What is processor,assembler and programming language

How does the processor (CPU) work?

You might know that the CPU (Central Processing Unit, or simply processor) is the “brain” of the computer, controlling all other parts of the computer and performing various calculations and operations with data. But how does it achieve that?

Processor is a circuit that is designed to perform single instructions: actually a whole series of them, one by one. The instructions to be executed are stored in some memory, in a PC, it’s the operating memory. Imagine the memory like a large grid of cells. Each cell can store a small number and each cell has its own unique number – address. The processor tells the memory address of a cell and the memory responds with the value (number, but it can represent anything – letters, graphics, sound… everything can be converted to numerical values) stored in the cell. Of course, the processor can tell the memory to store a new number in a given cell as well.

Instructions themselves are basically numbers too: each simple operation is assigned its own unique numeric code. The processor retrieves this number and decides what to do: for example, number 35 will cause the processor to copy data from one memory cell to another, number 48 can tell it to add two numbers together, and number 12 can tell it to perform a simple logical operation called OR.

Which operations are assigned to which numbers is decided by the engineers who design a given processor, or it’s better to say processor architecture: they decide what number codes will be assigned to various operations (and of course, they decide other aspects of the processor, but that’s not relevant now). This set of rules is then called the architecture. This way, manufactures can create various processors that support a given architecture: they can differ in speed, power consumption, and price, but they all understand the same codes as same instructions.

Once the processor completes the action determined by the code (the instruction), it simply requests the following one and repeats the whole process. Sometimes it can also decide to jump to different places in the memory, for example to some subroutine (function) or jump a few cells back to a previous instruction and execute the same sequence again – basically creating a loop. The sequence of numerical codes that form the program is called machine code.

What are instructions and how are they used?

As I already mentioned, instructions are very simple tasks that the processor can perform, each one having its unique code. The circuit that makes up the processor is designed in a way to perform the given operations according to the codes it loads from the memory. The numeric code is often called opcode.

The operations that the instructions perform are usually very simple. Only by writing a sequence of these simple operations, can you make the processor perform a specific task. However, writing a sequence of numeric codes is quite tedious (though that’s how programming was done long ago), so the assembly programming language was created. It assigns opcodes (the numeric code) a symbol – a name that sort of describes what it does.

Given the previous examples, where number 35 makes the processor move data from one memory cell to another, we can assign this instruction name MOV, which is a short for MOVe. Number 48, which is the instruction that adds two numbers together gets the name ADD, and 12, which performs the OR logical operation, gets the name ORL.

The programmer writes a sequence of instructions – simple operations that the processor can perform, using these names, which are much easier to read than just numeric codes. Then he executes a tool named assembler (but often the term “assembler” is used also for the programming language, though technically it means the tool), which will convert these symbols to the appropriate numeric codes that can be executed by the processor.

However, in most cases, the instruction itself isn’t sufficient. For example, if you want to add two numbers together, you obviously need to specify them, the same goes for logical operations, or moving data from a memory cell to another: you need to specify the address of the source and the target cell. This is done by adding the so-called operands to the instruction – simply one or more values (numbers) that will provide additional information for the instruction needed to perform a given operation. The operands are stored in the memory too, along with the instruction opcodes.

For example, if you want to move data from a location with address 1000 to a location 1258, you can write:

Hide   Copy Code

MOV 1258, 1000

The first number being the target address and the second the source (in assembly, you usually write the target first, and the source as the second one, it’s quite common). The assembler (the tool that converts the source to the machine code) stores these operands too, so when the processor first loads the instruction opcodes, which will tell it that it must move data from one location to another. Of course, it needs to know from which location to move to what destination, so it will load the operand values from the memory too (they can be at addresses right after the instruction opcodes), and once it has all the necessary data, it will perform the operation.