rk/kernel/multiboot.asm

%define MAGIC 0x1BADB002
%define FLAGS 0b110

; Multiboot v1 Specification 
; https://www.gnu.org/software/grub/manual/multiboot/multiboot.html
section .multiboot
align 4
dd MAGIC 
dd FLAGS
dd -(MAGIC + FLAGS)

dd 0
dd 0
dd 0
dd 0
dd 0

dd 1
dd 80
dd 25
dd 0

section .text
global _start
_start:
    cli

    ; Check if EAX contains the multiboot magic number the bootloader is supposed to give us
    cmp eax, 0x2BADB002
    jne halt

    lgdt [gdtp]
    mov ax, 0x10 ; Offset to kernel data descriptor
    mov es, ax
    mov ds, ax
    mov fs, ax
    mov gs, ax
    mov ss, ax
    jmp 0x8:.use_code_seg ; Offset to kernel code descriptor
.use_code_seg:
    mov esp, stack_bottom
    mov ebp, esp
    push ebx 
    extern kmain
    call kmain

; If the kernel function somehow returns, disable interrupts and hang
    cli
halt:
    hlt
    jmp halt

extern exception_handler
%macro isr_err_stub 1
isr_stub_%+%1:
    cli
    push %1
    pusha
    call exception_handler
    popa
    add esp, 8
    iret 
%endmacro
%macro isr_no_err_stub 1
isr_stub_%+%1:
    cli
    push 0
    push %1
    pusha
    call exception_handler
    popa
    add esp, 12
    iret 
%endmacro

isr_no_err_stub 0
isr_no_err_stub 1
isr_no_err_stub 2
isr_no_err_stub 3
isr_no_err_stub 4
isr_no_err_stub 5
isr_no_err_stub 6
isr_no_err_stub 7
isr_err_stub    8
isr_no_err_stub 9
isr_err_stub    10
isr_err_stub    11
isr_err_stub    12
isr_err_stub    13
isr_err_stub    14
isr_no_err_stub 15
isr_no_err_stub 16
isr_err_stub    17
isr_no_err_stub 18
isr_no_err_stub 19
isr_no_err_stub 20
isr_no_err_stub 21
isr_no_err_stub 22
isr_no_err_stub 23
isr_no_err_stub 24
isr_no_err_stub 25
isr_no_err_stub 26
isr_no_err_stub 27
isr_no_err_stub 28
isr_no_err_stub 29
isr_err_stub    30
isr_no_err_stub 31

global isr_stub_table
isr_stub_table:
%assign i 0 
%rep    32 
    dd isr_stub_%+i
%assign i i+1 
%endrep

global ps2_isr
ps2_isr:
    extern ps2_handler
    pushad
    cld
    call ps2_handler
    popad
    iret

global test
test:
    push ebp
    mov ebp, esp 

    mov eax, 1 
    mov edi, 0
    mov esi, buf
    int 0x80
    jmp $
buf: db "Hello from stdout!", 0

global syscall_isr
syscall_isr:
    xchg bx, bx
    pushad
    mov ecx, esp
    sysenter
after_syscall:
    popad
    iret

global syscall
syscall:
    imul eax, eax, 4
    add eax, syscall_table
    pushad
    call [eax]
    popad
    mov edx, after_syscall
    sysexit

; GDT for a flat memory layout
; We get access to all memory and can utilize paging
global gdt
gdt:
gdt_null:
    dq 0
gdt_kern_code:
    dw 0xffff
    dw 0x0000
    db 0x00
    db 0b10011010
    db 0b11001111
    db 0x00
gdt_kern_data:
    dw 0xffff
    dw 0x0000
    db 0x00
    db 0b10010010
    db 0b11001111
    db 0x00
gdt_user_code:
    dw 0xffff
    dw 0x0000
    db 0x00
    db 0b11111010
    db 0b11001111
    db 0x00
gdt_user_data:
    dw 0xffff
    dw 0x0000
    db 0x00
    db 0b11110010
    db 0b11001111
    db 0x00
tss:
    dq 0
gdt_end:

gdtp:
    dw gdt_end - gdt - 1
    dd gdt

global syscall_table
syscall_table:
    extern syscall_read
    dd syscall_read

    extern syscall_write
    dd syscall_write

; Reserve 16KiB of kernel stack space
section .bss
align 16
stack_top:
    resb 16384 
stack_bottom: