Download A history of computers

Computer History
Year/Enter
Computer History
Inventors/Inventions
Computer History
Description of Event
1936
Konrad Zuse - Z1 Computer
First freely programmable
computer.
1942
John Atanasoff & Clifford
Berry
ABC Computer
Who was first in the
computing biz is not always
as easy as ABC.
1944
Howard Aiken & Grace
Hopper
Harvard Mark I Computer
The Harvard Mark 1
computer.
1946
John Presper Eckert & John
W. Mauchly
ENIAC 1 Computer
20,000 vacuum tubes later...
1948
Frederic Williams & Tom
Kilburn
Manchester Baby
Computer & The Williams
Tube
Baby and the Williams Tube
turn on the memories.
1947/48
John Bardeen, Walter
Brattain & Wiliam Shockley
The Transistor
No, a transistor is not a
computer, but this invention
greatly affected the history of
computers.
1951
John Presper Eckert & John
W. Mauchly
UNIVAC Computer
First commercial computer &
able to pick presidential
winners.
1953
International Business
Machines
IBM 701 EDPM Computer
IBM enters into 'The History
of Computers'.
1954
John Backus & IBM
FORTRAN Computer
Programming Language
The first successful high level
programming language.
Stanford Research Institute,
Bank of America, and
General Electric
ERMA and MICR
The first bank industry
computer - also MICR
(magnetic ink character
recognition) for reading
checks.
1958
Jack Kilby & Robert Noyce
The Integrated Circuit
Otherwise known as 'The
Chip'
1962
Steve Russell & MIT
Spacewar Computer
Game
The first computer game
invented.
1964
Douglas Engelbart
Computer Mouse &
Windows
Nicknamed the mouse
because the tail came out the
end.
1969
ARPAnet
The original Internet.
1955
(In Use 1959)
1970
Intel 1103 Computer
Memory
The world's first available
dynamic RAM chip.
1971
Faggin, Hoff & Mazor
Intel 4004 Computer
Microprocessor
The first microprocessor.
1971
Alan Shugart &IBM
The "Floppy" Disk
Nicknamed the "Floppy" for
its flexibility.
1973
Robert Metcalfe & Xerox
The Ethernet Computer
Networking
Networking.
1974/75
Scelbi & Mark-8 Altair &
IBM 5100 Computers
The first consumer
computers.
1976/77
Apple I, II & TRS-80 &
Commodore Pet
Computers
More first consumer
computers.
1978
Dan Bricklin & Bob Frankston Any product that pays for
VisiCalc Spreadsheet
itself in two weeks is a
Software
surefire winner.
1979
Seymour Rubenstein & Rob
Barnaby
WordStar Software
Word Processors.
1981
IBM
The IBM PC - Home
Computer
From an "Acorn" grows a
personal computer revolution
1981
Microsoft
MS-DOS Computer
Operating System
From "Quick And Dirty"
comes the operating system
of the century.
1983
Apple Lisa Computer
The first home computer with
a GUI, graphical user
interface.
1984
Apple Macintosh
Computer
The more affordable home
computer with a GUI.
1985
Microsoft Windows
Microsoft begins the friendly
war with Apple.
SERIES
TO BE
CONTINUED
AN OPEN LETTER TO HOBBYISTS
By William Henry Gates III
February 3, 1976
An Open Letter to Hobbyists
To me, the most critical thing in the hobby market right now is the lack of good software
courses, books and software itself. Without good software and an owner who understands
programming, a hobby computer is wasted. Will quality software be written for the
hobby market?
Almost a year ago, Paul Allen and myself, expecting the hobby market to expand, hired
Monte Davidoff and developed Altair BASIC. Though the initial work took only two
months, the three of us have spent most of the last year documenting, improving and
adding features to BASIC. Now we have 4K, 8K, EXTENDED, ROM and DISK BASIC.
The value of the computer time we have used exceeds $40,000.
The feedback we have gotten from the hundreds of people who say they are using BASIC
has all been positive. Two surprising things are apparent, however, 1) Most of these
"users" never bought BASIC (less than 10% of all Altair owners have bought BASIC),
and 2) The amount of royalties we have received from sales to hobbyists makes the time
spent on Altair BASIC worth less than $2 an hour.
