Mapping higher-half kernel: checking my understanding
Posted: Mon Jun 16, 2014 6:45 pm
Ok, so I am trying to understand the mapping of the kernel to the 3gb mark within the series. As I understand it, the boot loader sets up a temporary page directory, copies the kernel to 3gb (1mb physical), and jumps to the image. Here is the relevant code:
So, if I am understanding this correctly, if I would copy my kernel BEFORE paging was enabled, I would change IMAGE_PMODE_BASE to 1mb rather than 3gb:
Additionally, I have a question. The kernel DISABLES paging to set up the physical memory manager. But now that the kernel has been built for address 0xc0000000 (3gb), how does this code not cause a triple-fault (any jmps-calls go to an incorrect address)? I think I have the answer, so if someone could check my guess and verify: The jumps/calls involved in setting up the physical memory manager are all relative jumps/calls, not fixed jumps/calls. But how can we ensure that this is the case? I think I am starting to majorly confuse myself, so if anyone can make sense out of my crazy thoughts please feel free to take a stab at it! Thanks 
Code: Select all
%define IMAGE_RMODE_BASE 0x3000
%define IMAGE_PMODE_BASE 0xC0000000
bits 32
%include "Paging.inc"
BadImage db "*** FATAL: Invalid or corrupt kernel image. Halting system.", 0
Stage3:
;-------------------------------;
; Set registers ;
;-------------------------------;
mov ax, DATA_DESC ; set data segments to data selector (0x10)
mov ds, ax
mov ss, ax
mov es, ax
mov esp, 90000h ; stack begins from 90000h
call ClrScr32
call EnablePaging
CopyImage:
mov eax, dword [ImageSize]
movzx ebx, word [bpbBytesPerSector]
mul ebx
mov ebx, 4
div ebx
cld
mov esi, IMAGE_RMODE_BASE
mov edi, IMAGE_PMODE_BASE
mov ecx, eax
rep movsd ; copy image to its protected mode address
TestImage:
mov ebx, [IMAGE_PMODE_BASE+60]
add ebx, IMAGE_PMODE_BASE ; ebx now points to file sig (PE00)
mov esi, ebx
mov edi, ImageSig
cmpsw
je EXECUTE
mov ebx, BadImage
call Puts32
cli
hlt
Code: Select all
%define IMAGE_RMODE_BASE 0x3000
[b]%define IMAGE_PMODE_BASE 0x100000[/b]
bits 32
%include "Paging.inc"
BadImage db "*** FATAL: Invalid or corrupt kernel image. Halting system.", 0
Stage3:
;-------------------------------;
; Set registers ;
;-------------------------------;
mov ax, DATA_DESC ; set data segments to data selector (0x10)
mov ds, ax
mov ss, ax
mov es, ax
mov esp, 90000h ; stack begins from 90000h
call ClrScr32
CopyImage:
mov eax, dword [ImageSize]
movzx ebx, word [bpbBytesPerSector]
mul ebx
mov ebx, 4
div ebx
cld
mov esi, IMAGE_RMODE_BASE
mov edi, IMAGE_PMODE_BASE
mov ecx, eax
rep movsd ; copy image to its protected mode address
TestImage:
mov ebx, [IMAGE_PMODE_BASE+60]
add ebx, IMAGE_PMODE_BASE ; ebx now points to file sig (PE00)
mov esi, ebx
mov edi, ImageSig
cmpsw
[b]call EnablePaging[/b]
je EXECUTE
mov ebx, BadImage
call Puts32
cli
hlt