Lec 11 - Memory Types

? LEARNING OBJECTIVES
After completion of this lecture, you should be able to:
? Describe how the computer representation numerical information.
? Describe the computer arithmetic
? Describe the floating-point representation

1. TYPES OF MEMORY
The two major types of memory are:

1.1. READ-WRITE OR RANDOM ACCESS MEMORY (RAM)

It has a variable content; also it is generally used to store the variable data. RAM can also be used to store frequently changed programs and other information. The term “random” means that any memory location can be accessed in the same amount of time, regardless of its position in the memory.
RAM allows the computer to store information quickly for later reference:
1- The active part of the operating system, the fundamentalالاساسي program that control the operation of the computer.
2- The application program being executed (for example a letter being written with the word processing program).
3- A representation of the data being presented on the video display

In RAM, the stored information will be lost when computer power supply is removed (even a short interruption). That is RAM, is volatile memory.
1.1.1. TYPES OF RAM

Various RAM chips have been developed to move data in and out of memory quicker, to avoid errors and to make collections of RAM chips smaller. Here are the most common types of RAM:
1- DRAM (Dynamic RAM): For years, the most common type of main memory. “ Dynamic” refers to the memory’s method of storage, basically storing the charge on a capacitor, which leaksتسرب the charge over time and must be refreshed every 10-100 ms.

2- SRAM (Static RAM): Unlike DRAM doesn’t need to have its electrical charges constantly refreshed because the contents of each location persistمستمرة as long as power is applied to the chips. SRAM is usually faster than DRAM but more expensive, so it is used for must speed-critical parts of a computer, such as the cache memory.

3- VRAM (Video): RAM optimized for video adapters. VRAM chips have two ports so that video data can be written to chips at the same time the video adapter continuously reads the memory to refresh the monitor’s current display.

4- EDO RAM (Extended Data Out RAM): Faster than DRAM. EDO memory can send data even while receiving instructions about what data to access next.

5- SDRAM (Synchronous DRAM): RAM designed to keep up with the bus speed of fast processors. SDRAMs are designed with two internal banks of transistors for storing data. This allows one bank to get ready for access while the other bank is being accessed. This is most often the RAM of choice for Pentium-class PCs.

6- DDR SDRAM (Double data-rate synchronous DRAM) : Enhancement of SDRAM that uses both edges as control signals.

7- SIMM (Single In-line Memory Modules): memory chips are put onto a small circuit board with pins along the bottom , which plugs into a connector on the motherboard.

8- DIMM (Dual In-line Memory Module): Similar to SIMMs, DIMM use memory chips and separate connector pins on both sides of the circuit board to increase the amount of memory that can be plugged into a single connector and to increase the size of the data path for faster data transfer.

9- ECC (Error Correcting Code): RAM that uses extra bits to detect errors. Types of memory such as DRAM, SDRAM and DIMM may also be ECC chips.


1.2. READ ONLY MEMORY (ROM)

The real difference between RAM and ROM is this: information that is stored in RAM can be changed. It is a nonvolatile memory, when the computer turns off, the contents of ROM are not change, that is another different between RAM and ROM. Sending data to a ROM memory address is ineffective, because the contents of ROM are permanentدائم.

1.2.1. TYPES OF ROM

1- ROMS: The first type of ROM the contents are determine by a masking operationاخفاء عملية that is performed while the chip is being manufactured such chip cannot be altered by the user.

2- PROM (Programmable ROM): The contents of this type allow the data to be loaded by the user if the proper equipment معدات مناسبةis a available. Once a memory is programmed, its contents can never be changed.
3- EPROM (erasable Programmable ROM) and EAPROM (electrically alterable PROM): this type of ROM can be erasable, reprogrammable by using special equipment (ultraviolet light)الاشعة فوق البنفسجية and can be reprogrammed as often as required .

4- Flash EPROMs (FEPROMs): This FEPROMs are more compact, faster, and removable compared to EPROMs. The erasure time of a FEPROM is far faster than that of an EPROM.

5- Electrically EPROM or EEPROM: In this type, the erasure of the EPROM can be done electrically and, moreover, selectively; that is, only the contents of selective cells can be erased, leaving the other cells’ contents untouched. Both FEPROMs and EEPROMs are used in applications requiring occasional updating of information, such as Programmable TVs, VCR, and automotives.


2. BYTE ORDERING

Storing data often requires more than a byte. For example, we need four bytes of memory to store an integer variable that can take a value between 0 and 232-1. Let us assume that the value to be stored is the one in Figure 45a.
Suppose that we want to store these 4-byte data in memory at locations 100 through 103.
How do we store them? Figure 45 shows two possibilities: least significant byte (Figure 45b) or most significant byte (Figure 45c) is stored at location 100. These two byte ordering schemes are referred to as the little endian and big endian. In either case, we always refer to such multibyte data by specifying the lowest memory address (100 in this example).
Is one byte ordering scheme better than the other? Not really! It is largely a matter of choice for the designers. For example, Pentium processors use the little-endian byte ordering. However, most processors leave it up to the system designer to configure the processor. For example, the MIPS and PowerPC processors use the big-endian byte ordering by default, but these processors can be configured to use the little-endian scheme.
The particular byte ordering scheme used does not poseلا تشكل any problems as long as you are working with machines that use the same byte ordering scheme. However, difficulties arise when you want to transfer data between two machines that use different schemes. In this case, conversion from one scheme to the other is required. For example, the Pentium provides two instructions to facilitate such conversion: one to perform 16-bit data conversions and the other for 32-bit data.