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An addressing mode specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere. In computer programming, addressing modes are primarily of interest to those who write in assembly languages and to compiler writers.
In computing, protected mode, also called protected virtual address mode, [1] is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as segmentation , virtual memory , paging and safe multi-tasking designed to increase an operating system's control over application software .
The 8086 [3] (also called iAPX 86) [4] is a 16-bit microprocessor chip designed by Intel between early 1976 [5] and June 8, 1978, when it was released. [6] The Intel 8088, released July 1, 1979, [7] 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.
Windows 3.0 actually had several modes: "real mode", "standard mode" and "386-enhanced mode"; the latter required some of the virtualization features of the 80386 processor, and thus would not run on an 80286. Windows 3.1 removed support for real mode, and it was the first mainstream operating environment which required at least an 80286 processor.
The range of addressing of memory depends on the bit size of the bus used for addresses – the more bits used, the more addresses are available to the computer. For example, an 8-bit-byte-addressable machine with a 20-bit address bus (e.g. Intel 8086 ) can address 2 20 (1,048,576) memory locations, or one MiB of memory, while a 32-bit bus (e.g ...
The IBM PC AT provided the hardware to do this (for full backward compatibility with software for the original IBM PC and PC/XT models), and so all subsequent "AT-class" PC clones did as well. 286 protected mode was seldom used as it would have excluded the large body of users with 8086/88 machines.
To use virtual 8086 mode, an operating system sets up a virtual 8086 mode monitor, which is a program that manages the real-mode program and emulates or filters access to system hardware and software resources. The monitor must run at privilege level 0 and in protected mode. Only the 8086 program runs in VM86 mode and at privilege level 3.
Intel's Nehalem microarchitecture contains multiple AGUs behind the CPU's reservation station.. The address generation unit (AGU), sometimes also called address computation unit (ACU), [1] is an execution unit inside central processing units (CPUs) that calculates addresses used by the CPU to access main memory.