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I'm making a custom boot loader, and currently in the stage of enabling paging after switching to protected mode. I tried to check all memory range is usable from 0x00000000 to 0xFFFFFFFF to make sure my kernel has full control over memories. Checking is done by copying value to the memory address and printing the value through video memory(0xB8000). But the address range from 0xC0000000 to 0xFFFFFFFF always contains 0x00(if executed by QEMU) or 0xFF(if executed in real hardware through legacy booting).
I've been searching for the reason for last 3 weeks but failed to find a clear answer. From searching, I've found some clues like physical-virtual address translation, kernel mode and user mode, MMU, reserved hardware memory range.
- The phenomenon is due to physical-virtual address translation.
The boot loader is in protected mode and paging is not enabled. So the address in the code, which is virtual(= linear) address, is translated by 1:1 mapping.
Virtual address : 0x12345678 -> Physical address : 0x12345678
It seems it is unrelated to the phenomenon-particular address range always containing 0x00 or 0xFF.
- The phenomenon is due to kernel mode and user mode.
There were numerous articles about kernel mode and user mode especially about linux. But my boot loader hasn't done enabled paging yet.
Therefore, there is no rule defined such as - user/kernel space range ex) User memory space : 0x00000000 to 0xBFFFFFFF Kernel memory space : 0xC0000000 to 0xFFFFFFFF - how virtual memory will be mapped virtual memory 0x00010000 -> 0xC0010000 - pages
The phenomenon is due to MMU
It seems it is related to translation, and nothing to do with particular memory range containing 0x00 or 0xFF.
The phenomenon is due to reserved memory.
I thought this as highly likely the main reason, and executed memory detection by BIOS interrupt 0x15 eax 0xE820. The result 'it was not'. Also, checked intel datasheet.
Now, I have no idea why is this happening.
Why is memory range from 0xC0000000 to 0xFFFFFFFF always contains 0x00 or 0xFF? (Describing 'always contain 0x00 or 0xFF' because I am unsure about whether - memory is inaccessible - memory is not writable - memory is not readable - etc )
The result and codes are posted below.
■ BootLoader.asm
%include "MBR.asm"
%include "BootSector2.asm"
■ MBR.asm
[ORG 0x0000]
[BITS 16]
; ──────────────────────────────────────────────────
; EQU
; ──────────────────────────────────────────────────
MBRBaseAddress: equ 0x7C00
ReadSectorCount: equ 0x0001
; ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
; Code Section
; ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
SECTION .text
MBR:
; ──────────────────────────────────────────────────
; Initialize Registers
; ──────────────────────────────────────────────────
jmp 0x0000:(MBRBaseAddress + 0x0005) ; Initialize CS (Code Segment) Register / Absolute Far Jump / set cs = 0x0000, jump to 0x07C05 (0x7C00 + 0x0005)
mov ax, word 0x0000
mov ds, ax ; Initialize DS (Data Segment) Register
mov ss, ax ; Initialize SS (Stack Segment) Register
mov ax, word 0x1000
mov es, ax ; Initialize ES (Extra Segment) Register
mov ax, word 0xB800
mov fs, ax ; Initialize FS Register
mov ax, word 0x0000 ; Initialize AX (Accumulator) Register
mov bx, word 0x0000 ; Initialize BX (Base) Register
mov cx, word 0x0000 ; Initialize CX (Count) Register
mov dx, word 0x0000 ; Initialize DX (Data) Register
mov si, word 0x0000 ; Initialize SI (Source Index) Register
mov di, word 0x0000 ; Initialize DI (Destination Index) Register
mov sp, word 0xFFFE ; Initialize SP (Stack Pointer) Register
mov bp, word 0xFFFE ; Initialize BP (Base Pointer) Register
; ──────────────────────────────────────────────────
; Clear Screen
; ──────────────────────────────────────────────────
.Clear1Digit:
