Whatever else the boot process involves, what is most important is what you see it do, especially on the screen. I will limit details to what to expect physically.

The boot process is performed by the mother board's BIOS chips, a set of one or two chips, somewhat on the large size, perhaps with 28 pins each. These are almost always identified with a manufacturer's name, like "American Megatrends" or "Phoenix."

Different users have distinct preferences for one name brand over another, especially since early boards (through the late 80's) often included wacky BIOS chips. (My BIOS gets the date wrong since the beginning of 2000.)

The BIOS chip is replaceable (although I have never heard of anyone ever getting a replacement), because -- supposedly -- a BIOS manufacturer can't guess what type of processor you might install in the future.

The whole purpose of the boot-up process, called the "Power On System Test" (POST), is to ascertain what is attached to the mother board, that is, how much memory, if there is a video card, a keyboard, what the hard drive and floppy drive specifications are. After all, while the computer was asleep, the user could have made changes.

The POST program is thus hardwired into the BIOS chips, and only starts up (that is, it is run by the CPU) when the CPU senses that power has been turned on.

In addition, the BIOS POST process tests everything, although you can set things up to not test certain components, or set instructions not to warn you. More on that below.

One of the first things the BIOS does during a boot-up is to check for the existence of a video card, and start it up. Without a working video, the BIOS cannot tell you much of anything.

What the BIOS's POST program cannot do is to guess what file system or OS may be on board. Thus the POST stops once it has found the boot sector of the boot device.

OK, follow this sequence...

• On the "power up" condition, the CPU clears its registers and loads the addresss F0000 (hex) into the "next instruction pointer" register.

• This happens to be the address of the system BIOS ROM chip. The BIOS ROM chip is just a program which stores many different instructions which can be executed by the CPU.

• Although "F0000" is a memory address in RAM, the CPU knows enough to look toward the "other end" of the system bus, that is, among the motherboard chips and extension slots, rather than toward the end which connects to RAM. In fact the system at this point doesn't even know if it has any RAM.

• The CPU thus starts executing the instructions in the BIOS chip ROM, one after another. The BIOS will perform the "POST" portion of its program on power-up condition (that is, the first time it is run).

• The POST program will perform an inventory of attached equipment, check it against the list held by the CMOS chip, and eventually turn things over to a boot-sector program stored at a boot-device.

• The first thing the POST program does is to check for the existance of other ROM chips which might be held on various expansion cards (in the slots of the expansion card bus).

  • These will have addresses in the C0000 to EFFFF range. Hopefully two cards will not have the same address. Only certain intervals are checked, and the actual check is to look for two consecutive hex bytes which read AA,55 (hex).

  • When found, the program of the ROM at that location will actually be executed by the CPU, that is, the BIOS's POST program will be put on hold, and the CPU will execute the program of the ROM.

  • The video card's ROM will always be located at C0000, so it will be the first to "run." When first run, the video ROM (video BIOS) will initialize the monitor, usually with a little song and dance on the display. Good thing, too, for it gives you something to look at.

  • When the video BIOS is done, the system BIOS regains controll of the CPU, and continues to find other ROM chips at higher addresses (for example, the ROM for the HDC, the Ethernet card, the SCCI controller card) and will have the CPU execute these.

  • Many peripheral devices, however, have their own "stored programs" on board, which will do local testing of things as soon as the power is on. That explains why you will hear the hard drive do a self-test, as well as ZIP drives and backup tapes.

• Next comes the more familiar display of things: a count (and check) of RAM is made, the keyboard is tested, the drives are located (except certain streamer Tape drives, ZIP drives, and CDROM drives which don't get initialized till later), secondary video cards are identified, Com ports are counted (and initialized by most POST programs, but not all), parallel ports are found, what kind of CPU is on board, how much CPU cache should be counted on, what ROMs can be translated to RAM memory, etc. Even a check is made of the battery attached to the CMOS chip.

• All the while this information is being checked against the list of things held at the CMOS chip. If the actual list of things does not match the remembered inventory, the BIOS will complain and ask you to rewrite the information at the CMOS chip.

• At this point the system will know what (regular) drives are attached, and what their specifications (heads, cylinders, sectors) are. But it will not know anything about the organization of the file system on these drives, or what OS will be in use.

• The POST process at this point gives up, but first it tells the CPU to take as the next instruction the information written in the boot-sector of the first boot device (floppy or hard drive) that the POST program has found.

• The first instruction of the first sector of the boot device is usually a "jump" to a location (on the drive) for further instructions. For a DOS system these are the instructions to load into memory the files IO.SYS, MSDOS.SYS, and COMMAND.COM, and execute them.

• IO.SY and MSDOS.SYS are boot files which remain in memory. IO.SYS will set up (for DOS) the methods of reading and writing data on the drives, and how to make sense of the DOS record keeping of the file system. MSDOS.SYS will determine how to install device drivers (the stuff in the config.sys file), and knows about additional tasks specific to the DOS Operating System.

• Once command.com has been executed, it remains in memory to handle all other requests from the keyboard. It will, in fact just sit there and wait forever for you to type something.

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