COMPUTER ARCHITECTURE

I. The central processing unit (CPU) and its elements

Computer systems follow the concept of the input, process, output and storage model.

Data or instructions are processed by the central processing unit (CPU), which is a hardware component of a computer system and can perform basic arithmetic, logical or input/output operations, in order to process data from input devices into useful information. The CPU is the brain of a computer system and can process data very quickly, but it cannot think for itself (it can only process data by following instructions).

The CPU contains:
> control unit (CU)
> arithmetic logic unit (ALU)
> memory address register (MAR)
> memory data register (MDR)

The CU controls the retrieval of instructions from the primary memory as well as the sequence of their execution.
The ALU performs all the basic arithmetic, logical or input/output operations.
The MAR holds the memory address of the data to be used in ALU, so that ALU can fetch the corresponding content from the memory. The MAR may also hold the memory address of where data will need to be stored. In order for the MAR to communicate with the primary memory, a connection is necessary (Memory (Address) Bus).
The MDR holds the data that is to be used by the ALU and then saved to the RAM.

II. The RAM and ROM

The primary memory is the only storage that is directly accessible by the CPU. The primary memory consist of two types of memory:

Random Access Memory (RAM) is a general-purpose storage area, so the data stored can be over-written. However, RAM is volatile, which means that whenever power is lost the contents of its memory are wiped clean.

On the other hand, Read Only Memory (ROM) is used to store instructions and data that cannot be over-written, and it is considered non-volatile. ROM is much smaller than RAM.

III. The cache memory

RAM has two main types:
>Dynamic RAM (DRAM)
>Static RAM (SRAM)
SRAM is faster but more expensive than DRAM, and as such DRAM is preferred for the main RAM. However, a small amount of SRAM is placed between the main RAM and the processor and it is called cache. Cache holds the information from the RAM that is most actively used, and accessed most frequently. The computer system will run faster as the slower main memory will need to be accessed less frequently.
There are two types of cache. L1 cache is placed on the microprocessor itself whereas L2 cache is placed between the primary memory and the microprocessor.

IV. The machine instruction cycle

1. fetch instruction from primary memory to control unit
2. decode instruction in control unit
3. execute instruction
4. store result and check for next instruction