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The first x86 processor, the Intel 8086, had a 20-bit address bus for its memory, as did its Intel 8088 variant. [9] This allowed them to access 2 20 bytes of memory, equivalent to 1 megabyte. [9]
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.
The Intel x86 computer instruction set architecture has supported memory segmentation since the original Intel 8086 in 1978. It allows programs to address more than 64 KB (65,536 bytes) of memory, the limit in earlier 80xx processors.
The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20-bit segmented memory address space (giving 1 MB of addressable memory) and unlimited direct software access to all addressable memory, I/O addresses and peripheral hardware. Real mode provides no ...
Below is the full 8086/8088 instruction set of Intel ... User Move – perform data moves that can access user memory while in In-circuit emulation HALT mode.
The Global Descriptor Table (GDT) is a data structure used by Intel x86-family processors starting with the 80286 in order to define the characteristics of the various memory areas used during program execution, including the base address, the size, and access privileges like executability and writability.
Virtual 8086 mode, introduced with the Intel 80386, allows the A20 wrap-around to be simulated by using the virtual memory facilities of the processor; physical memory may be mapped to multiple virtual addresses. Thus, the memory mapped at the first megabyte of virtual memory may be mapped again in the second megabyte of virtual memory.
Unreal mode is used by BIOS code as this is the initial mode of modern Intel processors. [8] Furthermore, the System Management Mode (SMM) in Intel 386SL and later processors places the processor in huge real mode. [9] Some boot loaders (such as LILO) use the unreal mode to access up to 4 GiB of memory.