LOADALL

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LOADALL is the common name for two different, undocumented machine instructions of Intel 80286 and Intel 80386 processors, which allow access to areas normally outside of the IA-32 API scope, like descriptor cache registers. The LOADALL for 286 processors is encoded 0Fh 05h, while the LOADALL for 386 processors is 0Fh 07h.

Both variants - as the name implies - load all CPU internal registers in one operation. LOADALL had the unique ability to set up the visible part of the segment registers (selector) independently of their corresponding cached part, allowing the programmer to bring the CPU into states not otherwise allowed by the official programming model.

As an example of the usefulness of these techniques, LOADALL can set up the CPU to allow access to all memory from real mode, without having to switch it into protected mode. Programs such as the RAMDRIVE and HIMEM drivers in MS-DOS, AboveDisk (a commercial package by Above Software which converted hard disk or extended memory into expanded memory), and OS/2 used LOADALL. Examination of the virtual machine monitor code in Windows/386 2.1 shows it uses both the 286 and the even less known 386 variant.

Another interesting usage of LOADALL, signalled in the book The Design of OS/2, would have been to allow running former real mode programs in 16-bit protected mode. Marking all the descriptor caches in the GDT and LDTs "not present" would allow the operating system to trap segment register reloads as well as attempts at performing real-mode specific "segment arithmetic" and emulate the desired behavior by updating the segment descriptors (LOADALL again). This "virtual 8086 mode" for the 80286 was however too slow to be practical. The idea had to be discarded furthermore due to an errata in some early Intel 80286 processors. As a result, OS/2 1.x - and Windows in "standard" mode as well - had to run DOS programs in real-mode. Nevertheless the idea was not lost, it led Intel to introduce the virtual mode of the 80386, allowing the implementation of "DOS-boxes" at last in a relatively efficient and documented way.