Why is this? As the majority of hobbyists must be aware, most of you steal your
software. Hardware must be paid for, but software is something to share. Who cares if the
people who worked on it get paid?
Is this fair? One thing you don't do by stealing software is get back at MITS for some
problem you may have had. MITS doesn't make money selling software. The royalty paid
to us, the manual, the tape and the overhead make it a break-even operation. One thing
you do do is prevent good software from being written. Who can afford to do
professional work for nothing? What hobbyist can put 3-man years into programming,
finding all bugs, documenting his product and distribute for free? The fact is, no one
besides us has invested a lot of money in hobby software. We have written 6800 BASIC,
and are writing 8080 APL and 6800 APL, but there is very little incentive to make this
software available to hobbyists. Most directly, the thing you do is theft.
What about the guys who re-sell Altair BASIC, aren't they making money on hobby
software? Yes, but those who have been reported to us may lose in the end. They are the
ones who give hobbyists a bad name, and should be kicked out of any club meeting they
show up at.
I would appreciate letters from any one who wants to pay up, or has a suggestion or
comment. Just write to me at 1180 Alvarado SE, #114, Albuquerque, New Mexico,
87108. Nothing would please me more than being able to hire ten programmers and
deluge the hobby market with good software.
Bill Gates
General Partner, Micro-Soft
MS-DOS OVERVIEW http://www.computerhope.com/overview.htm
Below is a listing of each of the MS-DOS commands currently listed on Computer Hope
and a brief explanation of what each of the commands do. The following commands are
all MS-DOS commands, which means not all the below commands will work in your
version of MS-DOS / Windows. Clicking on the command will open the help page for
that command.
COMMAND DESCRIPTION
Defines functions that change display graphics, control cursor movement,
ANSI.SYS
and reassign keys.
Causes MS-DOS to look in other directories when editing a file or
APPEND
running a command.
Displays, adds, and removes arp information from network devices.
ARP
Assign a drive letter to an alternate letter.
ASSIGN
View the file associations.
ASSOC
Schedule a time to execute commands or programs.
AT
Lists connections and addresses seen by Windows ATM call manager.
ATMADM
Display and change file attributes.
ATTRIB
Recovery console command that executes a series of commands in a file.
BATCH
BOOTCFG Recovery console command that allows a user to view, modify, and
rebuild the boot.ini
Enable / disable CTRL + C feature.
BREAK
View and modify file ACL's.
CACLS
Calls a batch file from another batch file.
CALL
Changes directories.
CD
Supplement the International keyboard and character set information.
CHCP
Changes directories.
CHDIR
Check the hard disk drive running FAT for errors.
CHKDSK
Check the hard disk drive running NTFS for errors.
CHKNTFS
Specify a listing of multiple options within a batch file.
CHOICE
Clears the screen.
CLS
Opens the command interpreter.
CMD
Easily change the foreground and background color of the MS-DOS
COLOR
window.
COMMAND Opens the command interpreter.
Compares files.
COMP
COMPACT Compresses and uncompress files.
CONTROL Open control panel icons from the MS-DOS prompt.
CONVERT Convert FAT to NTFS.
Copy one or more files to an alternate location.
COPY
Change the computers input/output devices.
CTTY
View or change the systems date.
DATE
DEBUG
DEFRAG
DEL
DELETE
DELTREE
DIR
DISABLE
DISKCOMP
DISKCOPY
DOSKEY
DOSSHELL
DRIVPARM
ECHO
EDIT
EDLIN
EMM386
ENABLE
ENDLOCAL
ERASE
EXIT
EXPAND
EXTRACT
FASTHELP
FC
FDISK
FIND
FINDSTR
FIXBOOT
FIXMBR
FOR
FORMAT
FTP
FTYPE
GOTO
GRAFTABL
HELP
IF
IFSHLP.SYS
IPCONFIG
KEYB
LABEL
Debug utility to create assembly programs to modify hardware settings.
Re-arrange the hard disk drive to help with loading programs.
Deletes one or more files.
Recovery console command that deletes a file.
Deletes one or more files and/or directories.
List the contents of one or more directory.
Recovery console command that disables Windows system services or
drivers.
Compare a disk with another disk.
Copy the contents of one disk and place them on another disk.
Command to view and execute commands that have been run in the past.
A GUI to help with early MS-DOS users.
Enables overwrite of original device drivers.
