1 2 Main memory Memory Stores data and

1. 2) Main memory

Memory: • Stores data and instructions in the beginning, intermediate and final stages of the processing.

Main memory • For storing data, a computer contains a large collection of circuits, each capable of storing a single bit. • Also known as RAM “Random Access Memory” Islamic University Of Gaza, Nael Aburas 3

Memory organization • A computer main memory is organized in manageable units called cells. • Cell: A unit of main memory (typically 8 bits which is one byte) • Each cell’s size equal to eight (8) bits. • higher-order end: the left end of the row of bits on the memory • lower-order end: the right end • most significant bit: the last bit at the highorder end. • least significant bit: the bit at the right end. Islamic University Of Gaza, Nael Aburas 4

Memory organization Islamic University Of Gaza, Nael Aburas 5

Memory organization • To identify individual cell in a computer’s main memory, each cell is assigned a unique “name”, called its address. • Address: A “name” that uniquely identifies one cell in the computer’s main memory. As shown in Figure Islamic University Of Gaza, Nael Aburas 6

Measuring memory capacity • Kilobyte = 1024 byte • Megabyte = 1024 kilobyte • Gigabyte = 1024 megabyte Islamic University Of Gaza, Nael Aburas 7

Technology of memory building: There are two significant types for technologies used in memory building ( design ): 1. Magnetic core 2. Semiconductor memory

1 - Magnetic core: • Consists of million of rings, each ring in the size of pinhead, these rings connected to each other forming a mesh, electricity flows through these rings, the rings are magnetized. If the electricity flows in clock wise, the ring represents 1 but in counter clock wise it represents 0. In this kind of technology data are not lost even if the power goes off. • Note: magnetic core is non-volatile memory

Magnetic core: 1 0

2 - Semiconductor memory: • Is an integrated circuit, is put on a silicone board, each cell contains transistor which works as switch and a capacitor which keeps the charge. If the capacitor is charged, it means the value stored is 1, other wise the value is 0.

Advantages of semiconductor: • • • less bulky than the magnetic core. survive for a longer period (less damage) data can be stored in a smaller size less cost faster in information retrieval

Disadvantages of semiconductor: • it is volatile, it losses its contents as soon as the power goes off. For this purpose UPS is used

3 - Bubble memory: • It has come to take the advantages of magnetic cores and semiconductor technology

Memory contains: 1. Address register: contains the address of the data to be brought or put in the memory ( the address value is brought from the CPU ) 2. Word register: it contains the data brought or to put from/in the memory 3. LCU (local control unit): supervises all the parts of the memory 4. The memory cells ( matrix ) ( The cell size is eight bits )

Memory contains: Least significant bit ( Lower order end ) Most significant bit ( High order end ) CPU ADDRESS REGISTE R LCU MATRIX WOR D REGIS TER

Measuring Memory capacity: • • • Bit = 0 or 1 Byte = 8 bits KByte = 1024 Byte = 210 byte MByte = 1024 * 1024 Byte = 220 byte GByte = 1024 * 1024 Byte = 230 byte

Ex: given the address register of length 16 bits and the length of the word is 4 bits, computes the size of the memory in bytes Sol. : No. of address locations = 216 = 65536 No. of bits in the memory = 65536*4 = 262144 bits Size of the memory in bytes = 262144 / 8 = 32768 bytes

Types of memory: • RAM ( Random Access Memory ) – – – It is a memory where data can be read or written. It is a volatile memory It is important in putting the computer specification – DRAM ( Dynamic RAM ) or SDRAM ( Synchronous DRAM ): Is used in reference to DRAM that applies additional techniques to decrease the time needed to retrieve the contents from its memory cells ROM ( Read Only Memory ) • • • – It is only for reading, it comes from the manufacture on a chip. For example the ASCII code is store in ROM This memory is non-volatile PROM ( Programmable Read Only Memory) Can be programmed by the manufacture or by the user (can be programmed for one time only ) – EPROM (Erasable Programmable Read Only Memory ) Can be erased and reprogrammed