Sec (1.2):

1. Sec (1.2): Main memory

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

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

4. 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

5. 1 0 Magnetic core:

6. 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.

7. 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

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

9. 3- Bubble memory: • It has come to take the advantages of magnetic cores and semiconductor technology

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

11. Least significant bit ( Lower order end ) Most significant bit ( High order end ) ADDRESS REGISTER MATRIX LCU CPU WORD REGISTER Memory contains:

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

13. 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

14. 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