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What is the sequence for POST

Posted on 2004-09-02
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Last Modified: 2011-10-03
Hello,

I was hoping someone could provide a general sequence to POST.  For example I am trying to figure out the steps that occur.  

1.  Power is turned on at the power supply and then a power on signal is sent to the MOBO.  

2.  This activates the CPU wich then activates the ROM BIOS etc.

Can you help out fill in the details and be as verbose as possible.

thanks

jon
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Question by:stewartje
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kiranghag earned 250 total points
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by:woodendude
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Homework?
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by:_
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: D
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by:Wakeup
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could be
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by:yavooza
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System Boot Sequence

The system BIOS is what starts the computer running when you turn it on. The following are the steps that a typical boot sequence involves. Of course this will vary by the manufacturer of your hardware, BIOS, etc., and especially by what peripherals you have in the PC. Here is what generally happens when you turn on your system power:

   1. The internal power supply turns on and initializes. The power supply takes some time until it can generate reliable power for the rest of the computer, and having it turn on prematurely could potentially lead to damage. Therefore, the chipset will generate a reset signal to the processor (the same as if you held the reset button down for a while on your case) until it receives the Power Good signal from the power supply.
   2. When the reset button is released, the processor will be ready to start executing. When the processor first starts up, it is suffering from amnesia; there is nothing at all in the memory to execute. Of course processor makers know this will happen, so they pre-program the processor to always look at the same place in the system BIOS ROM for the start of the BIOS boot program. This is normally location FFFF0h, right at the end of the system memory. They put it there so that the size of the ROM can be changed without creating compatibility problems. Since there are only 16 bytes left from there to the end of conventional memory, this location just contains a "jump" instruction telling the processor where to go to find the real BIOS startup program.
   3. The BIOS performs the power-on self test (POST). If there are any fatal errors, the boot process stops. POST beep codes can be found in this area of the Troubleshooting Expert.
   4. The BIOS looks for the video card. In particular, it looks for the video card's built in BIOS program and runs it. This BIOS is normally found at location C000h in memory. The system BIOS executes the video card BIOS, which initializes the video card. Most modern cards will display information on the screen about the video card. (This is why on a modern PC you usually see something on the screen about the video card before you see the messages from the system BIOS itself).
   5. The BIOS then looks for other devices' ROMs to see if any of them have BIOSes. Normally, the IDE/ATA hard disk BIOS will be found at C8000h and executed. If any other device BIOSes are found, they are executed as well.
   6. The BIOS displays its startup screen.
   7. The BIOS does more tests on the system, including the memory count-up test which you see on the screen. The BIOS will generally display a text error message on the screen if it encounters an error at this point; these error messages and their explanations can be found in this part of the Troubleshooting Expert.
   8. The BIOS performs a "system inventory" of sorts, doing more tests to determine what sort of hardware is in the system. Modern BIOSes have many automatic settings and will determine memory timing (for example) based on what kind of memory it finds. Many BIOSes can also dynamically set hard drive parameters and access modes, and will determine these at roughly this time. Some will display a message on the screen for each drive they detect and configure this way. The BIOS will also now search for and label logical devices (COM and LPT ports).
   9. If the BIOS supports the Plug and Play standard, it will detect and configure Plug and Play devices at this time and display a message on the screen for each one it finds. See here for more details on how PnP detects devices and assigns resources.
  10. The BIOS will display a summary screen about your system's configuration. Checking this page of data can be helpful in diagnosing setup problems, although it can be hard to see because sometimes it flashes on the screen very quickly before scrolling off the top.
  11. The BIOS begins the search for a drive to boot from. Most modern BIOSes contain a setting that controls if the system should first try to boot from the floppy disk (A:) or first try the hard disk (C:). Some BIOSes will even let you boot from your CD-ROM drive or other devices, depending on the boot sequence BIOS setting.
  12. Having identified its target boot drive, the BIOS looks for boot information to start the operating system boot process. If it is searching a hard disk, it looks for a master boot record at cylinder 0, head 0, sector 1 (the first sector on the disk); if it is searching a floppy disk, it looks at the same address on the floppy disk for a volume boot sector.
  13. If it finds what it is looking for, the BIOS starts the process of booting the operating system, using the information in the boot sector. At this point, the code in the boot sector takes over from the BIOS. The DOS boot process is described in detail here. If the first device that the system tries (floppy, hard disk, etc.) is not found, the BIOS will then try the next device in the boot sequence, and continue until it finds a bootable device.
  14. If no boot device at all can be found, the system will normally display an error message and then freeze up the system. What the error message is depends entirely on the BIOS, and can be anything from the rather clear "No boot device available" to the very cryptic "NO ROM BASIC - SYSTEM HALTED". This will also happen if you have a bootable hard disk partition but forget to set it active.

