This document discusses basic computer structure and operation. It describes the main components of a CPU including registers, ALU, and control unit. It explains how the CPU fetches, decodes and executes instructions in a repeating cycle. It also discusses memory types including RAM, ROM, flash and hard disks. The document provides an overview of operating systems, programming languages, and how user input such as a keypress is processed by the computer system.
3. Bus
Control bus:
Signals for maintaining and
status information.
e.g. sets processor in a
specific operation mode.
Address bus:
Unidirectional bus, (16 or
32 bits parallel wires) used
for transmitting addresses
to memory.
Data bus:
8 / 16 / 32 / 64 bits bi-
directional bus. Data stored
or loaded to / from the
memory.
4. Central Processor Unit
Instructions and data
are stored in RAM.
CPU loads and
decodes an
instruction.
Loads data needed
from RAM (according
to the instruction) and
stores them in
registers.
Make calculations if
necessary in registers,
according to the
instruction.
Stores data back in
RAM.
5. ARM Register Set
Registers:
Small and very fast memories.
Usually 32 bits each one.
R15 is the Program Counter: stores the
address of the next instruction.
CPSR (Current Program Status Register)
used to monitor and control internal
operations.
e.g. A calculation has been made (e.g. an
addition). Is the result negative? Did it lead to
overflow? Did a carry occur?
6. ARM Assembly Example
Very simple and low level commands.
The CPU decodes them to machine language (i.e.
binary).
Each instruction requires a different number of CPU
cycles.
7. CPU Instruction Set
Each different kind of CPU has each own set of
instructions.
4 types of instructions:
Transfer: e.g. Load data from memory to a CPU register for
processing.
Arithmetic: e.g. add data stored in two registers and put the
result to a 3rd register.
Logic: e.g. compare data stored in two registers and set the
appropriate flag in the Status Register.
Control: Set the program counter to a specific memory address.
9. CPU fetch-decode-execute cycle
5 basic operations:
Fetch: load a
memory instruction.
Decode: decode the
instruction.
Execute: perform
ALU operations.
Memory access: (In
fact, usually wait for
one CPU cycle, to
ensure consistency).
Write back: store
results in a register.
This is the CPU pipeline.
The operations of the CPU are
decomposed in independent
steps to allow concurrent
operations and increased CPU
performance.
10. CPU Clock
Each time the CPU clock “ticks” CPU moves to the
next pipeline stage.
So, in an 1GHz CPU each pipeline stage takes
1nsec.
12. Types of Memories
Most common widths: 8, 16, 32, 64 bits. If the memory width is the
same as the instructions length, the processors needs one memory
access to fetch a new instruction.
ROM (Read Only Memory): Contains an image set at production time
and cannot be reprogrammed (e.g. boot code).
Flash ROM: Can be rewritten. Used to store device firmware or long-
term data that needs to be preserved after power is off.
RAM (Random Access Memory).
DRAM (Dynamic RAM): The storage cells are refreshed after every few
milliseconds, so it needs a DRAM controller.
SRAM (Static RAM): faster than DRAM, does not require refreshing. Used
in caches.
SDRAM (Synchronous DRAM): Very fast, it is clocked and synchronizes
itself with the processor clock.
Memories that lose their data when power is off are called volatile.
(i.e RAM). ROM is non-volatile.
13. Read Only Memory
ROM
PROM (Programmable ROM):
It’s purchased already
programmed. Once
programmed, can never be
reprogrammed.
EPROM (Erasable PROM):
Can be reprogrammed using UV
light.
EEPROM (Electrically EPROM)
are reprogrammed electrically.
(Flash Memories)
Flash Memories:
Designed to replace
ROM and RAM.
Not as fast as
SRAM and ROM.
14. Flash EEPROM
Floating gate
transistors.
Electrons are
“trapped” inside the
float gate and open
the channel between
the n regions.
15. Hard Disk Drives &
Solid State Drives
Two types:
Hard Disk Drive
Solid State Drive
Hard disk Drive:
They contain a number of
platters.
The drive head reads or
writes a circular ring of
data.
One circular ring is called
track.
Sections within each track
are called sectors (512
bytes).
16. Cluster
The smallest part of the
hard disk space that is used
to store a file (also called
“allocation space”).
A file stored on the hard
disk uses one or more
clusters.
Fragmentation:
Unused space inside a
cluster is lost. (e.g. in a file
of 2048Bytes, 512 bytes are
unused). (Internal
fragmentation).
Clusters used to store a file
may not be contiguous.
(external fragmentation).
In this case, extra time is
needed to access the whole
file.
17. Hard Disk vs. Solid State Drive
Comparison is difficult.
SSDs do not rotate to seek data, so the access time is much smaller than HDDs time.
However, their performance degrades over time (even few weeks of use).
HDD drives have larger capacities. However, SSD capacity will increase in the future
and prices will fall.
HDD drives are more sensitive than SSD. However, in case of an SSD failure, all data
will be lost. There is no recovery process as happens with HDD.
SSD do not suffer from fragmentation problems. Time to access data does not
depend on their location.
SSD based on flash technology require half to one third energy in comparison with
HDD.
18. Software
Operating System:
The interface between the user of the
system and the hardware.
Operations: Scheduling and allocating
tasks to the CPU, allocating and freeing
memory, providing a user interface,
resource management, etc.
Programming Languages:
The programming language that tells
the hardware what to do (operating
systems are also written in
programming languages).
e.g. assembly language command
add r0, 3 tells the CPU to add 3 to the
value stored in register r0 and store
the result to register r0.
There are high level and low level
languages.
C, C++, Basic, Java are high level
languages.
#include<stdio.h>
int main(){
int a,b,sum;
scanf("%d %d",&a,&b);
sum = a + b;
printf("%d",sum);
return 0;
}
Programming languages
make use of a specific
program called “compiler”,
which translates the source
code of the programming
language to assembly code
that the CPU understands.
The assembler converts the
assembly program to
machine code (i.e. 0 & 1).
19. What happens when I press “a” in the
keyboard?
All I/O devices of the computer are “memory mapped”.
That means that a specific area in RAM is allocated to the specific
I/O device. (The operating system is responsible for mapping the
device to the memory).
Keyboard drivers are the interface between the keyboard and the
operating system.
When I press “a” a specific value is written in the memory mapped
area assigned to the keyboard.
The operating system scans many times per second all the
memory mapped areas of any device for changes. So, when it
detects a keyboard instruction it creates a specific group of
instructions and assigns it to the CPU.
CPU executes the commands and stores memory data to memory
mapped areas, according to the initial instructions (e.g. the
memory mapped area of the screen).
Then the operating system, detects memory mapped area
changes and creates new set of instructions to be assigned to the
CPU, and so on…