mov [fs:di], word 0x0A00 ; [0xB800:di] 0x00 / [0xB800:di + 1] 0x0A / 0 -> 0000 (Background Attribute) / A -> 1010 (Text Attribute)
add di, 2
cmp di, 2 * 80 * 25 ; Compare di to 2 * 80 * 25 (= 4000 = 0x0FA0)
jl .Clear1Digit
mov di, word 0x0000 ; Clear di
; ──────────────────────────────────────────────────
; Print String : Real Mode String
; ──────────────────────────────────────────────────
push RealModeString
push 0
call .PrintString
mov sp, bp
; ──────────────────────────────────────────────────
; Print String : Start Reading Sectors String
; ──────────────────────────────────────────────────
push StartReadingSectorsString
push 1 * 2 * 80
call .PrintString
mov sp, bp
; ──────────────────────────────────────────────────
; Read Sectors (via BIOS interrupt)
; ──────────────────────────────────────────────────
.ReadSectors:
mov si, word 0x0000 ; Initialize si (Read sector count)
mov al, 1 ; Initialize read sector count (1)
mov dl, byte 0x80 ; Initialize drive to read from (Hard Disk C:)
mov bx, word 0x0000 ; Offset to read to
mov ah, byte 0x02 ; BIOS interrupt : Read sector
.Read1Sector:
mov ch, byte [MBRBaseAddress + CylinderIndex] ; Set cylinder value to read from
mov dh, byte [MBRBaseAddress + HeadIndex] ; Set head value to read from
mov cl, byte [MBRBaseAddress + SectorIndex] ; Set sector value to read from
cmp si, ReadSectorCount ; Compare si to ReadSectorCount
jge .ReadSectorsEnd ; Jump to .ReadSectorsEnd if
int 0x13 ; Call BIOS interrupt
add si, 1
add cl, 1
mov [MBRBaseAddress + SectorIndex], cl
add bx, 512
jmp .Read1Sector
.ReadSectorsEnd:
; ──────────────────────────────────────────────────
; Print Value : Sectors Read Count
; ──────────────────────────────────────────────────
push si
push 2 * 2 * 80
call .PrintValue
mov sp, bp
; ──────────────────────────────────────────────────
; Print String : Sectors Read String
; ──────────────────────────────────────────────────
push SectorsReadString
push (2 * 2 * 80) + (2 * 5)
call .PrintString
mov sp, bp
; ──────────────────────────────────────────────────
; Switch to Protected Mode
; ──────────────────────────────────────────────────
cli
lgdt [MBRBaseAddress + GDT]
;mov eax, dword 0x4000003B
mov eax, cr0
or al, 1
mov cr0, eax
jmp dword 0x0008:0x00010000
; ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
; Functions
; ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
; ──────────────────────────────────────────────────
; Print Value
; ──────────────────────────────────────────────────
.PrintValue:
push bp ; Copy current bp to memory whose address is current sp
mov bp, sp ; Set new bp by current sp
mov cl, byte 12 ; Initialize cl (Shift right count)
mov di, [bp + 4] ; Initialize di (2nd argument / Video memory offset where to print)
.PrintValueLoop:
mov si, [bp + 6] ; Copy 1st argument (Value to print) to si
shr si, cl ; Shift right si cl times
and si, word 0x000F ; Keep last 4 bits and discard (fill) other bytes to 0
mov bl, byte [MBRBaseAddress + ASCIIString + si] ; Copy ASCII value, corresponding to si, to bl
mov [fs:di], bl ; [0xB800:di] bl / Copy 1 character to video memory
sub cl, 4
add di, 2
cmp cl, 0
jg .PrintValueLoop
mov si, [bp + 6] ; Copy 1st argument (Value to print) to si
and si, word 0x000F ; Keep last 4 bits and discard (fill) other bytes to 0
mov bl, byte [MBRBaseAddress + ASCIIString + si] ; Copy ASCII value, corresponding to si, to bl
mov [fs:di], bl ; [0xB800:di] bl / Copy 1 character to video memory
.PrintValueEnd:
pop bp ; Copy memory whose address is current sp to bp
ret
; ──────────────────────────────────────────────────
; Print String
; ──────────────────────────────────────────────────
.PrintString:
push bp ; Copy current bp to memory whose address is current sp
mov bp, sp ; Set new bp by current sp
mov bx, word [bp + 6] ; Initialize bx (1st argument / String offset)
mov si, 0 ; Initialize si (String index)