Displays messages and enables and disables echo.
View and edit files.
View and edit files.
Load extended Memory Manager.
Recovery console command to enable a disable service or driver.
Stops the localization of the environment changes enabled by the setlocal
command.
Erase files from computer.
Exit from the command interpreter.
Expand a Microsoft Windows file back to it's original format.
Extract files from the Microsoft Windows cabinets.
Displays a listing of MS-DOS commands and information about them.
Compare files.
Utility used to create partitions on the hard disk drive.
Search for text within a file.
Searches for a string of text within a file.
Writes a new boot sector.
Writes a new boot record to a disk drive.
Boolean used in batch files.
Command to erase and prepare a disk drive.
Command to connect and operate on a FTP server.
Displays or modifies file types used in file extension associations.
Moves a batch file to a specific label or location.
Show extended characters in graphics mode.
Display a listing of commands and brief explanation.
Allows for batch files to perform conditional processing.
32-bit file manager.
Network command to view network adapter settings and assigned values.
Change layout of keyboard.
Change the label of a disk drive.
Load a device driver in to high memory.
Recovery console command that displays the services and drivers.
Load a program above the first 64k.
Load a device driver in to high memory.
Lock the hard disk drive.
Recovery console command to list installations and enable administrator
login.
Displays the device name of a drive.
MAP
Command to create a new directory.
MD
Display memory on system.
MEM
Command to create a new directory.
MKDIR
Modify the port or display settings.
MODE
Display one page at a time.
MORE
Move one or more files from one directory to another directory.
MOVE
Early Microsoft Virus scanner.
MSAV
Diagnostics utility.
MSD
Utility used to load and provide access to the CD-ROM.
MSCDEX
Displays protocol statistics and current TCP/IP connections using NBT
NBTSTAT
Update, fix, or view the network or network settings
NET
Configure dynamic and static network information from MS-DOS.
NETSH
Display the TCP/IP network protocol statistics and information.
NETSTAT
Load country specific information.
NLSFUNC
NSLOOKUP Look up an IP address of a domain or host on a network.
View and modify the computers path location.
PATH
PATHPING View and locate locations of network latency.
Command used in batch files to stop the processing of a command.
PAUSE
Test / send information to another network computer or network device.
PING
Changes to the directory or network path stored by the pushd command.
POPD
Conserve power with computer portables.
POWER
Prints data to a printer port.
PRINT
View and change the MS-DOS prompt.
PROMPT
Stores a directory or network path in memory so it can be returned to at
PUSHD
any time.
Open the QBasic.
QBASIC
Removes an empty directory.
RD
Renames a file or directory.
REN
Renames a file or directory.
RENAME
Removes an empty directory.
RMDIR
View and configure windows network route tables.
ROUTE
Enables a user to execute a program on another computer.
RUNAS
SCANDISK Run the scandisk utility.
Scan registry and recover registry from errors.
SCANREG
Change one variable or string to another.
SET
LH
LISTSVC
LOADFIX
LOADHIGH
LOCK
LOGON
SETLOCAL Enables local environments to be changed without affecting anything
else.
Change MS-DOS version to trick older MS-DOS programs.
SETVER
Installs support for file sharing and locking capabilities.
SHARE
Changes the position of replaceable parameters in a batch program.
SHIFT
SHUTDOWN Shutdown the computer from the MS-DOS prompt.
SMARTDRV
SORT
START
SUBST
SWITCHES
SYS
TELNET
TIME
TITLE
TRACERT
TREE
TYPE
UNDELETE
UNFORMAT
UNLOCK
VER
VERIFY
VOL
XCOPY
Create a disk cache in conventional memory or extended memory.
Sorts the input and displays the output to the screen.
Start a separate window in Windows from the MS-DOS prompt.
Substitute a folder on your computer for another drive letter.
Remove add functions from MS-DOS.
Transfer system files to disk drive.
Telnet to another computer / device from the prompt.
View or modify the system time.
Change the title of their MS-DOS window.
Visually view a network packets route across a network.
View a visual tree of the hard disk drive.
Display the contents of a file.
Undelete a file that has been deleted.
Unformat a hard disk drive.
Unlock a disk drive.
Display the version information.
Enables or disables the feature to determine if files have been written
properly.
Displays the volume information about the designated drive.
