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Extend explanation for long mode test and improve code comments

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Philipp Oppermann
2016-02-02 23:09:44 +01:00
parent d6efe62c80
commit bb4d64dc99
1 changed files with 16 additions and 9 deletions
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posts/2015-08-25-entering-longmode.md
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@@ -150,20 +150,27 @@ Now we can use CPUID to detect whether long mode can be used. I use code from [O
```nasm
check_long_mode:
mov eax, 0x80000000 ; Set the A-register to 0x80000000.
cpuid ; CPU identification.
cmp eax, 0x80000001 ; Compare the A-register with 0x80000001.
jb .no_long_mode ; It is less, there is no long mode.
mov eax, 0x80000001 ; Set the A-register to 0x80000001.
cpuid ; CPU identification.
test edx, 1 << 29 ; Test if the LM-bit is set in the D-register.
jz .no_long_mode ; They aren't, there is no long mode.
; test if extended processor info in available
mov eax, 0x80000000 ; implicit argument for cpuid
cpuid ; get highest supported argument
cmp eax, 0x80000001 ; it needs to be at least 0x80000001
jb .no_long_mode ; if it's less, the CPU is too old for long mode
; use extended info to test if long mode is available
mov eax, 0x80000001 ; argument for extended processor info
cpuid ; returns various feature bits in ecx and edx
test edx, 1 << 29 ; test if the LM-bit is set in the D-register
jz .no_long_mode ; If it's not set, there is no long mode
ret
.no_long_mode:
mov al, "2"
jmp error
```
It tries to invoke a CPUID function to test if the long mode is available. But this function is not part of CPUIDs core functions, therefore we need to test if the so-called extended functions are available before. So the first `cpuid` call tests whether the function is available that checks long mode support. And then the second `cpuid` call uses that function to test whether long mode support is available. Whew!
Like many low-level things, CPUID is a bit strange. Instead of taking a parameter, the `cpuid` instruction implicitely uses the `eax` register as argument. To test if long mode is available, we need to call `cpuid` with `0x80000001` in `eax`. This loads some information to the `ecx` and `edx` registers. Long mode is supported if the 29th bit in `edx` is set. [Wikipedia][cpuid long mode] has detailed information.
[cpuid long mode]: https://en.wikipedia.org/wiki/CPUID#EAX.3D80000001h:_Extended_Processor_Info_and_Feature_Bits
If you look at the assembly above, you'll probably notice that we call `cpuid` twice. The reason is that the CPUID command started with only a few functions and was extended over time. So old processors may not know the `0x80000001` argument at all. To test if they do, we need to invoke `cpuid` with `0x80000000` in `eax` first. It returns the highest supported parameter value in `eax`. If it's at least `0x80000001`, we can test for long mode as described above. Else the CPU is old and doesn't know what long mode is either. In that case, we directly jump to `.no_long_mode` through the `jb` instruction (“jump if below”).
### Putting it together
We just call these check functions right after start: