Download week 24 - Memory, storage, processors and operating systems

What is memory?
Memory is used to store information within a computer,
either programs or data. Programs and data cannot be
used directly from a disk or CD, but must first be moved
in memory:
RAM and ROM
Cache
Memory Hierarchies
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Each part of memory has a separate memory location,
which can be referred to using a memory address.
Size of memory is measured in bytes (or multiples such
as kilobytes (KB) or megabytes (MB)).
The two main types of memory (RAM and ROM) act as
if they were two parts of a continuous list of memory
addresses.
Read only Memory (ROM)
This is memory whose contents are not lost if the
machine is switched off. This is also called non-volatile
memory.
There are variations on this basic idea.
Programmable ROM: Programmed after manufacture.
Once they are programmed, the basic ROM and PROM
cannot be changed.
Random Access Memory (RAM)
For most PCs, main memory is stored using RAM chips.
These are volatile so switch the machine off and the
contents in this form of memory are lost.
There are 3 basic types of RAM
– Dynamic RAM (DRAM)
– Static RAM (SRAM)
– Non-volatile RAM (NVRAM)
Cache
Cache is faster type of memory than is found in main
memory. In other words, it takes less time to access
something in cache than in main memory.
It ‘sits’ between external memory (main memory) and
the processor. Runs at the speeds close to that of the of
the processor and has two main types.

L1 which stores instructions going to the processor.
Often split into two L1cache for instructions and L1
cache for data.

L2 which is used buffer data between L1 cache and
main memory, sometimes called unified cache.
Principle of locality
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The nature of programs and structure of data often
means that requests to memory are not random, but
localised.
Programs tend to reuse data and instructions that have
been recently used – temporal locality.
Instructions and data referenced closed together in
time, or often close together physically in memoryspatial locality.
When data is transferred between main memory and
the processor, a copy of the data is saved to cache.
When the processor needs more data, the addresses
are checked to see if the data is already in the cache, if
it there is no need to transfer the data from main
memory which is slower.
This is a cache hit and the data is taken directly from
the cache.
If the data is not, in the cache, this is known as a
cache miss and a memory transfer is performed.
The ability to store more data in a cache will makes the
computer faster, so bigger caches are an advantage.
However, cost is a limiting factor as this type of memory
is more expensive than main memory.
Virtual Memory
In its simplest form:
virtual memory uses a portion of the hard disk as extra
memory.
This is slower than using RAM.
Virtual memory is useful when the programs and data in
use are bigger than main memory’s capacity. We will
meet this in more detail later in the module.
Memory Hierarchies
Memory systems (a collection of various forms of
memory) are constructed in a hierarchy.
As a rule of thumb the faster the memory the higher the
cost in terms of price, making it very expensive to make
all the memory out of the fastest memory devices.
Slower technologies are less expensive, making it more
practical to make larger memories out of these devices.
At
the top of hierarchy is a type of memory called
registers these are fast,
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The
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but only provide a very limited and temporary storage
usually part of the processor.
expensive.
next level is cache memory which is
expensive
fast access time (time taken to access the data stored).
The amount that can be stored (capacity) in cache is greater
than is stored in the registers, but is slower than the registers
to access.
The next level is main memory. This has a greater
capacity than cache, but is slower than cache to access.
At the bottom of the hierarchy is virtual memory. This has
potentially the greatest capacity, but is the slowest to
access.
So the higher level in the hierarchy a device the less that
can be stored in but the quicker it is to access.
The goal of a memory hierarchy is to keep the data that is
accessed most high up the hierarchy, so it can be
accessed quickly, the least used at the bottom of the
hierarchy.
Magnetic
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
All magnetic disk storage allows direct access
to the data. When data is written to or read
from disk, a particular part of the disk can be
identified and use by its address.
Disk can usually be read from or write on both
the top and bottom of the disk.

A disk address uses sector and track. To access a
location on disk:
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the read/write heads have to move to the appropriate track
Wait for correct sector to appear under the head.
The speed at which the data can be read from or
written to the disk is called the seek time and is
usually referred to as the average seek time and is
measured in milliseconds (ms) and is the time taken to
travel to the appropriate track.

