High memory
Encyclopedia
High Memory is the part of physical memory
in a computer
which is not directly mapped by the page tables of its operating system
kernel
. The phrase is also sometimes used as shorthand for the High Memory Area, which is a different concept entirely.
Some operating system kernels, such as Linux
, divide their virtual address space
into two regions, devoting the larger to user space
and the smaller to the kernel. In current 32-bit
x86 computers, this commonly (although does not have to, as this is a configurable option) takes the form of a 3GB/1GB split of the 4GB address space, so kernel virtual addresses start at 0xC0000000 and go to 0xFFFFFFFF. Some sources also refer to 896 MB of Normal Memory and the rest being high memory. In this case the kernel virtual addresses start at 0x07000000 and go to 0xFFFFFFFF. When in user mode, translations are only effective for the first region, thus protecting the kernel from user programs
, but when in kernel mode, translations are effective for both regions, thus giving the kernel an easy way to refer to the buffers of processes—it just uses the process' own mappings.
However, if the kernel needs to refer to physical memory for which a userspace translation has not already been provided, it has only 1GB (for example) of virtual memory to use. On computers with a lot of physical memory, this can mean that there exists memory that the kernel cannot refer to directly—this is called high memory. When the kernel wishes to address high memory, it creates a mapping on the fly and destroys the mapping when done, which incurs a performance penalty.
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
in a computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
which is not directly mapped by the page tables of its 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...
kernel
Kernel (computing)
In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...
. The phrase is also sometimes used as shorthand for the High Memory Area, which is a different concept entirely.
Some operating system kernels, such as Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, divide their virtual address space
Virtual address space
Virtual address space is a memory mapping mechanism available in modern operating systems such as OpenVMS, UNIX, Linux, and Windows NT...
into two regions, devoting the larger to user space
User space
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
and the smaller to the kernel. In current 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
x86 computers, this commonly (although does not have to, as this is a configurable option) takes the form of a 3GB/1GB split of the 4GB address space, so kernel virtual addresses start at 0xC0000000 and go to 0xFFFFFFFF. Some sources also refer to 896 MB of Normal Memory and the rest being high memory. In this case the kernel virtual addresses start at 0x07000000 and go to 0xFFFFFFFF. When in user mode, translations are only effective for the first region, thus protecting the kernel from user programs
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
, but when in kernel mode, translations are effective for both regions, thus giving the kernel an easy way to refer to the buffers of processes—it just uses the process' own mappings.
However, if the kernel needs to refer to physical memory for which a userspace translation has not already been provided, it has only 1GB (for example) of virtual memory to use. On computers with a lot of physical memory, this can mean that there exists memory that the kernel cannot refer to directly—this is called high memory. When the kernel wishes to address high memory, it creates a mapping on the fly and destroys the mapping when done, which incurs a performance penalty.