OS support The protected mode of the 80286 was not routinely utilized in PC applications until many years after its release, in part because of the high cost of adding extended memory to a PC, but also because of the need for software to support the large user base of 8086 PCs. For example, in 1986 the only program that made use of it was VDISK, a RAM disk driver included with PC DOS 3.0 and 3.1. A DOS could utilize the additional RAM available in protected mode (extended memory) either via a BIOS call (INT 15h, AH=87h), as a RAM disk, or as emulation of expanded memory. The difficulty lay in the incompatibility of older real-mode DOS programs with protected mode. They simply could not natively run in this new mode without significant modification. In protected mode, memory management and interrupt handling were done differently than in real mode. In addition, DOS programs typically would directly access data and code segments that did not belong to them, as real mode allowed them to do without restriction; in contrast, the design intent of protected mode was to prevent programs from accessing any segments other than their own unless special access was explicitly allowed. While it was possible to set up a protected-mode environment that allowed all programs access to all segments (by putting all segment descriptors into the Global Descriptor Table (GDT) and assigning them all the same privilege level), this undermined nearly all of the advantages of protected mode except the extended (24-bit) address space. The choice that OS developers faced was either to start from scratch and create an OS that would not run the vast majority of the old programs, or to come up with a version of DOS that was slow and ugly (i.e., ugly from an internal technical viewpoint) but would still run a majority of the old programs. Protected mode also did not provide a significant enough performance advantage over the 8086-compatible real mode to justify supporting its capabilities; actually, except for task switches when multitasking, it yielded only a performance disadvantage, by slowing down many instructions through a litany of added privilege checks. In protected mode, registers were still 16-bit, and the programmer was still forced to use a memory map composed of 64 kB segments, just like in real mode.
os support the protected mode of the 80286 was not routinely utilized in pc applications until many years after its release in part because of the high cost of adding extended memory to a pc but also because of the need for software to support the large user base of 8086 pcs for example in 1986 the only program that made use of it was vdisk a ram disk driver included with pc dos 30 and 31 a dos could utilize the additional ram available in protected mode (extended memory) either via a bios call (int 15h ah=87h) as a ram disk or as emulation of expanded memory the difficulty lay in the incompatibility of older real-mode dos programs with protected mode they simply could not natively run in this new mode without significant modification in protected mode memory management and interrupt handling were done differently than in real mode in addition dos programs typically would directly access data and code segments that did not belong to them as real mode allowed them to do without restriction; in contrast the design intent of protected mode was to prevent programs from accessing any segments other than their own unless special access was explicitly allowed while it was possible to set up a protected-mode environment that allowed all programs access to all segments (by putting all segment descriptors into the global descriptor table (gdt) and assigning them all the same privilege level) this undermined nearly all of the advantages of protected mode except the extended (24-bit) address space the choice that os developers faced was either to start from scratch and create an os that would not run the vast majority of the old programs or to come up with a version of dos that was slow and ugly (ie ugly from an internal technical viewpoint) but would still run a majority of the old programs protected mode also did not provide a significant enough performance advantage over the 8086-compatible real mode to justify supporting its capabilities; actually except for task switches when multitasking it yielded only a performance disadvantage by slowing down many instructions through a litany of added privilege checks in protected mode registers were still 16-bit and the programmer was still forced to use a memory map composed of 64 kb segments just like in real mode