The data on a disk is stored in a binary form as
magnetic dots along the tracks, often as one polarity
for a 1 and the other polarity for 0. The read/write head
(R/W head) detects the polarity, because there are only
two polarities, there are only two states. The pattern of
pulses produced by the R/W head in relation to the
drive timing is translated into data and set to the CPU.
Types of magnetic media
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Floppies
Hard disks
ZIP drives
Tape and JAZ drives
Optical media
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Magnetism is not the only way data can be stored,
another option is to use optical media where the disk
has holes burnt in it by a laser to record the binary
code. A hole or pit on the disk reflects less light than
the surface of the disk. By detecting the difference in
reflected light in relation to the time of the drive, a
pattern of pulse is produced and this is translated into
data sent to the CPU.
Compact Disks

CDs are optical media storing approximately 650MB of
data, but are slower than a hard disk to access. The
head used for reading from a CD has two parts a laser
that shines on a small part of the disk and a light
sensor to measure the reflected light.
Digital versatile disk (DVD)

A DVD uses
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the same basic principle as a CD,
the data is packed more densely
Smaller and narrower pits
it has a larger capacity (4.7GB) as compared to a CDs 650MB,
it also has a quicker transfer rate than a CD.
Better compression algorithms help to improve the
capacity.
Fetch

All programs are stored in memory, and have to be
‘fetched’ from memory before they can be carried out
(executed).

Before the CPU can use an instruction, the instruction
must be brought to the CPU from the memory.
Starting with a program counter (PC) this points to
the memory location/address of the next
instruction to be executed.
Therefore, if the contents of the PC were 5, the next
instruction to be executed is stored in memory
location 5.
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The contents of the program counter are copied to the
memory address register (MAR).
The program counter contents are then incremented
to the next instruction.
The MAR holds the address of the memory
location where in memory the data is to be used is
located.
Note: The MAR does not just do this in the fetch part. If you
taking something out of memory or putting something into
memory the address of the memory location has to be there

The contents of MAR during this part of the cycle
contains address of the instruction to be executed, the
contents of this memory location are placed into the
memory data register (MDR).
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During this part of the cycle the data that is passed
from memory is an instruction.
The instruction is moved from the MDR to the
instruction register (IR) where it is divided into two
fields.

One field is the operation code often shortened to
opcode, which tells the CPU the instruction to carry
out.

The second field is the operand field, which contains
the address/data (or addresses) of data used by the
instruction. Sometimes the operand field is not used.
RTL (Register Transfer Language) for
fetch part of the cycle.
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Short hand notation for describing register
transfers
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[MAR][PC]
[PC][PC]+1
[MDR][M([MAR])]
[IR][MDR]
CU[IR(opcode)]

The next stage is the decode-execute cycle where
the control unit takes the opcode from the
instruction register, and generates the control
signals to control the various parts of the CPU.
The control unit is responsible during the fetch
cycle for all the operations to make the fetch
happen
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Pipelining is a key technique used to make
faster CPUs.
Processors allow instructions to be executed in
stages; stages implemented using separate
hardware.
Stages connected together forming an
instruction pipeline, allowing more than one
instruction to be processed at the same time.
Fetch-execute
cycle
stages
fetch
1
decode
execute
instruction 1
2
instruction 1
3
instruction 1
4
5
6
7
8
write-back
instruction 1
instruction 2
instruction 2
instruction 2
instruction 2
Pipelining
cycle
1
2
3
4
5
6
7
8
stages
fetch
instruction
instruction
instruction
instruction
instruction
instruction
instruction
instruction
decode
1
2
3
4
5
6
7
8
instruction
instruction
instruction
instruction
instruction
instruction
instruction
execute
1
2
3
4
5
6
7
instruction
instruction
instruction
instruction
instruction
instruction
write-back
1
2
3
4
5
6
instruction
instruction
instruction
instruction
instruction
1
2
3
4
5
Speed up
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If a k-stage pipeline executes
n instructions using a clock
with a cycle time t, without
overlapping instructions the
total time to execute
instructions will be
So if 4 stages are used (k=4),
4 instructions (n=4), and
t=1s, Ts=16s.
Ts  nkt
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If instructions are executed in
parallel, where kt is the time
to fill-up to the point where
the first instruction completes
and (n-1)t is time taken for
the remaining (n-1)
instructions at a rate of one
per clock cycle.
T p  kt  ( n  1)t
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Speedup factor,
S=Ts/Tp=(nk)/(k+n-1)
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If n=50 (50 instructions in a sequence)
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S= (50*4)/(4+50-1)=3.77
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If n=100 S=3.88
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Arithmetic and Logic Unit (ALU)
This carries out
arithmetic operations such as adding or subtracting,
and logical operations.
Control Unit (CU) This controls the execution of
instructions.
Input and Output Ports (I/O)
Communicates with the
outside world, either reading in data or sending data
out.
Clock
This produces regular pulses which controls
the rate at which instructions are carried out.
Registers
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A register is a small storage unit which holds a
single piece of data.
Program status word:This register contains
information about the result of operations such
as if the result is negative or zero.
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Initially these processors were designed with a
smaller number of instructions around 50, so
name Reduced Instruction set computer
(RISC) in contrast with Complex instruction set
Computer (CISC) (VAX, etc). Later RISC
processors may have larger set of instructions.
Why has RISC not replaced CISC
completely?
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Backward compatibility, billons pounds (or dollars)
have been invested in software for the Intel line
Intel employed some of the RISC ideas in the 486s
upwards. It contained a RISC core to execute simplest
and most common instructions, while the more
complicated instructions in the usual CISC way. So
common instructions are generally fast and less
common instructions are slow.
hybrid approach not a pure RISC design. This allows
higher performance but still allowing old software to run
unmodified.
ALU
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Arithmetic Logic Unit

