The System Bus

The System Bus is one of the four major components of a computer.

This logical representation is taken from the textbook.

The system bus is used by the other major components to communicate
data, addresses, instructions, and control signals.

Real computers have more than one bus, but this is all we need.

Notation for Bus Signal Levels

The system clock is represented as a trapezoidal wave to emphasize the fact
that it does not change instantaneously.

Here is a typical depiction. Others may be seen, but this is what our author uses.

Single control signals are depicted in a similar fashion, except (of course) that they
may not vary in “lock step” with the bus clock.

Notation for Multiple Signals

A single control signal is either low or high (0 volts or 5 volts).

A collection, such as 32 address lines or 16 data lines cannot be represented with
such a simple diagram. For each of address and data, we have two important states
address or data is valid
address or data is not valid

For example, consider the address lines on the bus. Imagine a 32–bit address.

At some time after T₁, the CPU asserts an address on the address lines. This means
that each of the 32 address lines is given a value.

When the CPU has asserted the address, it is valid until the CPU ceases assertion.

Reading Bus Timing Diagrams

Sometimes, we need to depict signals on a typical bus. Here we are looking at a
synchronous bus, of the type used for connecting memory.

This figure, taken from the textbook, shows the timings on a typical bus.

Note the form used for the Address Signals: between t₀ and t₁ they change value.
According to the figure, the address signals remain valid from t₁ through the end of t₇.

Attaching an I/O Device to a Bus

This figure shows a DMA Controller for a disk attached to a bus.
It is only slightly more complex than a standard controller.

Each I/O Controller has a range of addresses to which it will respond.

Specifically, the device has a number of registers, each at a unique address.

When the device recognizes its address, it will respond to I/O commands sent
on the command bus.

Bus Arbitration

A number of I/O devices are usually connected to a bus.

Each I/O device can generate an Interrupt, called “INT” when it needs service.
The CPU will reply with an acknowledgement, called “ACK”.

The handling by the CPU is simple. There are two signals only
INT some device has raised an interrupt
ACK the CPU is ready to handle that interrupt.

We need an arbitrator to take the ACK and pass it to the correct device.

The common architecture is to use a “daisy chain”, in which the ACK is passed
from device to device until it reaches the device that raised the interrupt.

Northbridge and Southbridge

In modern computers, the requirement to
manage I/O devices with a wide range of
data capacities has driven a new design.

This design has two controller hubs, called
“Northbridge” and “Southbridge”.

The Northbridge handles the faster devices
such as memory and graphics.

The Southbridge handles slower devices,
and communicates with the CPU through
the Northbridge.

The DASD and Evolution of Storage Devices

Rule 1: “Data processing requires data storage”. – I just made that up.

Data were originally stored on paper media, first as written documents but fairly soon
(Hollerith, late 19^th century) the storage medium was machine–readable.

In the 1950’s, New York Life Insurance Company was devoting an entire floor
of its main building to the storage of punched cards. Something had to change.

IBM quickly came out with two magnetic media for storing data

the magnetic tape

the DASD (disk)

The acronym “DASD” stands for Direct Access Storage Device.

Until recently, the standard disk drive was the only commercially viable example.

We now have another very popular example, these USB “flash drives”. While
different from standard disk drives, these are managed as if they were disk drives
and are considered disk drives.

Structure of a Large Disk Drive

The typical large–capacity (and physically small) disk drive has a number of
glass platters with magnetic coating. These spin at a high rate (7,200 rpm
or 120 / second)

This drawing shows a disk with three platters and six surfaces. In general,
a disk drive with N platters will have 2·N surfaces, the top and bottom
of each platter.

On early disk drives, before the introduction of sealed drives, the top and
bottom surfaces would not be used because they would become dirty.

More on Disk Drive Structure

Each surface is divided into a number of concentric tracks.

Each track has a number of sectors.

A sector usually contains 512 bytes of data, along with a header and trailer part.

Seek Time and Rotational Latency

In order to read from a disk track, the read/write heads must be moved to the track.

This is a mechanical action, as the read/write heads are physical devices.

There are two seek times typically quoted for a disk.

Track–to–track: the time to move the heads to the next track over

Average: the average time to move the heads to any track.

The rotational delay is due to the fact that the disk is spinning at a fixed high speed.
It takes a certain time for a specific sector to rotate under the read/write heads.

Suppose a disk rotating at 12,000 RPM. That is 200 revolutions per second.
Each sector moves under the read/write heads 200 times a second,
once every 0.005 second or every 5 milliseconds.

The rotational latency, or average rotational delay, is one half of the time for a
complete revolution of the disk. Here it would be 2.50 milliseconds.

The Idea of a Cylinder

Fixed head disks have one head per track. The last time I heard of such a
device was 1977, when working with a 1 MB fixed head disk on a PDP–11/45.

I claim that fixed head disks are obsolete. Revisit the picture of a typical disk.

Question: How many tracks can be read before the read/write heads must be moved?

Answer: One track per surface can be read without moving the heads. Here it is 6.

Definition: A cylinder is that set of tracks that can be read without moving the disk
read write heads. A disk has as many cylinders as a surface has tracks.
A cylinder has as many tracks as the disk has surfaces.