Memory Subsystem in Computer Architecture

Microcomputer Memory

Memory is an essential component of the microcomputer system.
It stores binary instructions and datum for the microcomputer.

The memory is the place where the computer holds current programs and data that are in use. None technology is optimal in satisfying the memory requirements for a computer system.

Computer memory exhibits perhaps the widest range of type, technology, organization, performance and cost of any feature of a computer system. The memory unit that communicates directly with the CPU is called main memory. Devices that provide backup storage are called auxiliary memory or secondary memory.



 

Characteristics of memory systems

• The memory system can be characterised with their Location, Capacity, Unit of transfer, Access method, Performance,
• Physical type, Physical characteristics, Organisation.

RAM, ROM, DRAM, Flash Memory

• The main memory is a Random Access Memory. It is organized into words of fixed length.
• The length of a word is called word length.
• Each of these memory words has an independent address and each has the same number of bits.
• Normally the total numbers of words in memory are some power of 2. Some typical memory word size are 8 bits, 16 bits, 32 bits etc.
• The main memory can be both read and written into, therefore it is called read-write memory. 
• The block diagram of a read- write memory of RAM is shown in figure. 

The access time and cycle time in RAM are constant and independent of the location accessed. The computer places the address of the location into which the data are to be read into the memory address register. This register consist of n binary devices (generally flip-flops), where 2n is the number of words that can be stored in the memory. Example : A 32 × 4 RAM means that this RAM has 32 words, 5 address lines (25 = 32), and 4 bit data word size.

Dram (Dynamic Random Access Memory)

RAM technology is divided into two technologies : Dynamic and Static. A Dynamic RAM (DRAM) is made with cells that store data as charge on capacitors. The presence and absence of charge on capacitor is interpreted as binary 0 or 1. Since the capacitors have natural tendency to discharge Dynamic RAM requires periodic charge refreshing to maintain data storage.

ROM (Read-Only Memory)

A ROM is a memory or storage device in which a fixed set of binary information is stored. A ROM is a device with several input and output lines such that for each input value there is a unique output value. A block diagram of ROM is shown in the figure 3.

It consist n input lines and m output lines. Each combination of the input variables in called an address. Each bit combination that comes out of
the output lines in called a word. The number of bits per word is equal to the number of output lines m. The number of distinct address possible with n input
variable is 2n.

A ROM is characterized by the number of words (2n) and number of bits (m) per word. For example a 16 × 8 ROM which can be written as 24 × 8 consist of 16 words of 8 bit each, which mean there are 8 output lines and 16 distinct words stored in the unit. There are only 4 input lines because 16 = 24 and with 4 binary variable we can specify 16 address. A ROM is basically a combinational circuit and can be constructed as shown in figure 3 (c). On applying an Input I1 = 0, I2 = 0, the 00 line of the decoder is selected and we will get O1 = 0 and O2 = 1;
on applying I1 = 0 and I2 = 1 we will get O1 = 1 and O2 = 0. The same logic is used for constructing larger ROM.

Flash Memory :

Flash Memory is a form of semiconductor memory which was first introduced
in mid-1980. These memories can be reprogrammed at high speed and hence the
name flash. These are electronic Random Access Memory but non-volatile in nature. 

In flash memory the entire memory can be erased in a few seconds by using electric erasing technology. It is used in many I/O devices. It is also used to store data and programming algorithm in cell phones, digital cameras and MP3 music players. Basically flash memory falls in between EPROM and EEPROM.

There are two kinds of flash memory :


(i) Code storage flashmade by Intel, AMD, Atmel, etc. It stores
programming algorithms and is largely found in cell phones 

(ii) Data storage Flashmade by San Disk, Toshiba etc. It stores data and
comes in digital cameras and MP3 players.

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