os-k/docs/boot/loader/cpu

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#=----------------------------------------------------------------------------=#
# OS on Kaleid Documentation #
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# Desc: OS/K Loader : CPU Management functions #
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# Copyright © 2018-2021 The OS/K Team #
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This folder contains two files, `boot/loader/cpu/cpu.inc` and
boot/loader/cpu/cpu32.inc`.
The `boot/loader/cpu/cpu32.inc` is intented to provide 32 bits protected mode
function that can control wether the CPU is qualified to run the OS/K code.
The `boot/loader/cpu/cpu3.inc` provides 64 bits long mode temporization function
used by the loader when necessary.
There is two functions in `cpu32.inc` :
- Is64Bits(), intented to check if the CPU is "ok" with 64 bit code,
if it supports long mode and will not burn if we ask it to execute OS/K
(i.e trigger a #UD fault).
For this check, we use first the 0x80000000 function of the CPUID instruction
that returns the highest CPUID function available. It must return at least
0x80000001, otherwise we can't check the long mode presence and it is most
likely absent. We pass the function number in the eax register.
As soon as 0x80000001 is available, we use it and verify it flips to 1 the bit
29 in edx.
- Check_cpuid(), that check if the CPUID instruction is supported by the CPU
before use it (don't want #UD).
For this, we use the FLAGS register and check if we can flip (overwrite with
a 1 persistantly) the bit 21. If it is possible, then CPUID is supported.
The temporize(), bitemporize() and tritemporize() functions in `cpu.inc`
are juste loops that make the cpu busy to wait. These temporization are not
precise at all, and are not intented to wait for a determined time lapse.