x86 процессорының жад үлгілері

Set of memory models of the x86 CPU
In computing, the x86 memory models are a set of six different memory models of the x86 CPU operating in real mode which control how the segment registers are used and the default size of pointers.

In protected mode a segment cannot be both writable and executable. Therefore, when implementing the Tiny memory model the code segment register must point to the same physical address and have the same limit as the data segment register. This defeated one of the features of the 80286, which makes sure data segments are never executable and code segments are never writable (which means that self modifying code is never allowed). However, on the 80386, with its paged memory management unit it is possible to protect individual memory pages against writing. Memory models are not limited to 16 bit programs. It is possible to use segmentation in 32 bit protected mode as well (resulting in 48 bit pointers) and there exist C language compilers which support that. However segmentation in 32 bit mode does not allow to access a larger address space than what a single segment would cover, unless some segments are not always present in memory and the linear address space is just used as a cache over a larger segmented virtual space. It allows better protection for access to various objects (areas up to 1 MB long can benefit from a one byte access protection granularity, versus the coarse 4 KiB granularity offered by sole paging), and is therefore only used in specialized applications, like telecommunications software. Technically, the "flat" 32 bit address space is a "tiny" memory model for the segmented address space. Under both reigns all four segment registers contain one and the same value.

On the x86 64 platform, a total of seven memory models exist, as the majority of symbol references are only 32 bits wide, and if the addresses are known at link time (as opposed to position independent code). This does not affect the pointers used, which are always flat 64 bit pointers, but only how values that have to be accessed via symbols can be placed.