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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.
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.
With the release of the 386, protected mode offers what the Intel manuals call virtual 8086 mode. Virtual 8086 mode is designed to allow code previously written for the 8086 to run unmodified and concurrently with other tasks, without compromising security or system stability. [34] Virtual 8086 mode, however, is not completely backward ...
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 ...
To support old software, the processor starts up in "real mode", a mode in which it uses the segmented addressing model of the 8086. There is a small difference though: the resulting physical address is no longer truncated to 20 bits, so real mode pointers (but not 8086 pointers) can now refer to addresses between 100000 16 and 10FFEF 16.
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.
In computer engineering, an orthogonal instruction set is an instruction set architecture where all instruction types can use all addressing modes.It is "orthogonal" in the sense that the instruction type and the addressing mode may vary independently.