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.[1]

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 (the so-called "unreal mode"). Programs such as the RAMDRIVE and HIMEM drivers in MS-DOS, AboveDisk (a commercial package by Above Software that converted hard disk or extended memory into expanded memory), and OS/2[2] used LOADALL. Examination of the virtual-machine monitor code in Windows/386 2.1 shows that it uses both the 286[citation needed] and the even less known 386 variant[citation needed]. Microsoft's HIMEM.SYS version 2.06[3] 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,[4] would have been to allow running former real-mode programs in 16-bit protected mode, as utilized by Digital Research's Concurrent DOS 286 since 1985,[5] as well as FlexOS 286 and IBM 4680 OS since 1986/1987. 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 mostly discarded, furthermore, due to errata in some early Intel 80286 processors before the E-2 stepping.[5][6] 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.

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.[7]

As the two LOADALL instructions were never documented and do not exist on later processors, the opcodes were reused in the AMD64 architecture.[8] 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 cemented even on Intel CPUs with the introduction of the Intel 64 implementation of AMD64.[9]

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. In SMM mode, 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.[10] 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.

80286[edit]

Opcode 0F05. The instruction reads data from addresses 00800–00866, whatever the content of the segment registers.

Address number of bytes register register register register
00800 6 not used
00806 2 MSW (machine status word)
00808 14 not used
00816 2 TR (task register)
00818 2 flags
0081A 2 IP (instruction pointer)
0081C 2 LDTR (local descriptor table register)
0081E 4x2 DS (data segment) SS (stack segment) CS (code segment) ES (extra segment)
00826 4x2 DI (destination index) SI (source index) BP (base pointer) SP (stack pointer)
0082E 4x2 BX DX CX AX
00836 4x6 ES segment descriptor CS segment descriptor SS segment descriptor DS segment descriptor
0084E 4x6 GDT, global descriptor table LDT, local descriptor table IDT, interrupt descriptor table TSS, task state segment

Note that the 80286 LOADALL instruction can not be used to switch from protected back to real mode[11] (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[edit]

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

Address number of bytes register register register register
ES:EDI+00 4 CR0, control register 0
ES:EDI+04 4 EFLAGS
ES:EDI+08 4 EIP, instruction pointer
ES:EDI+0C 4x4 EDI, destination index ESI, source index EBP, byte pointer ESP, stack pointer
ES:EDI+1C 4x4 EBX EDX ECX EAX
ES:EDI+2C 2x4 DR6 DR7
ES:EDI+34 4 TR, task state selector
ES:EDI+38 4 LDTR, local descriptor table
ES:EDI+3C 4x2 GS, extra segment not used FS, extra segment not used
ES:EDI+44 4x2 DS, data segment not used SS, stack segment not used
ES:EDI+4C 4x2 CS, code segment not used ES, extra segment not used
ES:EDI+54 4x12 TSS descriptor, task state selector IDT descriptor, interrupt descriptor table GDT descriptor, global descriptor table LDT descriptor, local descriptor table
ES:EDI+84 4x12 GS segment descriptor FS segment descriptor DS segment descriptor SS segment descriptor
ES:EDI+B4 2x12 CS segment descriptor ES segment descriptor

See also[edit]

References[edit]

  1. ^ Van Gilluwe, Frank (1994). The Undocumented PC. pp. 62–70. ISBN 0-201-62277-7. 
  2. ^ Necasek, Michal (2011-03-18). "HIMEM.SYS, unreal mode, and LOADALL". OS/2 Museum. Archived from the original on 2017-01-03. Retrieved 2017-01-03. 
  3. ^ "Microsoft HIMEM.sys 2.06 source code". 
  4. ^ Deitel, Harvey M.; Kogan, Michael S. (1992). The Design of OS/2. Addison-Wesley. ISBN 0-201-54889-5. 
  5. ^ a b Foster, Edward (1985-05-13). "Super DOS awaits new 80286 – Concurrent DOS 286 – delayed until Intel upgrades chip – offers Xenix's power and IBM PC compatibility". InfoWorld. InfoWorld Media Group. 7 (19): 17–18. ISSN 0199-6649. 
  6. ^ Foster, Edward (1985-08-26). "Intel shows new 80286 chip – Future of DRI's Concurrent DOS 286 still unclear after processor fixed". InfoWorld. InfoWorld Media Group. 7 (34): 21. ISSN 0199-6649. 
  7. ^ Frank Van Gilluwe, The Undocumented PC, 1994, page 69, ISBN 0-201-62277-7.
  8. ^ Necasek, Michal (2013-10-18). "LOADALL Strikes Again". OS/2 Museum. Archived from the original on 2017-01-03. Retrieved 2016-01-27. 
  9. ^ "Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 2B" (PDF). Intel Corporation. December 2015. Retrieved 2016-01-27. 
  10. ^ Collins, Robert R. (January 1997). "Intel's System Management Mode". Archived from the original on 2017-01-03. Retrieved 2017-01-03. 
  11. ^ Slater, Michael (October 1987). "Secret 286 LOADALL instruction allows access to extended memory in real mode". Microprocessor Report. 

Further reading[edit]