Basic Architecture
the main components of a simple computer
and their interconnections.
The component parts of the computer are:
Processor. Carries out computation and has overall control of the computer.
Main memory. Stores programs and data while the computer is running. Is fast
access, directly accessible by the processor, limited in size and non-permanent.
External memory. Holds quantities of information too large for storage in main
memory. Slower access than main memory, not accessible directly by the
processor but can be used to keep a permanent copy of programs and data.
Peripheral devices (input/output devices). These allow the computer to
communicate with the outside world.
External system bus. This allows communication of information between the
component parts of the computer.
a schematic diagram of a typical microcomputer system built around the
6502 microprocessor.
The 6502 microprocessor (introduced in about 1975) was among the first
microprocessors to be used in early home computers.
The 6502 processor included the usual Arithmetic/Logic Unit with some internal
registers and a Control Unit all on the same chip. It had an external crystal-controlled
clock to generate timing signals.
The external bus was a combination of an 8-bit-wide data bus, a 16-bit-wide address
bus and some control lines that carried synchronisation signals throughout the system.
Hence only 8 bit data could be moved around the system, but 16-bit addresses could
be used to address memory.
Early models ran at a clock-rate of 1 MHz but some later processors ran at 6 MHz.
However with its limitation to an 8-bit-wide data bus and 8-bit arithmetic with no multiply
or divide operations, it could not compete with later 16- and 32- bit microprocessors.
The Intel 8086 series of processors were introduced in 1978 and were used in the early
IBM PC.
The 8086 had the capability to address 1 megabyte of memory since
220 = 1,048,576.
the 8086/8088 processors ran at a clock rate of 4.7 MHz and could
execute some instructions in as little as 400 nanoseconds compared
with the two microseconds of earlier 8-bit personal computers
Memory
Computers can store data in internal memory (within the computer itself)
or external memory (backing storage).
Internal memory is any memory device located within the computer
system itself. This includes:Registers (storage within the processor)
Cache memory (may be in the processor, or just outside it but on the
(motherboard)
Main memory (mostly RAM, in separate memory chips on the
motherboard)
Registers
A typical processor may have hundreds of registers.
A few of these have very specific purposes, and have names,
including:
the memory address register (MAR)
the memory data register (MDR) - sometimes called the memory
buffer register (MBR)
the program counter (PC)
the instruction register (IR)
The other registers within the processor are called general
purpose registers, and can be used by the processor for storing
and manipulating any data as required.
registers are part of the processor chip
access to the data in them is almost instantaneous, measured in nanoseconds (10-9s)
they are limited in number, anything from a few in early microprocessors up to a
few thousand in more recent processors
they are used to hold data temporarily which is needed immediately by the processor
The main memory consists of individual memory locations,
each able to store several bytes (typically 4 or 8 in current systems),
and each with its own unique memory address, so that the processor
can access any item of data directly.
a typical motherboard layout. The position of the main processor chip, soldered on RAM
chips, slots for additional main memory, and cache memory chips.
Typical access time to get data from main memory to the processor
would be in the range 10-100 nanoseconds, with data transfer rates of
up to 3Gb per second for the latest RAM chips.
One solution to this problem would be to increase the number of
registers on the microprocessor However, this solution is impractical,
leading to over complex and large microprocessor chips.
Cache Memory
Cache is simply very fast memory chips located on the motherboard
very close to the processor.
cache memory has access times of 10-50ns, with data transfer rates to
the processor of over 10Gb per second.
External Memory
Backing storage is provided by external memory devices. There is a
wide range of devices available, and the technology is continually
improving.
• magnetic discs
• optical disc
• magnetic tape
Magnetic Disc
The 1.4 Mb capacity made the floppy disc less and less useful
The hard disc, still continues to play an important role as the main
backing storage device in most computer systems
removable hard disc could be purchased for , much cheaper than RAM.
