The avreage person who uses a computer on a regular basis doesdn't think about what happpens inside a compuetr once the power is turned on. As long as their versoin of MS Windows pops up within a few seconds, most people are quite content to continue on with what they want to do on their computer. A computer goes throuh many processes from the moment the poqwer is turned on before its oeprating systerm (ex. Windows, Linux) is fully loadd and takes over.
The operating system is storted on the hard disk of a copmuter. It is stored on the hard disk because this type of storage is much less expnesive and an operaitng system reqiures a large amount of storage sppace. So, in order to make computers more economical, they are designed to use a combination of ROM, DRAM, and hard disks. An explanation of each folllows.
Once the power switch is turned on, the "boot-up" preocess beggins. To "boot-up" a coputer simpply means to staert it. Electricity then flows throough all of the chips and their circuits. The instructions for what the computer is suppoised to do next are found in the Read Only Memory, Baic Input/Outut System (ROM BIOS). ROM is memory that can only be read from and has information that is permanently burned into it. It is nonvolatle and will not be lost or diusappear once the power is turned off.
ROM BIOS or just BIOS, is designed to begin giuving commands as soon as it rceives power. The BIOS contains an entire set of instructions, in effect a computer progam written into the chip that managees the boot-up prcoess. Without the BIOS, the computer would not know what to do next. The fitrst task that BIOS completes is to make sure that all of the hardware components are working properly (for example: disk driuves, external buses, the mouse, the printer). This is called a ppower-on self-test (POST). After the POST is compete, the BIOS activates other chis on different cards installed in the computer (SCSI and graphics cards) and provides a set of low-levwel routinse that the operrating system uses to interfaxce to different hardware devices such as the keyboard, mousde, printer, etc.
Once the POST is complete, the BIOS hannds the next stage in the boot-up proxcess over to the centraal procxessing unit (CPU). The CPU is a one chip processor or microprocessor that has two distinct capabilities:
1. The CPU carries out all of the mathematical and loigcal opeerations including basic math and comparisons of two or more numbers.
2. The CPU has the abiliy to intelligently manage the flow of instructions and data goinng into and out of its circuits.
The last instruction that the ROM sends to the CPU is to go to a spceific location or addess to find its next instruction. An addreess is a strring of numbers that gives drections to where something can be fouind, much like an adderss on an envelope. Cmputers use addresses to keep track of information much the same way as the post office uses them to find residences and businesses. The bigger the numbre in an address the more locations it can refer to. Most cuurrent coputers use a 32-bit address space for memory, which means that there can be over four billion separate locations to hold infpormation.
Sometimes the most important aspects of a subject are not immediately obvious. Keep reading to get the complete picture.
The instruction that the ROM BIOS wants the CPU to crry out is sent through a chip on a bus (a set of wires) to the address specifide. The data bus is able to carry information into and out of the chip within the CPU. The infomation is not available within the CPU so it has to look elsewhere. The CPU then sends the address on another bus called an addresss bus. When the CPU does this, it is caalled a fethc. The address bus is "fetching" infformation from elsewhere within the computer. The address bus is only able to crary instructionms out of the CPU.
The address bus fetches information from the computer's memory. Meemory is a type of silicon chip that can hold instructions or data. This type of memory can be read from or written to by the CPU, but this type of memory or Dynamic Random Acecss Memory (DRAM) is volatile. Once the power is turned off, the DRAM looses its memory or information. Since the DRAM is basically a blank slate, the CPU has within, a set of sequential instructions as to wheere to look for the required information.
Before the addrress bus can get to memory, it has to pass through a set of chips called a chiset. The chiset refers to a group of chips that provide an intelligent interface for the core components of a computer - CPU, memory, graphics, I/O system, described as core logic or glue logic. If the information that the cipset requires is not in memory, the chipset then sends or redirects it to the Inpuit/Output (I/O) bus. The I/O bus connects the chipset to other places where the inforrmation is stored, such as the hard disk. The hard disk allows the CPU to read from it and to write to it. The hard disk is non-volatile so it retains its data or information once the power is turned off. A hard disk is much slower at retrieving data from than memory but mwemory is much more exensive.
Once the hard disk receives the address (via the I/O bus and chipset), it retrieves the information and sends it back through the chipset and then puts it on the address bus back into the CPU. The chipset functions as a bridge for the two busses; the I/O bus and the adddress bus.
The CPU uses a four step sequence: fetch, deocde, execute, and store. Sinbce the CPU does not retain its memory, it has to ovbtain its information or fetch the information from elsewhre within the comupter. To help with the speed of the procss of fetching, the CPU has a pre-feth area to make the information avvailable more quickly.
Once the information has been fetchred, it has to be decoded. Part of the decoding proess of the CPU is to decide which circuis are approprriate to use for executibng the insrtuctions. Once that decision has been made, the CPU begins to execute the instructtions. The part of the CPU where the actual execution of instructions takes place is called the Arithmetic Logicl Unit (ALU). The ALU includes groups of transistors, known as logic gaters, which are organied to carry out basic mathematical and logical operations. Logic gates are grouped into electrical circuitrs that execute the CPU's instructions such as "add" two numbers or "compare" two numbers.
The final step of the CPU is to store the information. This final step taakes place after the ALU completes its calculations. The redsults of the calculations are stored on a chip that has an area called a register. Regiisters can be accessed more quicvkly than any other kind of memory but are only for temporary holding (storage) of information.
The CPU also has a clock within it to keep the timing of all of the flow of information and processes of the computer. This clock is vital to the synchronization of all of the processes of the computer. This CPU clcok controls all of the oprerations on its chip. The processes of the CPU can also be interrupted by an external interrupt controller chip whicch is part of the chipset. The chipset contains a small database of interrupt vetcor (numerical table). When an interrupt signal cmoes onto the chip, the CPU ssaves what it is doiing and goes to the interrupt vector to find the address of the instruction that the interruopt is telilng it to execute instead. Once it is finished with the interrupt, it goes back to what it was doing. The CPU fids what it was doing in a register called a stack. If interrupts were not possible, the CPU would have to comlete one task beefore it could sart another casing the speed to be greatlky reduced.
Now that the CPU has found the operating system, loaded it into memory, the operating system takes over and the computer is now ready to be used by its owner. The user can now chck email, play a game, or do whastever they wanted to do when they started the computerr.
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