How to reboot in x86 assembly from 16 bit real mode?

Question

APM shutdown has been covered at X86 instructions to power off computer in real mode?

How to reboot instead of shutting down the computer? Please quote and explain the relevant documentation / standard in your answer.

I've understood that APM and ACPI are the two main methods, and I'm interested in both. Let me know if there are others.

I have found the ACPI spec is at http://uefi.org/specifications but it's taking too much brainpower to parse it that I found it worthy of a question.

I could not find the URL for the APM spec, only a Google cache of a Microsoft RTF here.

If you need a way to test your answer, this minimal APM shutdown boot sector example may be a good starting point. The goal is to convert it to a reboot example instead.

I have read the Linux kernel 4.2 code at arch/x86/realmode/rm/reboot.S but I don't understand how it works, since I can't find the difference between those commands and my working shutdown example.

Related more generic on superuser: https://superuser.com/questions/294681/how-does-a-computer-restart-itself

Related question for protected mode: Rebooting in Protected Mode

Kono

Answer 1

I use the following code in a floppy-disk boot image to reset the pc if the kernel files aren't present on the boot media.

Reboot:
    db 0x0ea 
    dw 0x0000 
    dw 0xffff 

Which essentially assembles to the following instruction:

jmp     far ptr 0FFFFh:0 

I think it's a warm-reboot, i.e equivalent to hitting Ctrl-Alt-Delete, as opposed to hitting the reset button on the front of the machine.

Su

That worked! GAS syntax: ljmpw $0xFFFF, $0x0000. Working minimal example. Linux kernel is doing ljmpw $0xf000, $0xfff0 on the linked file, which also works. But why does it work? Where is it documented? – Kono

Brilliant. According to the answer here: stackoverflow.com/questions/31296422/… and the contained link, here: en.wikipedia.org/wiki/Reset_vector it's because the address 0xFFFF0 is the address of the reset-vector, or the address of the first instruction the cpu should execute when it begins operation. I guess that means it has to be a warm-boot - the RAM still contains the data that was already in it. – Su

If you are using ljmpw $0xFFFF, $0x0000 be aware that this is fine if your target CPU is an 8088/8086. On a 286 it was recommended to ljmpw $0xF000, $0xFFF0 . Although this looks the same, most 386+ now boot in unreal mode with CS segment descriptor 0xF000 pointing at the physical address at the REAL top area of memory where special firmware often resides (not necessarily region <1mb). It use to be advised that enabling the A20 was a good idea before such a far jmp. For what its worth the entry point for the BIOS is 0xf000:0xe05b which is why some versions of Linux jump there instead. – Janessajanet

And if one wants to be pedantic about this prior to jumping to 0xF000:0xFFF0 you can always consider writing to the BDA at 0x0000:0x0472.If you store the word 0x1234 @ that location a warm boot should be done when you jmp to 0xF000:0xFFF0.Typically if you store the word 0x0000 @ 0x0000:0x0472 it should cold reboot when you jmp to the BIOS.Jumping to the BIOS like this isn't guaranteed to work so if ACPI is present issuing the reboot that way is preferred, if that fails drop back to the method of resetting via keyboard controller (just like on the 286). If that fails then try BIOS – Janessajanet

Is rebooting a PC easy?Not according to Linux Developer Matthew Garrett.I discovered He wrote this blog on the subject.He also wrote this email about how Windows certified machines get rebooted by a Windows OS and how even Linux will often hang a PC during reboot.The 286 KBD reset discussed is document on OSDev. If you review all the reboot.c code you find they resort to specific methods depending on computer model – Janessajanet

@MichaelPetch please put those comments in an answer and get rep! :-) – Kono

Answer 2

Works on my old pc 8086:

mov ds, ax
mov ax,0000
mov [0472], ax
jmp FFFF:000

Isocracy

Where did you get this from? Why do each operation? – Kono

I believe that a warm reboot should be set by moving 0x1234 to 0x0000:0x0472. You should be setting DS to 0, and putting 0x1234 into memory location DS:0x0472. I also believe that you meant 0472h not 0472 (which is decimal) – Janessajanet

I used debug. com to compile it, no need 0472h in that case source: computer-programming-forum.com/46-asm/92f051fd8d60ee50.htm – Isocracy

Even if the decimal value 0472 wasn't a problem (because of the use of DEBUG.COM) you should clearly state what is the value in AX that you move into the DS register. – Hypoblast

ok. but I have no issue, I just answered the question, on real 8086 hardware, that code works fine – Isocracy