Carries out all arithmetic operations
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Carries out all logic operations, including
comparison (>,<.equal to, not equal to, etc) not
just ANDs, Ors, etc.
Example AVR instructions
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ldi
ldi
ldi
r18, 0xff
r18, 0b11111111
r18, 255
These all do the same thing they put the value
(load) of 255 into register r18, as either a
hexadecimal, binary, or decimal number.
Look up some of these other instructions you used
in the practical.
What is an operating system?

A short definition of an operating system: An
operating system is a collection of system
programs that manage the resources of a
computer and control the running of user
programs.

Kernel
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Central part of the operating system.
All hardware requests go through the kernel as
system calls
Shell
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The interfaces for the applications programs and
users, with the operating system.
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Process manager deals with
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hardware/software interrupts.
Processor errors
Scheduling tasks
Communication between tasks
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Memory Management
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Allocate memory
Ensures processes (see next week’s lecture) stay
memory boundaries
Controlling virtual memory
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I/0 System
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Communicates with peripherals and hardware
components
Co-ordinates i/o systems such as interrupts and
direct memory access
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Files system
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Organises and accesses files
Maintains on a multi-user system user file quotas
Controls file/record access
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Application Program Interface
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Provides systems services for applications
An interface between the applications and the
operating system.
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User interface
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Allows the user access to programs
Allows the user to view and change system settings.
A consistent interface between the user and the
operating system.

Most modern operating systems have the ability to
execute several programs at once, although there is
usually only one processor in the system. This is
multiprogramming and is made possible by rapidly
switching the processor between programs. Interrupting
the processor periodically, gives the programs control of
the processor for a short period.

This switching is triggered by a piece of hardware
called the interval timer, which generates an
interrupt (a time-out interrupt) when the
programmed period has elapsed. The interrupt
handler, then saves the context of the processor.
The context is the contents of registers that may be
overwritten by other programs. Control is passed
over to the dispatcher/low-level scheduler.

The dispatcher searches a set of potentially
'runnable' program using a scheduling algorithm to
select a suitable program to run next. The context
of the processor, which existed when that program
last ran, is restored to the registers.

A process consists of an executable program,
the data associated with the program, and it’s
execution context. The execution context
includes the processor context, but also
information such as the process identifier,
priority level, and a process state
Process-3 states

When a user process is in the running state,
the processor is executing the program code. If
a timer interrupt occurs, the running process is
moved into ready state, and another process
form the list of processes in the ready state is
moved into the running state.

If a process in the running state requests an
operating system service and it must wait for it,
then the process is moved into the blocked
state.

A process will remain in the blocked state until
the event required has taken place, the
process can then be moved back into the
ready state. If a process is blocked the
operating system is free to reschedule another
processor.