more work to make our assembly initialization more beautiful

arch/x86/kernel/entry64.asm

@@ -89,10 +89,10 @@ SECTION .data
 global boot_pml4
 ALIGN 4096 ; of course, the page tables have to be page aligned
 MAP_ENTRIES equ 512
-boot_pml4 times 512 DQ 0
-boot_pdpt times 512 DQ 0
-boot_pd times 512 DQ 0
-boot_pt times (MAP_ENTRIES*512) DQ 0
+boot_pml4 times MAP_ENTRIES DQ 0
+boot_pdpt times MAP_ENTRIES DQ 0
+boot_pd times MAP_ENTRIES DQ 0
+boot_pt times (MAP_ENTRIES*MAP_ENTRIES) DQ 0 ; 1GB kernel space
 
 SECTION .text
 ALIGN 8
@@ -228,10 +228,27 @@ cpu_init:
     mov cr4, eax
     ret
 
+; identity map a single page at address eax
+identity_page:
+    push edi
+    push ebx
+
+    and eax, 0xFFFFF000
+    mov edi, eax
+    shr edi, 9                    ; (edi >> 12) * 8 (index for boot_pt)
+    add edi, boot_pt
+    mov ebx, eax
+    or ebx, 0x13                  ; set present, writable and cache disable bits
+    mov DWORD [edi], ebx
+
+    pop ebx
+    pop edi
+    ret
+
 ALIGN 4
 stublet:
     mov esp, startup_stack-4
-; save pointer to the multiboot structure
+; save pointer to the Multiboot structure
     push ebx
 ; initialize cpu features
     call cpu_init
@@ -240,7 +257,7 @@ stublet:
 ; check if lapic is available
     call check_lapic
 
-; find MP Floating Pointer Structure
+; find MP Floating Pointer structure
     push DWORD 0x100000
     push DWORD 0xF0000
     call search_mps
@@ -258,60 +275,37 @@ stublet:
     je map_kernel
 
 map_mps:
-; map MP Floating Pointer Structure
+; map MP Floating Pointer structure
     mov DWORD [apic_mp], eax
-    mov edi, eax
-    and edi, 0xFFFFF000
-    shr edi, 9   ; (edi >> 12) * 8
-    add edi, boot_pt
-    mov ebx, eax
-    and ebx, 0xFFFFF000
-    or ebx, 0x00000013
-    mov DWORD [edi], ebx
+    call identity_page
 
-; map mp_config
-    mov edi, [eax+4]
-    and edi, 0xFFFFF000
-    shr edi, 9   ; (edi >> 12) * 8
-    add edi, boot_pt
-    mov ebx, [eax+4]
-    and ebx, 0xFFFFF000
-    or ebx, 0x00000013
-    mov DWORD [edi], ebx
+; map MP Configuration table
+    mov eax, [apic_mp+4]
+    call identity_page
 
 %ifdef CONFIG_VGA
-map_vga:
-    mov edi, 0xB8000
-    shr edi, 9   ; (edi >> 12) * 8
-    add edi, boot_pt
-    mov ebx, 0xB8000
-    or ebx, 0x00000013
-    mov DWORD [edi], ebx
+; map VGA textmode plane
+    mov eax, 0xB8000
+    call identity_page
 %endif
 
-map_multiboot:
-    mov edi, [esp]
-    and edi, 0xFFFFF000
-    shr edi, 9   ; (edi >> 12) * 8
-    add edi, boot_pt
-    mov ebx, [esp]
-    and ebx, 0xFFFFF000
-    or ebx, 0x00000003
-    mov DWORD [edi], ebx
+; map Multiboot structure
+    mov eax, [esp]                ; pointer is still on the stack
+    call identity_page
 
 map_kernel:
     mov edi, kernel_start
-    shr edi, 9   ; (kernel_start >> 12) * 8
+    shr edi, 9                    ; (edi >> 12) * 8 (index for boot_pt)
     add edi, boot_pt
     mov ebx, kernel_start
-    or ebx, 0x00000003
+    or ebx, 0x00000003            ; set present and writable flags
     mov ecx, kernel_end           ; determine kernel size in number of pages
     sub ecx, kernel_start
     shr ecx, 12
     inc ecx
 
 .l1:
-    mov DWORD [edi], ebx          ; Set the double word at the destination index to the B-register.
+    mov DWORD [edi], ebx
     add edi, 8
     add ebx, 0x1000
     loop .l1
@@ -319,18 +313,24 @@ map_kernel:
 init_paging:
     mov edi, boot_pml4
     mov cr3, edi
-; So lets make PML4T[0] point to the PDPT and so on:
-    mov DWORD [edi], boot_pdpt    ; Set the double word at the destination index to pdpt.
-    or DWORD [edi], 0x00000003    ; Set present and writeable bit
+
+    mov DWORD [edi], boot_pdpt
+    or DWORD [edi], 0x3
+
+    add edi, MAP_ENTRIES-1        ; create self reference for recursive paging
+    mov DWORD [edi], boot_pml4    ; boot_pml4[511] -> boot_pml4
+    or DWORD [edi], 0x3           ; set present and writable flags
+
     mov edi, boot_pdpt
-    mov DWORD [edi], boot_pd      ; Set the double word at the destination index to pd.
-    or DWORD [edi], 0x00000003    ; Set present and writeable bit
+    mov DWORD [edi], boot_pd
+    or DWORD [edi], 0x3           ; set present and writable flags
+
     mov edi, boot_pd
     mov ebx, boot_pt
     mov ecx, MAP_ENTRIES
 .l1:
-    mov DWORD [edi], ebx          ; Set the double word at the destination index to pt.
-    or DWORD [edi], 0x00000003    ; Set present and writeable bit
+    mov DWORD [edi], ebx
+    or DWORD [edi], 0x3           ; set present and writable flags
     add edi, 8
     add ebx, 0x1000
     loop .l1