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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.
In x86 computing, unreal mode, also big real mode, flat real mode, or voodoo mode [1] is a variant of real mode, in which one or more segment descriptors has been loaded with non-standard values, like 32-bit limits allowing access to the entire memory. Contrary to its name, it is not a separate addressing mode that the x86 processors can ...
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.
Real Address mode, [37] commonly called Real mode, is an operating mode of 8086 and later x86-compatible CPUs. Real mode is characterized by a 20-bit segmented memory address space (meaning that only slightly more than 1 MiB of memory can be addressed [ p ] ), direct software access to peripheral hardware, and no concept of memory protection or ...
In real mode, in order to calculate the physical address of a byte of memory, the hardware shifts the contents of the appropriate segment register 4 bits left (effectively multiplying by 16), and then adds the offset. For example, the logical address 7522:F139 yields the 20-bit physical address:
A 386 CPU can be put back into real mode by clearing a bit in the CR0 control register, however this is a privileged operation in order to enforce security and robustness. By way of comparison, a 286 could only be returned to real mode by forcing a processor reset, e.g. by a triple fault or using external hardware.
Real mode ; 8080 emulation mode (NEC V20/V30 only) Protected mode (Intel 80286) Unreal mode (Intel 80286) Virtual 8086 mode (Intel 80386) System Management Mode (Intel 386SL) Long mode (AMD Athlon 64) x86 virtualization (Intel Pentium 4, AMD Athlon 64) AIS mode (VIA C3 only) First supported platform shown in parentheses
Real-mode software cared only about the area slightly above 1 MB, so the Gate-A20 line was enough. Enabling the Gate-A20 line is one of the first steps that a protected-mode x86 operating system does in the bootup process, often before control has been passed to the kernel from the bootstrap (in the case of Linux, for example).