Principles and importance of BIOS self-check

Http://blog.csdn.net/dog250/article/details/6246594

After the motherboard is powered on, the entire motherboard and the chip/Bus Controller on it constitute a map, which indicates where a certain device is placed, and then BIOS self-check begins. It should be noted that everything is difficult at the beginning. How does the BIOS start to run? Its command Code How do I start execution? This does not need to be concerned. It is standard-enforced. For example, after the BIOS occupies the range from address a to address B on the address bus, and the motherboard map is established (with stable voltage, the CPU starts to execute from a specific location in address A to B. This process is very simple. The CPU will take the command from C and send the address c to the address bus, the North Bridge routes the command to the BIOS chip according to the standard, and then the BIOS chip sends the command back to the CPU through the data bus, and the CPU executes the command, so that the BIOS command is executed. The BIOS is basically self-check, next, jump to another place. As for where to jump, the CPU and motherboard designers don't care. It is decided by the BIOS. At this time, the entire board has started to work. You want to jump to the Code in the disk partition, first, you need to load the code in the disk into the memory. This is another process of sending the address to the address bus and getting the command to execute.
So what is the principle of BIOS self-check? After the motherboard is powered on and the voltage is stable, the voltage of all the chips on the motherboard will be stable, and each chip will be in the reset state, that is to say, these chips follow their respective standards, the circuit is in a stable state, and each chip's stable circuit has its own standards. For example, the stability standard of chip X is to emit a high level for its 1st and 3rd pins, if a high level can be obtained from its 5th pins, it indicates that the chip is normal, and then a 10-bit level sequence will be obtained from its 10th to 20 pins, for example: high-high-low, binary is 1101010010, And the binary number is related to the chip X, for example, to detect the ram capacity, it may be to send a level signal sequence to some pins of each ram storage chip, and then read the capacity of the chip from other pins, in this way, the total size of Ram will be displayed on the screen, just as we can see that the ram capacity will change from 0 to the maximum value when the computer is turned on. BiOS detects the features of all devices in a similar way. So how does the BIOS find a specific device? PreviousArticle As mentioned above, either a topology graph is saved in the BIOS or the topology graph is initialized in the first step at the start of running, almost sends level series signals to specific chip pins according to the standard. Note that it is still based on the standard ), the topology graph records the address information of each chip or device bus on the motherboard and other metadata, the chip, device, and bus on the motherboard map have their own addresses. The BIOS is the main manager. If the motherboard is regarded as a city, the BIOS is the city supervisor, record the address information of each household, and send a door-to-door greeting to everyone. Now everything is safe! This is the self-check process.
Note: Generally, the address information of the motherboard chip or device bus is hard stored in the BIOS chip, but most of the chip features are not stored in the BIOS chip, instead, it dynamically reads data from the corresponding address during self-check. The reading method is nothing more than getting the device standard based on the address information, and then sending some level sequences to some pins of the device according to the standard, then obtain the device feature information, which is stored in a small chip of the chip or device memory, or is called a device controller or device BIOS.
Linuxbios, now coreboot, implements an open-source bios and uses a large number of Linux kernel source code. It is equivalent to a highly streamlined Linux operating system, in addition to self-check, it can do a lot more, because control is always in the hands of linuxbios before it is handed over to other code, and it is based on Linux, so it has more functions. We usually do not have much access to bios, but we can control the entire computer. For example, if the operating system directly requires bios, if the BIOS tells the operating system that there is only 30 mb of memory, the operating system can only use 30 M. If the BIOS tells the operating system that there is 4g RAM and g RAM is retained for the device, you have no choice but to protest. Nowadays, the so-called IT practitioners focus on top-layer applications, while those who focus on bottom-layer mechanisms can all be regarded as mechanic, and there will be fewer viruses like CIH. However, once this happens again, I think few people can cope with it.
We have mentioned the address topology in the BIOS. This figure can be stored in the CMOS chip or dynamically detected. This also constructs two BIOS self-check methods, the first is that the address topology is stored in CMOS. the BIOS checks the device based on the address and attribute information in the topology. For example, the following entry is displayed in CMOS: if the PCI device is located at address a, the BIOS sends a level signal at address a according to the standard image of the PCI device, and then reads the reply. The second method is the detection method. The BIOS starts from address 0, the maximum address is always detected. Although the BIOS does not know what the device is, each device must provide a detection interface. For example, this interface is written to 10110, then, the BIOS writes 10110 to each address, and the response is the device type. Then, the self-check is performed based on the first method. You may be familiar with linuxbios. BIOS self-check is completely based on the motherboard design. Therefore, BIOS development technology has been controlled by the motherboard manufacturer. One Board corresponds to one BIOS, this is the truth! The BIOS still holds everything. There is an interface between the motherboard and the application software, that is, the BIOS.