Sec (1.2): Main memory

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

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: 10

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

Measuring Memory capacity: Bit = 0 or 1 Byte = 8 bits KByte = 1024 Byte = 2 10 byte MByte = 1024 * 1024 Byte = 2 20 byte GByte = 1024 * 1024 * 1024 Byte = 2 30 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 = 2 16 = No. of bits in the memory = 65536*4 = bits Size of the memory in bytes = / 8 = 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