Succesful ELF kernel boot!

This commit is contained in:
Robbe Van Herck 2019-12-30 13:59:02 +01:00
parent a9985ea925
commit bc3165eeb8
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GPG key ID: A66F76F7B81BD784

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@ -1,12 +1,12 @@
DISK_ID EQU 0x81
KERNEL_START EQU 0x100000
ELF_START EQU 0x8000
org 0x7C00
bits 16
; memory layout:
; 0x8000 -> ... = ELF file, unparsed
; 0x100000 -> ... = actual kernel
jmp .start
[bits 16]
@ -154,7 +154,7 @@ call puthex
.reset_disk_no_error:
mov cx, 1 ; start at sector 1
mov bx, 0x8000 ; write to 0x8000
mov bx, ELF_START ; write to ELF_START
push bx
push cx
.read_disk_loop:
@ -223,16 +223,80 @@ mov ss, ax
; set up stack
mov esp, 0x090000
; parse ELF file to 0x10000
; parse elf file at ELF_START to KERNEL_START
; load memory position from ELF file
mov eax, [ds:0x8018]
; verify ELF header
mov esi, ELF_START
cmp dword [esi], 464C457Fh ; ELF magic
jne .invalid_elf
cmp word [esi+4], 0101h ; lsb 32 bit, little endian
jne .invalid_elf
cmp word [esi+18], 03 ; x86 architecture
jne .invalid_elf
jmp eax
; read the entrypoint and store it
mov eax, dword [esi+0x18] ; program entry position
mov dword [.entrypoint], eax
mov cx, word [esi+0x2C] ; read phnum (number of program headers)
mov eax, dword [esi+0x1C] ; read phoff (offset of program header)
; ebx is now ELF_START, esi jumps to the start of the program header
mov ebx, esi
add esi, eax
; set up for loop
sub esi, 0x20
inc cx
.elf_ph_loop:
add esi, 0x20
dec cx
jz .invalid_elf ; there is no valid code block
cmp word [esi], 1 ; check if p_type is loadable
jne .elf_ph_loop
; add offset to ebx (ebx = pointer to code)
add ebx, dword [esi+0x04]
; store p_filesz (ecx = size of the segment)
mov ecx, dword [esi+0x10]
; calculate size of bss (edx = size of bss)
mov edx, dword [esi+0x14]
sub edx, ecx
; check if ecx is zero
or ecx, ecx
jz .invalid_elf
; set source
mov esi, ebx
; set destination
mov edi, KERNEL_START
; repeat ecx/4 times (because it moves 4 bytes at a time)
shr ecx, 2
; copy
repnz movsd
; jump to start of kernel
jmp [.entrypoint]
.end:
jmp .end
.invalid_elf:
mov byte [ds:0x0B8000], 'E'
mov byte [ds:0x0B8001], 0x40
mov byte [ds:0x0B8002], 'L'
mov byte [ds:0x0B8003], 0x40
mov byte [ds:0x0B8004], 'F'
mov byte [ds:0x0B8005], 0x40
jmp .end
.data:
.str_no_A20:
db "A20 not enabled"
@ -258,12 +322,12 @@ db 11001111b
db 0
.gdt_end:
.gdt_desc:
dw .gdt_end - .gdt - 1
dd .gdt
.entrypoint: dd 0
; print padding nullbytes
times 510 - ($ - $$) db 0