This process is called a "cold boot" (since the machine was off, or cold, when it started). A "warm boot" is the same thing except it occurs when the machine is rebooted using {Ctrl}+{Alt}+{Delete} or similar. In this case the POST is skipped and the boot process continues roughly at step 8 above.


BIOS Power-On Self Test (POST)

The first thing that the BIOS does when it boots the PC is to perform what is called the Power-On Self-Test, or POST for short. The POST is a built-in diagnostic program that checks your hardware to ensure that everything is present and functioning properly, before the BIOS begins the actual boot. It later continues with additional tests (such as the memory test that you see printed on the screen) as the boot process is proceeding.

The POST runs very quickly, and you will normally not even noticed that it is happening--unless it finds a problem (amazing how many things are like that, isn't it?) You may have encountered a PC that, when turned on, made beeping sounds and then stopped without booting up. That is the POST telling you something is wrong with the machine. The speaker is used because this test happens so early on, that the video isn't even activated yet! These beep patterns can be used to diagnose many hardware problems with your PC. The exact patterns depend on the maker of the BIOS; the most common are Award and AMI BIOSes. This part of the Troubleshooting Expert will help you figure out what the POST beep codes mean and what to do about them, if you are having this problem.

Note: Some POST errors are considered "fatal" while others are not. A fatal error means that it will halt the boot process immediately (an example would be if no system memory at all is found). In fact, most POST boot errors are fatal, since the POST is testing vital system components.

Many people don't realize that the POST also uses extended troubleshooting codes that you can use to get much more detail on what problem a troublesome PC is having. You can purchase a special debugging card that goes into an ISA slot and accepts the debugging codes that the BIOS sends to a special I/O address, usually 80h. The card displays these codes and this lets you see where the POST stops, if it finds a problem. These cards are obviously only for the serious PC repairperson or someone who does a lot of work on systems.
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by:slink9
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Would someone do my homework for me, please?  I just need someone to type my thoughts since my fingers don't seem to want to work properly today.

Even after this thing is tagged as probably homework an exact abswer is given.  I hope this person gives you an A after receiving it from the teacher!!!
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by:crazijoe
ID: 11972046
This definitly sounds like homework.
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by:DoTheDEW335
ID: 11972469
Don't be a hater. I bet you guys are jellous you couldn't do it back when you were in school! (I'm just jokin) It probably is homework and if so you should be ashamed
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by:buckeyes33
ID: 11972629
Slink, what is your question?   ;)
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by:slink9
ID: 11974050
I have no question.  I did my homework when I was in school.  If you want help on Outlook homework, you could buy my book.  It is finally available!!!!

Actually I guess my request was actually for the route between my brain and fingers to improve for the rest of the day (and the rest of my life) so I can quit using the backspace key so much.
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by:buckeyes33
ID: 11974091
>>It is finally available!!!!
that cool where is it?

lol to the other comment.
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by:buckeyes33
ID: 11974117
would you reccommend your book to an older, computer iliterate person?  I hate having to explain everything to my Grandma.
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by:slink9
ID: 11974265
It is aimed more at the intermediate user, but at $19.95 or less it is still a good investment.  It is on Outlook, though, not OE.

It is listed as currently available at www.wal-mart.com, www.bn.com, and www.bamm.com and will probably be available at the other places listed in my profile in the next couple of days.
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by:buckeyes33
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>>It is on Outlook, though, not OE.
 that stinks
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by:stewartje
ID: 12006947
Skeptics.

 It is not homework at all.  Seeing that I am a paying customer to this site I thought I would ask the experts to assist me in deciphering the actual steps taken in a POST.

As for Yavooza's comments, I too can cut and paste this information from about three different websites.  No disrespect intended but I was looking for more.  

Jon
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by:crazijoe
ID: 12007092
I'm kinda confused. You asked to "provide a general sequence to POST" and Yavooza has provided that. You also state "I too can cut and paste this information from about three different websites.""but I was looking for more. " more being having somebody write a thesus for you? All the information you should need is in the post that Yavooza has posted. What is the "more" you are looking for?
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by:kiranghag
ID: 12007215
if he has taken look at the links provided above, he could have got "more" than enough he wanted to know.
no hard feelings but i would like him to comment on the information in the links provided..
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by:stewartje
ID: 12008555

Crazijoe,

The information provided by Yavooza was taken from another website.  There are many out there that have the exact same information.  Your quips and comments are not appreciated.  Please do not post unless you have some constructive information pertaining the question.  

