.
In summary, the nanotube is a durable, compact, low power, compact, high capacity, and low latency storage device. The NRAM can be used in mobile and high performance systems in which the system requirements are stringent.