Copy multiple files, directories, and/or drives from one location to
another.
Types of hard disk drive connections.
IDE. IDE or Integrated Drive Electronics uses the ATA (AT Attachment interface) for cable
lengths up to 2 feet. A single IDE ATA channel can support up to two drives, master & slave. IDE
can only access one drive per channel at a time. There are three IDE drive capabilities, IDE
ATA33, IDE ATA66 and the latest IDE ATA100. These refer to the peak bandwidth of each type,
so IDE ATA33 = 33MB/s, IDE ATA66 = 66MB/s, & IDE ATA100 = 100MB/s. IDE ATA66 & IDE
ATA100 use a special 80-pin cable. ATA133 = 133MB/sec.
Actual performance of hard disk drives using IDE ATA is such that there are no hard drives that
can use the entire 66MB/s of the IDE ATA66 bandwidth today. Most hard disk drives barely use
the 33MB/s bandwidth that is available in the IDE ATA33, it is only the hard disk drives cache
that can make use of the increased bandwidth to give you increased performance. Serial ATA
uses two wires. One wire sends and receives data to and from the IDE hard disk drive at 1.5GB/s
and faster.
SCSI. SCSI can be up to 12 meters. Narrow SCSI has 8 addresses, wide SCSI has 16. SCSI can
support up to 15 devices on a single bus & bus speeds range from Ultra SCSI at 20MB/s to Ultra
160 SCSI at 160MB/s with further increase on the way.
Fibre. Fibre channel interface is similar to switched Ethernet & InfiniBand as it is not only for
connecting hard disk drives but also peripherals in a system. Fibre is also used for networking,
sharing drive resources and other high-bandwidth needs. Fibre channel is often used to connect
a SCSI RAID or RAID's to a network, workstations or servers. Fibre channel gives 106MB/s or
1.06Gbps. The next generation of Fibre channel will be 212MB/sec or 2.12Gbps. Some Ultra
High-end Fibre channel drives use multiple Fibre channels at once to give even higher bandwidth.
Fibre channel can be up to 10 kilometres with fibre optic cabling, less with copper cabling.
IEEE 1394. IEEE 1394, Apple's name (FireWire), Sony's name (iLink) is now becoming popular
for digital video data transfer and other devices that need a higher bandwidth than USB such as,
scanners, networking, digital cameras. IEEE FireWire can support up to 63 devices on a single
50MB/s channel. IEEE 1394b will support 100MB/s per channel. IEEE FireWire is hot pluggable so
you do not have to switch your computer off & on if you want to connect a FireWire device. It
does not provide power to devices. FireWire supports plug & play.
USB 1.1. USB, Universal Serial Bus. USB 1.1 has a data rate of about 1.5MB/s for approximately
5 meters. A single USB channel can have 127 devices connected in either pass through or using
USB hubs. USB uses a Master controller. This means that any signal sent from one device to
another must pass through the USB controller on the PC and then back to the other device. USB
devices cannot be shared by more than one computer. Two computers would have to be
networked together via a USB bridge device if you wanted to share USB devices between
computers. USB is hot pluggable as with IEEE 1394 and can supply power for USB devices. USB2
will increase the bandwidth to 60MB/s.
RAID. RAID, Redundant Array of Inexpensive Disks. RAID is an interface like IDE or SCSI. RAID
is a protocol or system for using existing IDE or SCSI drives and generally has two purposes. To
increase speed and/or reliability. Main types of RAID are: RAID 0, 1, & 0+1. RAID 0 uses two
hard drives at once, reading & writing from both at once. RAID 1 uses two hard disk drives but
mirrors the data on the first drive to the second drive. RAID 0+1 uses four hard disk drives
where the second two drives are mirrors.
RECOMMENDATIONS.
IDE gives you perhaps the best combination between price and performance for a general
purpose home or small business PC computer, especially with the increase in speed of modern
IDE hard disk drives. But, if your needs are for greater performance and more connectability then
SCSI drives & devices will be the best choice for home, business, server.
General Hard disk drive terms.
Areal Density: Measured in bytes/sq. inch. Refers to how densely packed the information is on
the hard disk drives platters is. Higher densities give greater storage per size and reduce time to
get the data.
Cache: Is a buffer between the hard disk drive and the bus. Cache's between 512k and 4MB are
usual and play an important part in the performance of the hard disk drive.
