Terms 3
Definitions and Questions
ROM
Memory hardware that allows fast access to permanently stored
data but prevents addition to or modification of the data
(Read Only Memory) A memory chip that permanently stores
instructions and data. Also known as "mask ROM," its content is
created in the last masking stage of the chip manufacturing process,
and it cannot be changed. Stand-alone ROM chips and ROM banks
in microcontroller chips are used to hold control routines for a myriad
of applications. ROMs were also widely used to hold the BIOS in
early PCs as well as plug-in cartridges for video games.
Although EPROMs, EEPROMs, and particularly flash memory, are
the kinds of non-volatile storage one hears about more often, ROM
technology is mature, inexpensive and easy to integrate into any
CMOS chip.
http://www.answers.com/topic/rom
RAM
Random access memory
RAM is considered "random access"
because you can access any memory cell
directly if you know the row and column
that intersect at that cell.
http://computer.howstuffworks.com/ram.htm
Types of RAM- Video RAM
Also called "VRAM," it is the
type of memory used in a
display adapter. Video RAM is
designed with dual ports so
that it can simultaneously
refresh the screen while text
and images are drawn in
memory. It is faster than the
common DRAM or SDRAM
chips used as main memory in
the computer
http://www.pcmag.com/encyclope
dia_term/0,2542,t=video+RAM
&i=53857,00.asp
Types of RAM- Static RAM
Static RAM is a type of
RAM that holds its data
without external refresh,
for as long as power is
supplied to the circuit
http://www.pcguide.com/ref/r
am/typesSRAM-c.html
Types of RAM- Non-Volatile
RAM
Nonvolatile (sometimes written as
"non-volatile") storage (NVS) - also
known as nonvolatile memory or
nonvolatile random access memory
(NVRAM) - is a form of static RAM
whose contents are saved when a
computer is turned off or loses its
external power source. NVS is
implemented by providing static RAM
with backup battery power or by
saving its contents and restoring them
from an electrically erasable
programmable ROM.
http://searchstorage.techtarget.com/sDefin
ition/0,,sid5_gci212679,00.html
Types of RAM- Dynamic RAM
A type of physical memory used in
most personal computers. The
term dynamic indicates that the
memory must be constantly
refreshed (reenergized) or it will
lose its contents. RAM (randomaccess memory) is sometimes
referred to as DRAM (pronounced
dee-ram) to distinguish it from
static RAM Static RAM is faster
and less volatile than dynamic
RAM, but it requires more power
and is more expensive.
http://www.webopedia.com/TERM/D/d
ynamic_RAM.html
Types of RAM- Dual-Ported
RAM
Dual-ported RAM (DPRAM) is a type of
Random Access Memory that allows
multiple reads or writes to occur at the same
time, or nearly the same time, unlike singleported RAM which only allows one access at
a time.
Video RAM or VRAM is a common form of dualported dynamic RAM mostly used for video
memory, allowing the CPU to draw the
image at the same time the video hardware
is reading it out to the screen.
Apart from VRAM, most other types of dualported RAM are based on static RAM
technology.
Most CPUs implement the processor registers as
a small dual-ported RAM.
http://en.wikipedia.org/wiki/Dual-ported_RAM
Byte
A byte (B) is a unit of measure used by computers for the
purposes of memory, storage, and processing
instructions that consists of a sequence of a specific
number of bits.
Most modern computers manipulate bytes that consist of
8 bits.
Bits (b) are single units of binary information (0's and 1's)
that serve as the most basic unit of computer storage
and carrying out instructions. 8-bit bytes can hold any of
256 possible values (2^8).
Bytes are typically used to represent characters (i.e.
letters, numbers, etc.) in encoding systems such as
ASCII, which is the standard character coding system on
most computers.
Kilobyte
Although kilo- is prefix meaning 1,000,
kilobytes (KB) consist of 1,024 bytes
(2^10).
Before 1990 this was a common unit of
measure available for memory and
secondary storage capacity on
microcomputers
Megabyte
Despite commonly being associated with a
value of 1 million bytes, a megabyte
actually contains 1,048,576 bytes (2^20).
A megabyte is equal to 1,024 kilobytes.
