highmem

HighMem refers to a memory management technique used in some operating systems, particularly older versions of Linux, to overcome limitations in how the system could access physical RAM. The core issue HighMem addresses is the "address space gap" that exists in 32-bit processors. A 32-bit processor can only directly address a maximum of 4 gigabytes (GB) of memory. Within this 4GB address space, a portion is reserved for the operating system kernel, leaving less than 4GB available for user applications. For systems with more than 4GB of physical RAM, a 32-bit OS without HighMem would be unable to utilize the excess memory effectively.

HighMem allows the operating system to map portions of physical RAM into the kernel's address space on demand. Instead of having all physical memory permanently mapped, parts of it are temporarily made accessible to the kernel as needed. This enables 32-bit systems to utilize more than 4GB of RAM, although access to this "high memory" is typically slower than accessing the memory directly mapped within the kernel's primary address space. The overhead associated with the mapping and unmapping of HighMem pages contributes to this performance difference. Modern 64-bit operating systems generally do not require HighMem due to their ability to address significantly larger amounts of memory, rendering the 4GB limitation obsolete.