This document defines common units of measurement for digital memory and storage. It explains that a byte is 8 bits, and larger units like kilobytes and megabytes are multiples of bytes. It describes how memory is addressed in computers and how this has increased from 20 to 32 to 64 bits. It also outlines different types of memory like RAM, ROM, cache and how data is organized and stored on hard disks in sectors and tracks.

1. Memory

2. BITS & BYTES
1 BIT - stands for Binary digIT
1 Byte = 8 bits
½ byte = 1 nibble = 4 bits
2x byte = 1 word = 16 bits
2x word = 1 long word = 32 bits
1KByte = 1,024 bytes = 210
1MByte = 1,048,576 bytes = 220
1GByte = 230
bytes
1TByte = 240
bytes
NOTE: http://en.wikipedia.org/wiki/Kilobyte
http://en.wikipedia.org/wiki/Kibibyte

3. Use of numbers to measure MEMORY (size, location)
128 64 32 16 8 4 2 1
+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
| | | | | | | | | | | | | | | |
+-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+
This represents 8-bits of memory in sequence.
Each 'box' represents a single binary digit and its position in a
storage location, known as 'memory'. The 'value' of a binary digit in
any position is shown.
A 32-bit computer processes four bytes at a time
A 64-bit computer processes eight bytes at a time

4. Memory Addressing
The ability of a computer to access any particular area of its memory
is referred to as "addressing".
Addressing is a function of the number of microprocessor address
lines on its "BUS", and is therefore, hardware dependant.
Memory location
● 8088/86 cpu can address up to 1MB with its 20 address lines
● 80286 cpu can address up to 16MB with its 24 address lines
● 80386 cpu can address up to 4GB with its 32 address lines
A processor with 64-bit memory addresses can directly access 2^64 bytes
(=16 exbibytes) of byte-addressable memory.
http://en.wikipedia.org/wiki/Exbibyte

5. EXTENDED MEMORY MAP
Size Location
2^32 4GB +---------+
| | This shows a 'grid' of
| | memory 'cells', 8-bits
| | wide by 4GB long.
2^24 16MB| |
| |
2^20 1MB |---------| FFFFF
|---------| E0000
|---------| C0000
640KB |---------| A0000
| |
| |
0KB +---------+ 00000
http://en.wikipedia.org/wiki/Conventional_memory

6. Why is A0000 = 640KB ?
Expanding:
A0000 = 1010 0000 0000 0000 0000 bytes
remove ten 0's to express in kilobytes
1010 0000 0000 0000 0000 bytes =
1010 0000 00 kilobytes =
1010000000 kilobytes
1010000000 = 512 + 128 = 640
So A0000bytes = 1010000000 kilobytes = 640 KB

7. Types of memory
+----------------------------------------------------------+
| RAM | ROM |
| Random Access Memory | Read Only Memory |
|----------------------------------------------------------|
| Both need power to access information |
|----------------------------------------------------------|
| When power is removed, | When power is removed, |
| loses data. | retains information. |
|----------------------------------------------------------|
| Has a read/write component. | Is read only. |
|----------------------------------------------------------|
|Faster than ROM (Static RAM- | Slower than RAM |
| SRAM is faster than | |
| Dynamic RAM-DRAM) | (Used in BIOS) |
|----------------------------------------------------------|
| Mounted on Single In-Line | Mounted on motherboard. |
| Memory Modules or Double | |
| In-Line Memory Modules. | |
| (SIMMs or DIMMs) | |
+----------------------------------------------------------+

8. DATA STORAGE

9. Disk Drives
Used to store the contents of RAM before computer is switched off.
PC Software is complex and large, and warrants at least one hard
disk drive ( HDD).
All modern software is written with HDD's in mind and a hard disk is
a necessity for a PC.
● Media stores data without power
● Data retrievable when power applied
● Read/Write access
● Slow compared to RAM - mechanical system, moving parts
● Can store huge amounts of data

10. DISK Technology
Circular platter covered with magnetic medium, such as a surface
coating of a metal oxide
Data is stored as changes in the induced magnetism of the metal
oxide, by the read/write heads. Same as for audio/video tape.
Disk access is faster than tape due to track hopping (i.e. as on a
vinyl record, or CD), as the read/write heads can skip between
tracks.
Note: solid-state drive (SSD) (also known as a
solid-state disk or electronic disk
http://en.wikipedia.org/wiki/Solid-state_drive