MTBF: Mean Time Between Failure. How long the hard disk drive is expected to last. Needless to
say, the higher the better. IDE hard disk drives tend to be lower than SCSI hard disk drives.
Platter: This is the actual disk of a hard disk drive and drives can and do have more than one
platter.
Rotational Speed: Measured in RPM. Range between 4,200rpm to 15,000rpm. Standard IDE
hard disk drive being 7,200 rpm while SCSI hard disk drives being 10,000rpm and now
15,000rpm.
Seek Time: This is the time the hard disk drive takes to find the track on the disk (platter). You
will see either track to track seek time, or an average seek time.
We hope you find our compact review helpful to you along with all our independent reviews of
computer hardware.
drive is the name for several types of storage media. There are also storage media, which
are not drives (RAM, Tape Streamers), but on these pages, we will discuss the drives.
Common to drive medium is:
 A file system can be assigned to them.
 They are recognized by the operating system and they are assigned a drive letter.
During start up, drives are typically recognized by the PC system software (ROM BIOS +
operating system). Thus, the PC knows which drives are installed. At the end of this
configuration, the appropriate drive letter is identified with each drive. If a drive is not
"seen" during start up, it will not be accessible to the operating system. However, some
external drives contain special soft-ware, allowing them to be connected during
operation.
Some examples of drives
Storage media
Drive letter
Floppy disks
A: B:
Hard disk
C: D: E:
CDROM/DVD
F:
MO drive
G:
Network drive
M:
RAM disk
O:
[top]
On this and the following pages, I will describe the various drive types, their history and
technology. The last two drive types in the above table will not be covered.
Storage principles
[top]
Storage: Magnetic or optic. Data on any drive are digitized. That means that they are
expressed as myriads of 0s and 1s. However, the storage of these bits is done in any of
three principles:
The physical drive
principle
Disk types
Magnetic
Floppy disks
Hard disk
Syquest
disks
Zip drive
LS-120 disks
Optic
CD-ROM
DVD
Magneto optic
High end
drives
[top]
Interface
Individual drives are connected to other PC components through an interface. The hard
disk interface is either IDE or SCSI, which in modern PCs is connected to the PCI bus.
Certain drives can also be connected through a parallel port or the floppy controller:
Interface
Drive
IDE and EIDE
Hard disks (currently up to 40 GB)
CD-ROM
SCSI
Hard disks (all sizes) and CD-ROM
ISA (internal)
Floppy drives
CDROM and super floppies connected through parallel port
Let us start evaluating the drives from the easy side:
The traditional floppy drive
[top]
We all know diskettes. Small flat disks, irritatingly slow and with too limited storage
capacity. Yet, we cannot live without them. Very few PCs are without a floppy drive.
Diskettes were developed as a low cost alternative to hard disks. In the 60s and 70s, when
hard disk prices were exorbitant, It was unthinkable to use them in anything but
mainframe and mini computers.
The first diskettes were introduced in 1971. They were 8" diameter plastic disks with a
magnetic coating, enclosed in a cardboard case. They had a capacity of one megabyte.
The diskettes are placed in a drive, which has read and write heads. Conversely to hard
disks, the heads actually touch the disk, like in a cassette or video player. This wears the
media.
Later, in 1976, 5.25" diskettes were introduced. They had far less capacity (only 160 KB
to begin with). However, they were inexpensive and easy to work with. For many years,
they were the standard in PCs. Like the 8" diskettes, the 5.25" were soft and flexible.
Therefore, they were named floppy disks.
In 1987 IBM's revolutionary PS/2 PCs were introduced and with them the 3½" hard
diskettes we know today. These diskettes have a thinner magnetic coating, allowing more
tracks on a smaller surface. The track density is measured in TPI (tracks per inch). The
TPI has been increased from 48 to 96 and now 135 in the 3.5" diskettes.