The main disadvantage of hard disc systems is that they are much
slower than RAM. Access time could be as much as 10 ms (10,000,000
nanoseconds), and the data transfer rate is typically in the range 30100Mb per second.
Pupil Task
Investigating a Hard Disc specification (20 min)
Find out the specification (price and capacity) of a currently available
hard disc.
If possible, find out its data transfer rate in Mb per second.
Calculate the cost per Megabyte.
For example: a 400GB costing £200 would give 0.05p per Mb
Optical Disc
Optical storage technology has developed rapidly since the early 1990s,
and looks likely to replace magnetic technology for many applications.
The most common is the CD-ROM. During the mastering of a CD, a
reasonably high powered laser is used to burn pits, typically 0.5 microns
wide, 0.83 to 3 microns long and 0.15 microns deep, in a spiral,
outwards from the centre of the disk. If the spiral was straightened out
then the data would stretch for four miles!!
Open Scholar booklet at page 15
The CD-R drive operates a laser light at one of 3 different levels. At low
levels it detects the presence or absence of pits, while at the highest
level it can burn data onto the surface.
CD-RW these operate using the highest power of laser to melt a small
region of the recording layer. After freezing, the middle power laser is
used to warm the surface to a temperature that is less than melting point
but high enough to create the highly reflective crystalline form. The
highest power is then used to write the data.
Capacity
CD-ROMs are typically 650 Mbytes in capacity. However, the future use
of blue lasers, instead of red laser light, will deliver increased CD-ROM
storage capacity.
Speed
In a single speed CD reader, data can be read at a rate of
150Kbytes/sec.
Multiple speed CD readers, such as 8xCD-ROM or 24xCD-ROM can
read data at a rate of 1.2 Mbytes/sec and 3.6 Mbytes/sec respectively.
48xCD-ROMs are available, at data rates of 7.2 Mbytes/sec,
they pale in comparison to SCSI hard disk drive rates.
Comparing optical discs with hard discs
Optical storage has many advantages over magnetic storage, but
current devices tend to be limited in capacity and have slower data
transfer rates than hard discs.
CD-ROM or CD-RW is 700Mb. DVD discs can store up to
4.7 or 9.4Gb, which is much more than a CD (enough to store 133
minutes of video on 1 side), but much less than a large hard disc
Optical storage devices tend to have lower data transfer rates than hard
discs, although speeds are increasing all the time.
At the time of writing, the fastest CD drives were rated as 52X, which
means 52 x 150Kbps, which is 7.8Mbps,
significantly slower than hard discs data transfer rates.
DVD data transfer rates are quoted as multiples of 1.3Mb per second,
so a16X DVD has data transfer rate of 16 x 1.3Mbps, which is 20Mbps.
However, this is still slower than hard disc speeds
If a CD-RW drive is referred to as 32X16X40X. Then this means:record data (for the first time) at 32 x 150Kbps
rewrite data at 16 x 150Kbps
read data (transfer it to the processor) at 40 x 150Kbps.
Pupil task
Investigating Optical Storage Devices (30 min)
Find out the current cost of a single CD-R, CD-RW, DVD-R and DVDRAM disc.
Express this as a cost per Megabyte.
Find out and calculate the data transfer rate of current CD and DVD
drives.
Magnetic tape
You are probably less familiar with magnetic tape as a backing
storage method, as it is mainly used on large commercial
systems or on network servers,
Type of tape
DAT
QIC (DC 600
or DC 2000)
8mm
DLT
Mammoth
AIT
Capacity
40Gb
Maximum data transfer rate
5Mb/s
Up to 2Gb
14Gb
70Gb
40Gb
50Gb
1.6Mb/s
2Mb/s
2Mb/s
6Mb/s
6Mb/s
magnetic tape is the cheapest form of data storage, but
also has the lowest data transfer rate. Therefore its only
benefit it is ideal for making and keeping long term backups of
commercial data, or backup copies of network hard drives
Pupil task
Collect comparison table of storage devices.
NEXT go onto Q6-36 on pages 19 through to 26