Measure of Microcomputer RAM memory
today (Random Access Memory)
Measure of CD-ROM storage capacity
Gigabyte
A gigabyte (GB) consist of 1,073,741,824
bytes (2^30), which is typically
approximated as 1 billion bytes.
A gigabyte is equal to 1024 megabytes or
1,073,741,824 bytes.
Measure of Microcomputer Hard Drive
storage capacity
Measure of DVD-ROM storage capacity
Hard Drive Capacity
Hard drive manufacturers rate hard drive sizes in terms of base 10
powers instead of base 2 powers because base 10 is a more
commonly understood system for the average person.
With this in mind, there are discrepancies in terms of storage for
megabytes, gigabytes, etc. The difference between the advertised
and actual storage capacity of a megabyte is 48,576 bytes. This
difference is equal to 73,741,824 bytes when considering a
gigabyte.
This leads to over reporting of disc space by manufacturers.
Another reason that advertised capacity is not what it appears to be
is formatting. Formatting, which is often done using such formats as
FAT16, FAT32 and NTFS, enables a computer to read and write
data to the drive. Storage space on the drive has to be designated
specifically for these files.
Virtual Memory
Virtual (or logical) memory is a concept that, when
implemented by a computer and its operating system,
allows programmers to use a very large range of
memory or storage addresses for stored data. The
computing system maps the programmer's virtual
addresses to real hardware storage addresses. Usually,
the programmer is freed from having to be concerned
about the availability of data storage.
In addition to managing the mapping of virtual storage
addresses to real storage addresses, a computer
implementing virtual memory or storage also manages
storage swapping between active storage (RAM) and
hard disk or other high volume storage devices. Data is
read in units called "pages" of sizes ranging from a
thousand bytes (actually 1,024 decimal bytes) up to
several megabyes in size. This reduces the amount of
physical storage access that is required and speeds up
overall system performance.
Virtual Memory Q/A
Q: Can the Virtual Memory be turned off on a really large
machine? Should it?
A: Strictly speaking Virtual Memory is always in
operation and cannot be “turned off.” What is meant by
such wording is “set the system to use no page file
space at all.”
Doing this would waste a lot of the RAM. The reason is
that when programs ask for an allocation of Virtual
memory space, they may ask for a great deal more than
they ever actually bring into use — the total may easily
run to hundreds of megabytes. These addresses have to
be assigned to somewhere by the system. If there is a
page file available, the system can assign them to it — if
there is not, they have to be assigned to RAM, locking it
out from any actual use.
Page File
 In storage, a pagefile is a reserved
portion of a hard disk that is used as an
extension of random access memory
(RAM) for data in RAM that hasn't been
used recently. A pagefile can be read
from the hard disk as one contiguous
chunk of data and thus faster than rereading data from many different original
locations.
 In other operating systems, the pagefile is
called a swap file or a swap partition.
Page File Q / A
Q: What is a page fault?
A: Only those parts of the program and data that are
currently in active use need to be held in physical RAM.
Other parts are then held in a page file. When a
program tries to access some address that is not
currently in physical RAM, it generates an interrupt,
called a Page Fault. This asks the system to retrieve the
the address from the page file. This — a valid page fault
— normally happens quite invisibly. Sometimes, through
program or hardware error, the page is not there either.
The system then has an ‘Invalid Page Fault’ error. This
will be a fatal error if detected in a program: if it is seen
within the system itself, it may manifest itself as a ‘blue
screen’ failure.
Emulation
Emulation is when hardware or software of a
system is modified or imitated with the
purpose of performing the same functions of
an existing system.
For example, many printers are designed to emulate Hewlett-Packard
LaserJet printers because so much software is written for HP printers. By
emulating an HP printer, a printer can work with any software written for a
real HP printer. Emulation tricks the software into believing that a device is
really some other device.
Communications software packages often include terminal
emulation drivers. This enables your PC to emulate a particular type of
terminal so that you can log on to a mainframe.
It is also possible for a computer to emulate another type of computer. For
example, there are programs that enable an Apple Macintosh to emulate a
PC.
Emulation Q/A
Q: What is the downside to emulating one
OS on another?
A: Because this involves a large overhead
as operations for one OS are translated
into operations for another, there is a large
performance hit associated with this sort of
emulation.