mov di, word [bp + 4] ; Initialize di (2nd argument / Video memory offset where to print)
.PrintStringLoop:
mov cl, byte [MBRBaseAddress + bx + si] ; Copy 1 character from memory (MBRBaseAddress + String offset + String index) to cl
cmp cl, 0 ; Compare cl to 0
je .PrintStringEnd
mov [fs:di], cl ; [0xB800:di] cl / Copy 1 character to video memory
add si, 1
add di, 2
jmp .PrintStringLoop
.PrintStringEnd:
pop bp ; Copy memory whose address is current sp to bp
ret
; ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
; Data (Read Only)
; ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
; ──────────────────────────────────────────────────
; ASCII String
; ──────────────────────────────────────────────────
ASCIIString:
db '0123456789ABCDEF', 0
; ──────────────────────────────────────────────────
; Strings
; ──────────────────────────────────────────────────
RealModeString:
db '[Current Mode : Real Mode]', 0
StartReadingSectorsString:
db 'Start Reading Sectors', 0
SectorsReadString:
db 'Sectors Read', 0
; ──────────────────────────────────────────────────
; Cylinder, Head, Sector Index
; ──────────────────────────────────────────────────
CylinderIndex:
db 0x00
HeadIndex:
db 0x00
SectorIndex:
db 0x02
; ──────────────────────────────────────────────────
; GDT (Global Descriptor Table)
; ──────────────────────────────────────────────────
GDTStart:
GDTNullDescriptor:
dq 0x0000000000000000
GDTCodeDescriptor:
dw 0xFFFF
dw 0x0000
db 0x00
db 0x9A
db 0xCF
db 0x00
GDTDataDescriptor:
dw 0xFFFF
dw 0x0000
db 0x00
db 0x92
db 0xCF
db 0x00
GDTEnd:
GDT:
dw GDTEnd - GDTStart
dd MBRBaseAddress + GDTStart
; ──────────────────────────────────────────────────
; Declare Zeros
; ──────────────────────────────────────────────────
times 510 - ($ - $$) db 0x00
; ──────────────────────────────────────────────────
; Boot Signature
; ──────────────────────────────────────────────────
dw 0xAA55
■ BootSector2.asm
[BITS 32]
; ──────────────────────────────────────────────────
; EQU
; ──────────────────────────────────────────────────
BootSector2BaseOffset: equ 0x0000FE00 ; ※ 0x00010000 (Read sector address) - 0x00000200 (Initial label offset) = 0x0000FE00 in order to use label as if it begins from 0x00000000 [ex) BootSector2Offset (0x0000FE00) + LABEL (0x000002XX) = 0x000100XX]
VideoMemory: equ 0x000B8000
BootSector2:
mov ax, word 0x0010
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
mov eax, dword 0x000B8000
mov ebx, dword 0x00000000
mov cl, 28
mov esi, dword 0x00000000
mov edi, dword VideoMemory + (3 * 2 * 80)
mov esp, dword 0xFFFC0004;0xBFFFFFFC;0xC0000000 + 0x00010000 + 0x000000E4
mov ebp, dword 0xFFFC0004;0xBFFFFFFC;0xC0000000 + 0x00010000 + 0x000000E4
.Loop1:
mov esi, esp
shr esi, cl
and esi, dword 0x0000000F
mov bl, byte [MBRBaseAddress + ASCIIString + esi]
mov [edi], bl
sub cl, 4
add edi, 2
cmp cl, 0
jg .Loop1
mov esi, esp
and esi, dword 0x0000000F
mov bl, byte [MBRBaseAddress + ASCIIString + esi]
mov [edi], bl
mov edi, dword VideoMemory + (4 * 2 * 80)
push dword 0xABCD1234
mov cl, 28
.Loop2:
mov esi, esp
shr esi, cl
and esi, dword 0x0000000F
mov bl, byte [MBRBaseAddress + ASCIIString + esi]
mov [edi], bl
sub cl, 4
add edi, 2
cmp cl, 0
jg .Loop2
mov esi, esp
and esi, dword 0x0000000F
mov bl, byte [MBRBaseAddress + ASCIIString + esi]
mov [edi], bl
mov edi, dword VideoMemory + (5 * 2 * 80)
mov cl, 28
.Loop3:
mov esi, dword [0xFFFC0000]
shr esi, cl
and esi, dword 0x0000000F
mov bl, byte [MBRBaseAddress + ASCIIString + esi]
mov [edi], bl
sub cl, 4
add edi, 2
cmp cl, 0
jg .Loop3
mov esi, dword [0xFFFC0000]
and esi, dword 0x0000000F
mov bl, byte [MBRBaseAddress + ASCIIString + esi]
mov [edi], bl
jmp $
; ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
; Data (Read Only)
; ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
ProtectedModeString:
db '[Current Mode : Protected Mode]', 0
SampleValue:
dd 0x12345678
times 1024 - ($ - $$) db 0x00
