The full name of mobile phone bl lock is "BootLoader", which is a small program that runs before the operating system kernel runs. It can initialize hardware devices and establish a memory space map, thereby bringing the system's software and hardware environment to a Appropriate state so that the correct environment is prepared for the final call to the operating system kernel. After BootLoader completes the initialization of the CPU and related hardware, it loads the operating system image or solidified embedded application into the memory and then jumps to the space where the operating system is located to start the operating system.
#The operating environment of this tutorial: Android 13 system, Xiaomi 12 mobile phone.
The full name of bl lock is BootLoader, is a small program that runs before the operating system kernel runs.
In embedded operating systems, BootLoader runs before the operating system kernel runs. It can initialize hardware devices and establish memory space mapping diagrams to bring the system's software and hardware environment to a suitable state so as to prepare the correct environment for the final call to the operating system kernel.
In embedded systems, there is usually no firmware program like BIOS (note, some embedded CPUs will also embed a short startup program), so the loading and startup tasks of the entire system are completely determined by BootLoader to complete. In an embedded system based on ARM7TDMI core, the system usually starts execution from the address 0x00000000 when it is powered on or reset, and what is arranged at this address is usually the system's BootLoader program.
Before unlocking the BL lock, users cannot freely perform flash operations and ROOT operations. In order to ensure the security of the mobile phone, some mobile phone officials have set up a BL lock for the mobile phone. If the BL lock is not unlocked, the user cannot flash the phone or obtain ROOT permissions by himself.
Bootloader is the first piece of code executed by the embedded system after power-on. After it completes the initialization of the CPU and related hardware, it then transfers the operating system image or solidification The embedded application is loaded into the memory and then jumps to the space where the operating system is located to start the operating system.
For embedded systems, Bootloader is implemented based on a specific hardware platform. Therefore, it is almost impossible to build a universal Bootloader for all embedded systems, and different processor architectures have different Bootloaders. Bootloader not only depends on the CPU architecture, but also depends on the configuration of the embedded system board-level device. For two different embedded boards, even if they use the same processor, if you want the Bootloader program running on one board to run on the other board, you generally need to modify the Bootloader source program.
In turn, most bootloaders still have many commonalities, and some bootloaders can also support embedded systems of multiple architectures. For example, U-Boot supports architectures such as PowerPC, ARM, MIPS, and X86 at the same time, and supports hundreds of boards. Generally, they can automatically boot from storage media, they can boot the operating system, and most of them can support serial and Ethernet interfaces.
Operating modes
Most bootloaders contain two different operating modes:
(1) Boot loading mode
In this mode, the Bootloader loads the operating system from a solid-state storage device of the target machine into RAM and runs it without user intervention. This mode is the normal working mode of the Bootloader, so when the embedded product is released, the Bootloader must work in this mode.
(2) Download mode
In this mode, the Bootloader on the target machine will download the kernel from the development host (Host) through communication means such as the serial port or network. The image and root file system image wait until they are in RAM, and then can be written to the solid-state storage media on the target machine by the Bootloader, or the system can be booted directly.
Boot loading mode is usually used when burning the kernel and root file system to solid-state storage media for the first time or during future system updates; download mode is mostly used by developers during the early development process to work on The bootloader in this mode usually provides a simple command line interface to its end users.
Bootloader transplantation
Bootloader is widely used in embedded devices such as handheld terminal devices, smart home appliances and set-top boxes with operating systems. It is responsible for Complete hardware initialization, operating system boot and system configuration, etc., which is equivalent to the BIOS on a PC. For an embedded Linux system, the Bootloader is the basis for the operation of the entire system. But the Bootloader used will be different for different ARM platforms. Completing the bootloader transplant is a crucial step to build and run the system on a specific hardware platform.
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