11. DATA STORAGE ON DISK
How data is organised and stored
● Data mapping is natural result of hardware geometry. Motionless
read/write head scans over a "ring" of magnetic material,
called a track
● Tracks are further subdivided into sectors of 512 bytes
● The several tracks located at any position of the heads is called
a cylinder
● Tracks located by cylinder number + read/write head.
#head #sector #track #cylinder

12. http://www.brokenthorn.com/Resources/OSDev20.html

13. http://www.brokenthorn.com/Resources/OSDev5.html

14. Example: Disk surface partitioned into 8 sectors per track of 1/2 KB per sector
x x x x x
x | x
x 0.5KB | 0.5KB / x
x | / x
x | / x
x 0.5KB | / 0.5KB x <---- track of 8 sectors
x / x with 512 bytes in
x ----------- O ---------- x each sector
x 0.5KB / x
x / | 0.5KB x
x / | x
x / | x
x / 0.5KB | 0.5KB x
x | x
x x x x x
DISK CAPACITY CYLS SEC/TRK HEADS KB/SEC
Disk 1.44MB 80 18 2 0.5
capacity = cylinders on disk x sectors/track x heads x kB/sector
NB: all hard disks use 1/2 kB per sector.
http://homepage.cs.uri.edu/courses/fall2004/hpr108b/FAT.htm

15. HARD DISK TECHNOLOGIES
IDE Integrated Drive Electronics.
• Parallel ATA Interface.
• Drive control electronics all mounted on drive itself.
• Cheap system
S ATA - Serial data transmission at very high rate
SCSI - Small Computer Systems Interface
not technically a disk controller interface
communication interface (parallel)
http://en.wikipedia.org/wiki/Parallel_ATA
http://en.wikipedia.org/wiki/Serial_ATA
http://en.wikipedia.org/wiki/SCSI

16. Processor CACHE memory
+--------+
| | Processor The internal cache is a
| | temporary memory storage area
| +-+ | where information is put to
| | | | ---Cache be processed quickly while the
| +-+ | main processor deals with the
+--------+ rest of the data. The cache
is fast RAM on the chip itself.
+--------+ +-----+ +---------+
| | |Cache| | |
| CPU | -------> |256KB| -----> | RAM |
+--------+ +-----+ +---------+
http://en.wikipedia.org/wiki/CPU_cache
Primary (Level 1) Cache -
a small high-speed cache on the CPU which stores recently-used data and instructions from memory
Level 2 cache -
memory that is external, and resides on a separate chip from the microprocessor. Newer CPU's
include L2 cache on chip.

17. Virtual memory, paging file, swap partitions.

18. Moving data around - DATA BUSES
For getting information to and from the video card and other plug in
cards, a standard bus structure is used. PCI
http://en.wikipedia.org/wiki/Peripheral_Component_Interconnect
● The Peripheral Component Interconnect standard (in practice
almost always shortened to PCI) specifies a computer bus for
attaching peripheral devices to a computer motherboard.
● The bus has been enhanced with PCI Express and other
technologies, which appear in new computers.
http://en.wikipedia.org/wiki/PCI_Express

19. Video Cards
+---------+
Digital -------| Digital |
signal -------| to |------- RED pixel
-------|analogue |------- GREEN pixel
-------|converter|------- BLUE pixel
| for VGA |
+---------+
Colours
16 2 to the power 4 bits, 4 bit colour
256 2 to the power 8 bits, 8 bit colour
32K 2 to the power 15 bits, 15 bit colour
64K 2 to the power 16 bits, 16 bit colour
16.7million 2 to the power 24 bits, 24 bit colour
(True photographic quality)

20. Further reading.
Intel® Desktop Processor Comparison Chart
http://compare.intel.com/PCC/default.aspx?familyid=1&culture=en-US
http://en.wikipedia.org/wiki/Intel_80186
http://en.wikipedia.org/wiki/Intel_80286
http://en.wikipedia.org/wiki/64-bit_computing
http://www.techsupportalert.com/content/32-bit-and-64-bit-explained.htm