الذاكرة الرئيسية

Main memory
By Noor Kadhum
Architecture For 2nd Stage
Computer Science

In the first lecture, we learned about the first level in the hierarchy of memory - registers – they are the only type of memory that resides inside the processor .
This lecture is dedicated to the study of another level in the hierarchicy. The main memory is the central storage unit in acomputer. It is relatively larg and fast memory used to store programs and data during the computer operation.
Random Access Memory(RAM)
Integrated circuit RAM chips are avalable in two possiple operating modes:
1- Static RAM : consists essentially of internal flip-flops that store the binary information.the stored information remains valid as long as power is applied to the unit.The static RAM is easier to use and has shorter read and write cycles.

2- Dynamic RAM: stores the binary information in the form of electric charges that are applied to capacitors(متسعات). The capacitors are provided inside the chip by MOS transistors. The charge on the capacitors tend to discharge with time and the capacitors must be periodically recharge by refreshing the dynamic memories. The dynamic RAM offers reduced power consumption and larger storage capacity in a single memory chip.
RAM was used to refer a Random Access Memory but it is use to designate a read / write memory to distiguish from read only memory (ROM).
Read Only Memory(ROM)
Most of main memory in a general purpose computer is made of RAM integrated circuit chips but a portion of the memory may be constructed with ROM chips.
ROM is needed for storing an initial program called bootstrap loader whosefunction is to start the computer software operating when the power is turened on .
The starup of computer consistes of : Turning the power on and starting the initial program.thus when power is turned on the hardwar of computer sets the program counter (PC) to the first address of the bootstrap loader.
The bootstrap program loads a portion of the operating system (OS) from disk to main memory and control is then transferred to OS which prepare the computer for general use.