/joh'liks/ n.,adj. 386BSD
A UNIX legacy, the "u." or per-process data structure, which held the kernel-related data of a process, was present on 386BSD prior to February 1991.
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As in the user space (and in UNIX executable files), kernel instructions and data are arranged consecutively. The stack and a new special region, the per-process data structure or user structure (u. for short), appear below the kernel. One advantage of this arrangement is that it becomes possible to share all portions of the page tables for address space above the kernel base address. Notice that through this is a vital part of the kernel, it is technically at the very top of user address space and is purposely left readable by the user process. Everything beneath the system base address is switched when a context switch to the next process occurs. Currently, the kernel address space starts at virtual address 0xfe000000, and allows up to 32 Mbyte of address to be reserved for use within the kernel. This boundary can be moved at a later date if more address space is needed. Access of the ISA bus device memory (screen and LAN buffers) is obtained through an allocated region of the kernel memory, known as a utility page map. This is similar to portions of on-demand physical memory used by the kernel through other utility page maps. The kernel also has a variety of data structures scaled and allocated at boot time (valloc) and a heap for dynamic demands (malloc).
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