Here you see the standard PC diskette configurations:
Diskette size
Name
Tracks per
side
Number of sectors
per tracks
Capacity
5.25" Single side
SD8
40
8
40 X 8 X 512 bytes =
160 KB
5.25" Double side
DD9
40
9
2 X 40 X 9 X 512 bytes
= 360 KB
5.25" Double side
High Density
DQ15
80
15
2 X 80 X 15 X 512 bytes
= 1.2 MB
3.5" DD
DQ9
80
9
2 X 80 X 9 X 512 bytes
= 720 KB
3.5" HD
DQ18
80
18
2 X 80 X 18 X 512 bytes
= 1.44 MB
3.5" XD ( IBM only)
DG36
80
36
2 X 80 X 36 X 512 bytes
= 2.88 MB
Diskette drives turn at 300 RPM. That results in an average search time (½ revolution) of
100 ms.
The super floppy drives are described in module 4d.
The floppy controller
[top]
All diskette drives are governed by a controller. The original PC controller was named
NEC PD765. Today, it is included in the chip set, but functions like a 765. It is a
programmable chip. It can be programmed to handle all the various floppy drive types:
5.25" or 3.5" drives, DD or HD etc.
The controller has to be programmed at each start up. It must be told which drives to
control. This programming is performed by the start up programs in ROM (read module
2a). So you don't have to identify available drive types at each start up, these drive
parameters are saved in CMOS RAM.
The floppy controller reads data from the diskette media in serial mode (one bit at a time.
like from hard disks). Data are delivered in parallel mode (16 bits at a time) to RAM via a
DMA channel. Thus, the drives should be able to operate without CPU supervision.
However, in reality this does not always work. Data transfer from a diskette drive can
delay and sometimes freeze the whole PC, so no other operations can be performed
simultaneously.
Memory Types
http://bugclub.org/eric/memory/computermemory.html
RAM
David Risley
Random Access Memory (RAM) is what most of us think of when we hear the word
memory associated with computers. It is volatile memory, meaning all data is lost
when power is turned off. The RAM is used for temporary storage of program data,
allowing performance to be optimum.
Like ROM, there are different types of RAM:


Static RAM (SRAM) This RAM will maintain its data as long as power is
provided to the memory chips. It does not need to be rewritten periodically.
SRAM is very fast but much more expensive than DRAM. SRAM is often used
as cache memory due to its speed.
Dynamic RAM (DRAM) DRAM, unlike SRAM, must be continually rewritten in
order for it to maintain its data. This is done by placing the memory on a
refresh circuit that rewrites the data several hundred times per second. DRAM
is used for most system memory because it is cheap and small.
There are several types of DRAM, complicating the memory scene even more:



Fast Page Mode DRAM (FPM DRAM) FPM DRAM is only slightly faster than
regular DRAM. It uses a slightly more efficient method of calling data from the
memory. FPM DRAM is not used much anymore due to its slow speed, but it is
almost universally supported.
Extended Data Out DRAM (EDO DRAM) EDO memory incorporates yet
another tweak in the method of access. It allows one access to begin while
another is being completed. While this might sound ingenious, the
performance increase over FPM DRAM is only slight. EDO DRAM must be
properly supported by the chipset, but it is the most common type of memory
for most users. Power users with high bus speeds typically opt for something
faster, though.
Burst EDO DRAM (BEDO DRAM) This is basically EDO DRAM with combined
pipelining technology. The result is a much faster EDO memory chip capable


of working with faster bus speeds. Support for the BEDO technology is rather
sparse. SDRAM has caught on faster.
Synchronous DRAM (SDRAM) SDRAM is the developing new standard for
PC memory. Its speed is synchronous, meaning that it is directly dependent
on the clock speed of the entire system. It works at the same speed as the
system bus, up to 100MHz. Although SDRAM is faster, the speed difference
isn't noticed by many users due to the fact that the system cache masks it.
Also, most users are working on a relatively slow 66MHz bus speed, which
doesn't use the SDRAM to its full capacity.
RAMBus DRAM (RDRAM) This is a technology still being developed by Intel
that may prove to surpass SDRAM. Its goal is to get rid of the latency, the
time taken to access memory. It does this by actually narrowing the bus path
and treating the memory bus as a separate communication channel.
This is read-only memory, memory that can only be read from but not written to.
ROM is used in situations where the data must be held permanently. This is due to
the fact that it is non-volatile memory. This means the data is "hard-wired" into the
ROM chip. You can store the chip forever and the data will always be there. The data
is very secure. The BIOS is stored on ROM because the user cannot disrupt the
information.
There are different types of ROM, too.



Programmable ROM (PROM) This is basically a blank ROM chip that can be
written to once. It is much like a CD-R drive that burns the data into the CD.
