LOADALL is the common name for two different, undocumented machine instructions of

Intel 80286 The Intel 80286 (also marketed as the iAPX 286 and often called Intel 286) is a 16-bit microprocessor that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non- multiplexed address and data buses and also the ...

and

Intel 80386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnation o ...

API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

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.

Usage

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

unreal mode In x86 computing, unreal mode, also big real mode, huge real mode, flat real mode, or voodoo mode 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 ...

(which requires switching into protected mode, accessing memory and finally switching back to real mode). Programs such as the pre- XMS versions of RAMDRIVE.SYS (1985), SMARTDRV.SYS (1986) as well as

HIMEM.SYS HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory according to the Extended Memory Specification (XMS). The memory beyond the first 1 MB of address space is required by Windows 9x/ Me in order to load ...

(2.03, 1988-08-04; 2.04, 1988-08-17) drivers in

MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...

, Uniform Software Systems' The Extender (1985) and The Connector (1985) for

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, Above Disk (1986) (a LIMulator by Above Software (formerly Tele-Ware West aka Los Angeles Securities Group) that converted

hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...

space or

extended memory In DOS memory management, extended memory refers to memory above the first megabyte (220 bytes) of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. D ...

into

expanded memory In DOS memory management, expanded memory is a system of bank switching that provided additional memory to DOS programs beyond the limit of conventional memory (640 KiB). ''Expanded memory'' is an umbrella term for several incompatible tec ...

), and OS/2 1.0 and 1.1 used the 286 LOADALL instruction. DOS 3.3 and 4.0 reserved a 102-byte buffer at 0070:0100h (which was normally occupied by

DOS BIOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicati ...

data) so that there was no need to save & restore it for LOADALL. Microsoft's

EMM386.EXE EMM386 is the expanded memory manager of Microsoft's MS-DOS, IBM's PC DOS, Digital Research's DR-DOS, and Datalight's ROM-DOS which is used to create expanded memory using extended memory on Intel 80386 CPUs. There also is an EMM386.EXE available ...

special-cases both the 286 and 386 LOADALL instructions in its

invalid opcode handler An illegal opcode, also called an unimplemented operation, unintended opcode or undocumented instruction, is an instruction to a CPU that is not mentioned in any official documentation released by the CPU's designer or manufacturer, which ne ...

. Examination of the virtual-machine monitor code in Windows/386 2.10 shows that it uses both the 286 and the even less known 386 variant. Microsoft's HIMEM.SYS version 2.06 also used LOADALL to quickly copy to and from extended memory on 286 systems. Another interesting usage of LOADALL, laid out in the book ''The Design of OS/2'', would have been to allow running former real-mode programs in 16-bit protected mode, as utilized by

Digital Research Digital Research, Inc. (DR or DRI) was a company created by Gary Kildall to market and develop his CP/M operating system and related 8-bit, 16-bit and 32-bit systems like MP/M, Concurrent DOS, FlexOS, Multiuser DOS, DOS Plus, DR DOS and ...

Concurrent DOS 286 Multiuser DOS is a real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers. An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 operating systems, it was originally developed by ...

since 1985, as well as

FlexOS 286 FlexOS is a discontinued modular real-time multiuser multitasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. Developed by Digital Research's Flexible Automation Business U ...

and

IBM 4680 OS FlexOS is a discontinued modular real-time multiuser multitasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. Developed by Digital Research's Flexible Automation Business U ...

since 1986. Marking all the descriptor caches in the GDT and LDTs "not present" would allow the operating system to

trap A trap is a mechanical device used to capture or restrain an animal for purposes such as hunting, pest control, or ecological research. Trap or TRAP may also refer to: Art and entertainment Films and television * ''Trap'' (2015 film), Fil ...

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 " 8086 emulation mode" for the 80286 was, however, too slow to be practical. The idea had to be mostly discarded, furthermore, due to errata in some early Intel 80286 processors before the E-2 stepping. 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 8086 mode In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is runnin ...

of the 80386, allowing the implementation of " DOS boxes" at last in a relatively efficient and documented way. Because LOADALL did not perform any checks on the validity of the data loaded into processor registers, it was possible to load a processor state that could not be normally entered, such as using real mode (PE=0) together with paging (PG=1) on 386-class CPUs. An in-circuit emulator (ICE) is a tool used for low-level debugging. On Intel 80386, asserting the undocumented pin at location B6 causes the microprocessor to halt execution and enter ICE mode. The microprocessor saves its entire state to an area of memory isolated from normal system memory. The layout of this area is suitable for LOADALL instruction, and this instruction is used by ICE code to return to normal execution. In later processors, this evolved into

System Management Mode System Management Mode (SMM, sometimes called ring −2 in reference to protection rings) is an operating mode of x86 central processor units (CPUs) in which all normal execution, including the operating system, is suspended. An alterna ...

(SMM). In SMM, the RSM instruction is used to load a full CPU state from a memory area. The layout of this memory area is similar to one used by the LOADALL instruction. 386-style LOADALL instruction can be executed on 486 too, but only in SMM mode. In later processors, RSM instruction, with a different encoding, took its role. Microsoft's

Codeview CodeView is a standalone debugger created by David Norris at Microsoft in 1985 as part of its development toolset. It originally shipped with Microsoft C 4.0 and later. It also shipped with Visual Basic (classic), Visual Basic for MS-DOS, Microso ...

3.0 and Borland's

Turbo Debugger Turbo Debugger (TD) is a machine-level debugger for DOS executables, intended mainly for debugging Borland Turbo Pascal, and later Turbo C programs, sold by Borland. It is a full-screen debugger displaying both Turbo Pascal or Turbo C source and c ...

2.0 correctly decode 286 and 386 LOADALL instructions. As the two LOADALL instructions were never documented and do not exist on later processors, the opcodes were reused in the

AMD64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mod ...

architecture. The opcode for the 286 LOADALL instruction, 0F05, became the AMD64 instruction SYSCALL; the 386 LOADALL instruction, 0F07, became the SYSRET instruction. These definitions were implemented even on Intel CPUs with the introduction of the

Intel 64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mo ...

implementation of AMD64.

80286

Opcode 0F05. The instruction reads data from addresses 0x00800–0x00866, whatever the content of the segment registers. The 80286 LOADALL instruction can not be used to switch from protected back to real mode (it can't clear the PE bit in the MSW). However, use of the LOADALL instruction can avoid the need to switch to protected mode altogether.

80386

Opcode 0F07. The instruction loads data from address ES:EDI. It actually uses ES, not the ES descriptor.

Usage

80286

80386

See also

References

Further reading