E820
Encyclopedia
e820 is shorthand to refer to the facility by which the BIOS
of x86-based computer systems reports the memory map
to the operating system
or boot loader.
It is accessed via the int
15h call
, by setting the AX register to value
It reports which memory address ranges are usable and which are reserved for use by the BIOS.
BIOS-e820 is often the first thing reported by a booting Linux kernel
, and it can also be seen with the dmesg
command.
Sometimes the BIOS is buggy and incorrectly reports the reserved memory.
This can cause memory testing software, like Memtest, to report errors.
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
of x86-based computer systems reports the memory map
Memory map
In computer science, a memory map is a structure of data that indicates how memory is laid out. Memory maps can have a different meaning in different parts of the operating system....
to the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
or boot loader.
It is accessed via the int
INT (x86 instruction)
INT is an assembly language instruction for x86 processors that generates a software interrupt. It takes the interrupt number formatted as a byte value.When written in assembly language, the instruction is written like this:...
15h call
BIOS interrupt call
BIOS interrupt calls are a facility that DOS programs and some other software, such as boot loaders, use to invoke the facilities of the Basic Input/Output System...
, by setting the AX register to value
E820 in hexadecimal.It reports which memory address ranges are usable and which are reserved for use by the BIOS.
BIOS-e820 is often the first thing reported by a booting Linux kernel
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
, and it can also be seen with the dmesg
Dmesg
dmesg is a command on most Linux and Unix based operating systems that prints the message buffer of the kernel.-Booting:...
command.
Sometimes the BIOS is buggy and incorrectly reports the reserved memory.
This can cause memory testing software, like Memtest, to report errors.