Some companies use special machinery to write PROMs for special purposes.
Erasable Programmable ROM (EPROM) This is just like PROM except that
you can erase the ROM by shining a special ultra-violet light into a sensor
atop the ROM chip for a certain amount of time. Doing this wipes the data
out, allowing it to be rewritten.
Electrically Erasable Programmable ROM (EEPROM) (Also called Flash
BIOS) This ROM can be rewritten through the use of a special software
program. Flash BIOS also operates this way, allowing users to upgrade their
BIOS.
ROM is slower than RAM, which is why some try to shadow it to increase speed.
Access Times
The average period of time (in nanoseconds) it takes for RAM to
complete one access and begin another. Access time is composed of latency
(the time it takes to initiate a request for data and prepare to access it) and
transfer times.
DRAM chips for personal computers have accessing times of 50 to 150
nanoseconds (billionths of a second). Static RAM (SRAM) has access times
as low as 10 nanoseconds. Ideally, the accessing times of memory should be
fast enough to keep up with the CPU. If not, the CPU will waste a certain
number of clock cycles, which makes it slower.
A nanosecond (ns or nsec) is = 10-9 one billionth of a second.
Async SRAM
(Asynchronous Static Random Access Memory)
Manufacturer: many
Year Introduced:
Burst Timing: 3-1-1-1
Voltage: 2.7-3.1v
Speed: 8.5ns
Frequency: 500 MHz
85-ball
Pins:
PBGA
Bandwidth:
Async SRAM has been with us since the days of the 386, and is still in
place in the L2 cache of many PCs. It's called asynchronous because it's not
in sync with the system clock, and therefore the CPU must wait for data
requested from the L2 cache. The wait isn't as long as it is with DRAM, but
it's still a wait.
BIOS
(Basic Input Output System)
BIOS stands for Basic Input Output System. It is defined as a firmware
that controls much of the computer´s input/output functions such as
communication with disk drives, the printer, RAM chips and the monitor. It
is a set of software instruction parameters that are hard coded into a chip.
These instruction parameters within BIOS versions have changed as
technology has developed in regard to hardware. The BIOS is typically
placed on a ROM chip that comes with the computer (it is often called a
ROM BIOS). This ensures that the BIOS will always be available and will
not be damaged by disk failures. It also makes it possible for a computer to
boot itself.
Because RAM is faster than ROM, though, many computer
manufacturers design systems so that the BIOS is copied from ROM to
RAM each time the computer is booted. This is known as shadowing. Most
modern PCs have a Flash BIOS, which means that the BIOS has been
recorded on a flash memory chip, which can be updated if necessary. The
PC BIOS is standardized, so all PCs are alike at this level (although there are
different BIOS versions). Additional DOS functions are usually added
through software modules. This means you can upgrade to a newer version
of DOS without changing the BIOS. PC BIOSes that can handle Plug-andPlay (PnP) devices are known as PnP BIOSes, or PnP-aware BIOSes. These
BIOSes are always implemented with flash memory rather than ROM.
The POST
Power On Self Test is the first instruction executed during start-up. It
checks the PC components and that everything works. You can recognize it
during the RAM test, which occurs as soon as you turn power on.
You may follow the checks being executed in this order, as the
information are gathered:
1. Information about the graphics adapter
2. Information about the BIOS (name, version)
3. Information about the RAM (being counted)
As users, we have only limited ability to manipulate the POST
instructions. But certain system boards enable the user to order a quick
system check. Some enable the user to disable the RAM test, thereby
shortening the duration of the POST. The duration of the POST can vary
considerably in different PCs. On the IBM PC 300 computer, it is very slow.
But you can disrupt it by pressing [Esc].
Error messages
If POST detects errors in the system, it will write error messages on the
screen. If the monitor is not ready, or if the error is in the video card, it will
also sound a pattern of beeps (for example 3 short and one long) to identify
the error to the user. If you want to know more of the beeps, you can find
explanations on the Award, AMI and Phoenix web sites. For instance you
will receive error messages if the keyboard is not connected or if something
is wrong with the cabling to the floppy drive.
Buffered Memory
A term used to describe a memory module that contains buffers. The
buffers re-drive the signals through the memory chips and allow the module
to be built with a greater number of memory chips. Buffered and unbuffered
RAM cannot be mixed. The design of the computer's memory controller
dictates which type of RAM must be used.
