Protected mode

In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as virtual memory, paging and safe multi-tasking designed to increase an operating system's control over application software.Where: In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as virtual memory, paging and safe multi-tasking designed to increase an operating system's control over application software. When a processor that supports x86 protected mode is powered on, it begins executing instructions in real mode, in order to maintain backward compatibility with earlier x86 processors. Protected mode may only be entered after the system software sets up one descriptor table and enables the Protection Enable (PE) bit in the control register 0 (CR0). Protected mode was first added to the x86 architecture in 1982, with the release of Intel's 80286 (286) processor, and later extended with the release of the 80386 (386) in 1985. Due to the enhancements added by protected mode, it has become widely adopted and has become the foundation for all subsequent enhancements to the x86 architecture, although many of those enhancements, such as added instructions and new registers, also brought benefits to the real mode. The Intel 8086, the predecessor to the 286, was originally designed with a 20-bit address bus for its memory. This allowed the processor to access 220 bytes of memory, equivalent to 1 megabyte. At the time, 1 megabyte was considered a relatively large amount of memory, so the designers of the IBM Personal Computer reserved the first 640 kilobytes for use by applications and the operating system and the remaining 384 kilobytes for the BIOS (Basic Input/Output System) and memory for add-on devices. As the cost of memory decreased and memory use increased, the 1 MB limitation became a significant problem. Intel intended to solve this limitation along with others with the release of the 286. The initial protected mode, released with the 286, was not widely used; for example, it was used by Microsoft Xenix (around 1984), Coherent and Minix. Several shortcomings such as the inability to access the BIOS or DOS calls due to inability to switch back to real mode without resetting the processor prevented widespread usage. Acceptance was additionally hampered by the fact that the 286 only allowed memory access in 16 bit segments via each of four segment registers, meaning only 4*216 bytes, equivalent to 256 kilobytes, could be accessed at a time. Because changing a segment register in protected mode caused a 6-byte segment descriptor to be loaded into the CPU from memory, the segment register load instruction took many tens of processor cycles, making it much slower than on the 8086; therefore, the strategy of computing segment addresses on-the-fly in order to access data structures larger than 128 kilobytes (the combined size of the two data segments) became impractical, even for those few programmers who had mastered it on the 8086/8088. The 286 maintained backwards compatibility with its precursor the 8086 by initially entering real mode on power up. Real mode functioned virtually identically to the 8086, allowing the vast majority of existing 8086 software to run unmodified on the newer 286. Real mode also served as a more basic mode in which protected mode could be set up, solving a sort of chicken-and-egg problem. To access the extended functionality of the 286, the operating system would set up some tables in memory that controlled memory access in protected mode, set the addresses of those tables into some special registers of the processor, and then set the processor into protected mode. This enabled 24 bit addressing which allowed the processor to access 224 bytes of memory, equivalent to 16 megabytes. With the release of the 386 in 1985, many of the issues preventing widespread adoption of the previous protected mode were addressed. The 386 was released with an address bus size of 32 bits, which allows for 232 bytes of memory accessing, equivalent to 4 gigabytes. The segment sizes were also increased to 32 bits, meaning that the full address space of 4 gigabytes could be accessed without the need to switch between multiple segments. In addition to the increased size of the address bus and segment registers, many other new features were added with the intention of increasing operational security and stability. Protected mode is now used in virtually all modern operating systems which run on the x86 architecture, such as Microsoft Windows, Linux, and many others. Furthermore, learning from the failures of the 286 protected mode to satisfy the needs for multiuser DOS, Intel added a separate virtual 8086 mode, which allowed multiple virtualized 8086 processors to be emulated on the 386. Hardware support required for virtualizing the protected mode itself, however, had to wait for another 20 years.