[24hoursupport] Re: Inside computers

  • From: "Ron Allen" <chizotz@xxxxxxxxxxx>
  • To: 24hoursupport@xxxxxxxxxxxxx
  • Date: Wed, 10 Apr 2002 02:25:03 -0500

Hello Allen,

>I have been messing with computers since TRS-40, TRS-80 and Coleco. 
>Being old but not too "smort", I have not been able to  follow the
>inside a computer.  Most explanations seem to start off with too much 
>detail.  Are there any "black box" explanations for, say, what happens in
>a boot operation, modems operations, midi etc.

The boot process is called that from the saying "pulling yourself up by
your bootstraps". Boots used to have straps that you pulled on to get them
on. The boot process is pretty complicated, so I'm probably going to miss
something along the way, but here goes.

When you first turn the power on, the computer first verifies that the
power is good, meaning that all of the voltages coming from the PC power
supply are within the operating range of the system. After that check
happens, the CPU begins processing. The first thing it does is to look at a
specific memory address, which is normally at the end of the ROM space.
Normally, this address contains a command to jump to another specific
memory location, which varies from system to system, which is the start of
the BIOS program. BIOS stands for "Basic Input/Output System", and is a
program that is hard wired into a special chip on your motherboard. This
checking a specific memory location, and then jumping to another memory
location based on the information stored there, is technically the entire
bootstrap operation. However there are more steps that the computer takes
that everyone normally considers part of the process as well. When the CPU
starts running the BIOS program, several things are done. The first thing
that usually happens is the POST, or "Power On Self Test". The POST
consists of a number of checks of core system components, including checks
of the CPU itself, the math co-processor (if present), and other major
chips. You usually will see a visual indicator on the screen when the BIOS
is testing the memory. The BIOS then queries the system to see if there are
any other device-specific BIOS chips, such as drive controller BIOS chips
and so on, in the loop. If so, it methodically runs through any startup
routines found in those chips. Next the BIOS looks through a specific
memory area for a video ROM. This memory range is set aside for this
purpose, and nearly all systems will have a video ROM in that address
range. If a video ROM is found, control is transferred to it so it can
initialize the video adapter. Up until this point, your screen has been
blank. If the video subsystem initializes correctly, you'll start to see
the boot screen at this point. This is where various information is
displayed, such as the maker and version of the BIOS and possibly a company
logo and so on. Control is then transferred back to the BIOS and the memory
test begins. You normally will see this memory check displayed as rolling
numbers on the screen. After the memory check is finished, the BIOS looks
for an operating system to load. There is a specific place on properly
formatted hard disks, called the MBR, or Master Boot Record. The BIOS looks
at the MBR to determine where to start loading the operating system from.
It then loads the operating system, and transfers control to it and the
boot process is complete.

Modem stands for "Modulator/Demodulator". The normal phone lines, also
called POTS for "plain old telephone service", and traditional telephone
equipment is analog in nature. Analog equipment operates on continuous
electronic signals. Analog signals change in voltage smoothly up and down,
and information is contained in the speed and frequency of the changes.
Computers are digital, which operates in discrete chunks of information,
either "on" or "off", with no changing middle ground. What a modem does is,
it takes a digital signal and converts it to analog and vice versa in order
to allow the digital device of your computer communicate with the analog
device of the POTS lines and equipment.

Midi I know little about, except that it stands for Musical Instrument
Digital Interface, and that is a standard that is designed to facilitate
recording of music played by live musicians on traditional instruments on
digital recording equipment on digital media.



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