Burst Mode
Bursting is a rapid data-transfer technique that automatically generates a
block of data (a series of consecutive addresses) every time the processor
requests a single address. The assumption is that the next data-address the
processor will request will be sequential to the previous one. Bursting can be
applied both to read operations (from memory) and write operations (to
memory).
CDRAM, Cached DRAM
Manufacturer:
Year Introduced:
Burst Timing:
Voltage:
Speed:
Frequency:
Pins:
Bandwidth:
Cache DRAM (CDRAM) is a development that has a localized, on chip
cache with a wide internal bus composed of two sets of static data transfer
buffers between cache and DRAM. This architecture achieves concurrent
operation of DRAM and SRAM synchronized with an external clock.
Separate control and address input terminals of the two portions enable
independent control of the DRAM and SRAM, thus the system achieves
continuous and concurrent operation of DRAM and SRAM. CDRAM can
handle CPU, direct memory access (DMA) and video refresh at the same
time, by utilizing half-time multiplexed interleaving through a high-speed
video interface. The system transfers data from DRAM to SRAM during the
CRT blanking period. Graphic memory, as well as main memory and cache
memory, are unified in the CDRAM. As you can see, CDRAM can replace
cache and main memory, and it is has already been proven that a CDRAM
based system has a 10 to 50 percent performance advantage over a 256kbyte
cache based system.
CMOS RAM
(Complementary Metal Oxide Semiconductor Random Access Memory)
CMOS (pronounced see-moss) stands for complementary metal-oxide
semiconductor. This is a type of memory chip with very low power
requirements, and in PCs it operates using small batteries. In PCs, CMOS is
more specifically referred to as CMOS RAM. This is a tiny 64-byte region
of memory that, thanks to the battery power, retains data when the PC is shut
off.
The function of CMOS RAM is to store information your computer needs
when it boots up, such as hard disk types, keyboard and display type, chip
set, and even the time and date. If the battery that powers your CMOS RAM
dies, all this information is lost, and your PC will boot with the default
information that shipped with the motherboard. In most cases, this means
you´ll have no access to your hard disks until you supply CMOS with the
necessary information. Without access to your hard disks, you won´t be able
to boot your operating system.
Fortunately, today´s CMOS RAM is protected by nickel cadmium
batteries, which the computer´s power supply recharges. Even so, it´s an
extremely good idea to keep a copy of all the information stored in CMOS,
in case disaster strikes.
The information stored in CMOS is required by your computer´s Basic
Input/Output System, or BIOS (pronounced bye-oss). Your PC contains
several BIOSes--the video BIOS that interfaces your CPU and video card,
for example--but the most fundamental is the system BIOS. The system
BIOS is stored on a ROM (read-only memory) chip on the motherboard and
is copied at boot time to a 64K segment of upper system RAM for faster
system access (RAM is faster than ROM). The role of the system BIOS is to
boot the system, recognize the hardware devices, and locate and launch the
operating system. Once the operating system is loaded, the BIOS then works
with it to enable access to the hardware devices.
Virtual memory provides applications with more RAM space than allocated in the
computer. A technique which operating systems use to load more data into RAM than it
can hold. Part of the data is kept on disk and is constantly swapped back and forth into
system memory. For instance, when your run a CD application.
Whenever the operating system needs a part of memory that is currently not in
physical memory, a VIRTUAL MEMORY MANAGER picks a part of physical RAM
that hasn´t been used recently, writes it to a SWAP FILE on the hard disk and then reads
the part of RAM that is needed from the swap file and stores it into real RAM in place of
the old block. This is called SWAPPING. The blocks of RAM that are swapped around
are called PAGES.
Virtual memory allows for the multitasking (opening more than one program) that we
do.
When the amount of virtual memory in use greatly exceeds the amount of real
memory, the operating system spends a lot of time swapping pages of RAM around,
which greatly hampers performance. This called THRASHING and you can see it in your
LED hard disk drive light. The hard disk is thousands of times slower than the system
RAM, if not more. A system that is thrashing can be perceived as either a very slow
system or one that has come to a halt. Hard disk access time is measured in thousandths
of a second; RAM access time is measured in billionths of a second.