Terms 3

1. Terms 3 Definitions and Questions

2. 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

3. 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

4. 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/encyclopedia_term/0,2542,t=video+RAM&i=53857,00.asp

5. 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/ram/typesSRAM-c.html

6. 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/sDefinition/0,,sid5_gci212679,00.html

7. 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 (random-access 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/dynamic_RAM.html

8. 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 single-ported RAM which only allows one access at a time. Video RAM or VRAM is a common form of dual-ported 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 dual-ported 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

9. 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.

10. 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

11. 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

12. 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

13. 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.

14. 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.

15. 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.

16. 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 re-reading data from many different original locations. • In other operating systems, the pagefile is called a swap file or a swap partition.

17. 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.

18. 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.

19. 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.