Kiranghag

I will look again at your links.  The last link in your list is a dead link.  


Please keep all sarcastic comments out of this thread.  This is not the forum for this and is quite rude.  I was hoping to gather information for my own use about POST procedures and use them on my equipment.  Your replies about homework and thesis are all inaccurate.

Thanks,
jon
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by:kiranghag
ID: 12012474
last link is not a dead link. if you are using Internet exploder, it might appear that the system/browser has hung for a while.
this is normal with PDF files being opened directly into IE.
just right click the link and select "ave target as", then save the file somewhere and open it using acrobat reader...

the file is related to award bios and gives
possible Award BIOS Error Messages (ISA and EISA BIOS)
and Award BIOS POST Codes
from the post codes, you can see what steps are performed by the bios in much finer details.
the post codes AFIK are output on the data bus during the system is initialised. these can be monitored via special pc troubleshooting expansion cards.

the bios code outputs the code after each corresponding stage is completed.
whenever a motherboard hangs with/without display, the special cards can check the last out put code and determine which part of the motherboard is faulty.

the modern motherboards combine all the minor chips into one or two chipsets, while very older designs had all the ICs separate on the motherboard...their the post codes were useful to directly isolate and replace the chip.
now we have to directly replace the chipsets..

hope this helps..

Kir@n
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by:crazijoe
ID: 12015370
Ummm.
 I guess you still haven't answered my question. What is the "more" you are looking for?
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by:slink9
ID: 12015437
I still can't see a true purpose for "more" or what has been given other than homework.  Please, put my mind at ease and explain what your purpose is in knowing such detail about a POST sequence.
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by:DoTheDEW335
ID: 12016068
You click POST in the left side of this page it has all the information you would need about the sequence of the POST process.
http://www.it.rit.edu/~netsyslb/bootprocess/boot1.htm

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by:stewartje
ID: 12017824
slnik9,

your comments lend one to think you are a bitter person and maybe this is not the right place for you.  


thanks kiranghag, I gave the links a more thorough second look and aggree that they are helpful.  

jon
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by:slink9
ID: 12018054
As I tell my kids - "Is that a 'fill in the blank' answer?"

I am certainly not bitter.  I subscribe to the belief that when someone will not explain their motives for something it is generally of a questionable nature.  I read your reluctance to explain one way.  Others may read it differently.  Because of the way this has developed, I stand by my statement "I did my homework when I was in school" and will not do yours for you.

By the way, you don't know me and I don't know you.  I can tell you I have a PhD in astrophysics and I can't prove it without a face-to-face.  I can, however, prove that I am the author of LINK EM UP ON OUTLOOK and you can get a bio on me from that.  There are others in the same situation here who have a proven (in print) background and bio, but most can say whatever they want and it is up to the listener to believe or not.

Why did I put that in here?  I guess to reiterate the well known anonymity of the web and it's groups.
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by:DoTheDEW335
ID: 12018437
stewartje,

did you have a chance to look over my link?
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by:konek2r
konek2r earned 250 total points
ID: 12033125
***COMPLETE POST PROCESS***
HOPE THIS WILL HELP

1. The user turns on the system by pressing the power button, which is part of the control panel.  
2. The control panel sends a signal through the system board to the processor.
 
3. The processor interprets the signal and, in turn, signals the soft-switched power supply to power up the system.
 
4. The power supply sends electrical current to all of its power leads.  At this point, any device receiving power could have an electrical problem that would stop the P.O.S.T. process.  

 
5. The BIOS initializes the core chipsets (PIIX, 810, 815, 820…) on the system board.
6. The BIOS checks and configures the memory.  Newer systems do not display the memory check. Systems that support the "instant-on" specification may bypass this step altogether.  
 
7. The BIOS enables L1 and L2 cache and checks the processor.
8. The BIOS copies itself into memory and passes control to the copy in memory.
9. The BIOS checks and configures the memory.  The LEDs flash when the BIOS tests the keyboard a little later in the process.  
 
10. The BIOS tests and initializes Video.  This is where you see the splash screen.  
 
11. The BIOS verifies resource assignments in ESCD* and reallocates resources if new hardware is found.  This step may be bypassed if PnP OS is enabled. Setting this option causes resource assignments to be handled by the operating system.  
 
12. Any ROMs (such as a NIC or SCSI controller) initialize.
 
13. Dell Power Management starts up.
 
14. The BIOS checks for a system password.
 
15. Control passes to the operating system, if present.
 
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