Paging problems

MakaAlbarn001 · **Joined:** Fri Jun 16, 2017 1:51 am **Posts:** 20

As to your first point, I'll do an "info reg" at _start to try to work out what's happening. On the point that you mentioned in your edit, I can't believe that I didn't notice that.

linuxyne · **Joined:** Sat Jul 02, 2016 7:02 am **Posts:** 210

It is also worthwhile to check the calling convention between boot32.s (which is compiled as code32) and paging.cpp (which is compiled as code64), as it applies when calling page_set.

MakaAlbarn001 · **Joined:** Fri Jun 16, 2017 1:51 am **Posts:** 20

head... meet desk.

I am SUCH and IDIOT. Why did I not think to specify gcc to compile those bits of code as 32-bit I have no idea.

linuxyne · **Joined:** Sat Jul 02, 2016 7:02 am **Posts:** 210

I think I realized the situation a bit late :oops:

.
A 32bit cpu, interpreting actual 64bit instructions as 32bit, seems to be the cause, unless something else turns up as the root cause for the weird behaviour.

MakaAlbarn001 · **Joined:** Fri Jun 16, 2017 1:51 am **Posts:** 20

Not quite. A 64-bit cpu, running what should be 32-bit instructions as 64-bit, while in protected(32-bit) mode.

MakaAlbarn001 · **Joined:** Fri Jun 16, 2017 1:51 am **Posts:** 20

Well. I managed to get my paging problem fixed by setting the page tables in my boot32.s file. Now I need to set the GDT. Can anyone translate this to AT&T assembly:

Code:

GDT64:                           ; Global Descriptor Table (64-bit).
    .Null: equ $ - GDT64         ; The null descriptor.
    dw 0                         ; Limit (low).
    dw 0                         ; Base (low).
    db 0                         ; Base (middle)
    db 0                         ; Access.
    db 0                         ; Granularity.
    db 0                         ; Base (high).
    .Code: equ $ - GDT64         ; The code descriptor.
    dw 0                         ; Limit (low).
    dw 0                         ; Base (low).
    db 0                         ; Base (middle)
    db 10011010b                 ; Access (exec/read).
    db 00100000b                 ; Granularity.
    db 0                         ; Base (high).
    .Data: equ $ - GDT64         ; The data descriptor.
    dw 0                         ; Limit (low).
    dw 0                         ; Base (low).
    db 0                         ; Base (middle)
    db 10010010b                 ; Access (read/write).
    db 00000000b                 ; Granularity.
    db 0                         ; Base (high).
    .Pointer:                    ; The GDT-pointer.
    dw $ - GDT64 - 1             ; Limit.
    dq GDT64                     ; Base.

linuxyne · **Joined:** Sat Jul 02, 2016 7:02 am **Posts:** 210

Would it not be better to understand what the bits and pieces of such a table mean, and then write one of your own directly in your desired assembly?

MakaAlbarn001 · **Joined:** Fri Jun 16, 2017 1:51 am **Posts:** 20

For the most part, it was only the offset sections, "equ $ - GDT64" that got me. I managed to figure most of it out.

Code:

GDT64:
   .Null = . - GDT64
   .short 0
   .short 0
   .byte 0
   .byte 0
   .byte 0
   .byte 0
   .Code = . - GDT64
   .short 0
   .short 0
   .byte 0
   .byte 0x9A
   .byte 0x20
   .byte 0
   .Data = . - GDT64
   .short 0
   .short 0
   .byte 0
   .byte 0x92
   .byte 0
   .byte 0
   .Pointer:
   .short = . - GDT64 - 1
   .quad GDT64

My only remaining question is if I need to use $GDT64 instead of GDT64 in .pointer?

Edit: I did just realize that I couldn't use ".Code" or ".Data" for the GDT offsets because they might be assembled weirdly.

Edit 2: Okay, for some reason, when I try to set my data segment, it jumps to an entirely different area. Weird.

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