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However, on the 80386, with its paged memory management unit it is possible to protect individual memory pages against writing. [4] [5] 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. [6]
For example, in an Intel 8086, as well as in later processors running 16-bit code, a far pointer has two parts: a 16-bit segment value, and a 16-bit offset value. A linear address is obtained by shifting the binary segment value four times to the left, and then adding the offset value. Hence the effective address is 21 bits [Note 1]. There can ...
The memory model concept derives from the setup of the segment registers. For example, in the tiny model CS=DS=SS, that is the program's code, data, and stack are all contained within a single 64 KB segment. In the small memory model DS=SS, so both data and stack reside in the same segment; CS points to a different code segment of up to 64 KB.
In computer architecture, 16-bit integers, memory addresses, or other data units are those that are 16 bits (2 octets) wide.Also, 16-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers, address buses, or data buses of that size. 16-bit microcomputers are microcomputers that use 16-bit microprocessors.
The advantage over 8-bit or 16-bit integers is that the increased dynamic range allows for more detail to be preserved in highlights and shadows for images, and avoids gamma correction. The advantage over 32-bit single-precision floating point is that it requires half the storage and bandwidth (at the expense of precision and range). [5]
The Intel 80286 and later processors add "286 protected mode", which retains 16-bit addressing, and adds segmentation (without paging) and per-segment memory protection. For backward compatibility, all x86 CPUs start up in "real mode", with the same fixed overlapping 64 KiB segments, no memory protection, only 1 MiB physical address space, and ...
The 8086 [3] (also called iAPX 86) [4] is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, [5] is a slightly modified chip with an external 8-bit data bus (allowing the use of cheaper and fewer supporting ICs), [note 1] and is notable as the processor used in the original IBM PC design.
Therefore, modern 32-bit x86 operating systems use the LDT very little, primarily to run legacy 16-bit code. Should 16-bit code need to run in a 32-bit environment while sharing memory (this happens e.g. when running OS/2 1.x programs on OS/2 2.0 and later), the LDT must be written in such a way that every flat (paged) address has also a ...