Extend some explanations

[phil-opp]

Some issues that were brought up by rylev on IRC:

• "CPUID check" too fast
• more assembly explanations, especially cmp and jmp

Update: below @mtnygard mentioned some issues with Printing to Screen:

• too quick
• unclear code fragments (which file? function or impl method?)
• better document the lib.rs and Cargo.toml changes required for Unique and spin
• type conflict: byte strings vs definition of write_str

[mtnygard]

The part titled "Printing to Screen" also goes very quickly. The code fragments could use some more context: which file does it go in, is it top-level or inside a type? (E.g., the text doesn't state that write_str should be inside the Writer type definition.)

Using Unique and sping also require changes to the crate attributes and Cargo.toml. The text could use some clarification there as well.

[mtnygard]

Section "Printing Strings" talks about adding b to make something a byte string, but then there is a type conflict with the definition of write_str.

I suspect this part assumes more Rust knowledge than I have here on day 3 of my journey. :-)

[phil-opp]

Thanks a lot! I'll try to clarify these things.

[sarahhodne]

I was going through the CPUID check myself to try to understand what was going on, and here's the version I ended up with, with some different comments:

check_cpuid:
    ; Check if CPUID is supported by attempting to flip the ID bit (bit 21) in
    ; the FLAGS register. If we can flip it, CPUID is available.

    ; Copy FLAGS in to EAX via stack
    pushfd
    pop eax

    ; Copy to ECX as well for comparing later on
    mov ecx, eax

    ; Flip the ID bit
    xor eax, 1 << 21

    ; Copy EAX to FLAGS via the stack
    push eax
    popfd

    ; Copy FLAGS back to EAX (with the flipped bit if CPUID is supported)
    pushfd
    pop eax

    ; Restore FLAGS from the old version stored in ECX (i.e. flipping the ID bit
    ; back if it was ever flipped).
    push ecx
    popfd

    ; Compare EAX and ECX. If they are equal then that means the bit wasn't
    ; flipped, and CPUID isn't supported.
    xor eax, ecx
    jz .no_cpuid
    ret
.no_cpuid:
    mov al, "1"
    jmp error

[mtnygard]

@henrikhodne I had to do something similar. The comments in the original code didn't explain why the operations were being done that way.

[phil-opp]

@henrikhodne This is very useful! Mind if I include it in the tutorial?

I think you should also update the page in the OSDev Wiki. Your comments are way better!

[sarahhodne]

@henrikhodne This is very useful! Mind if I include it in the tutorial?

Not at all, go ahead.

And I'll see if I can update the wiki page too.