Fake IDE Code Editor Simulation - Look Busy Programming

; ============================================================================
; OS - Project Omega Kernel Dispatcher
; Author: C. Hacker
; Date:   2024-07-26
; Target: x86_64 Bare Metal
; ============================================================================

section .text
    global _start
    extern kmain ; Defined in C or another ASM file
    extern handle_syscall
    extern schedule_next_process
    extern timer_interrupt_handler

%include "constants.inc" ; Include system constants
%include "macros.inc"    ; Useful macros

; --- Kernel Entry Point ---
_start:
    cli                     ; Disable interrupts during setup
    mov ax, cs              ; Setup segment registers
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    ; Setup stack pointer (assuming stack grows down)
    mov esp, KERNEL_STACK_TOP

    ; Initialize essential subsystems (placeholder calls)
    call initialize_gdt
    call initialize_idt
    call initialize_memory_manager
    call initialize_scheduler
    call initialize_drivers

    sti                     ; Re-enable interrupts

    ; Jump to higher-level kernel main function
    call kmain

    ; If kmain returns (it shouldn't), halt the system
    hlt_loop:
        hlt
        jmp hlt_loop

; --- System Call Dispatcher --- 
; Expects syscall number in EAX, args in EBX, ECX, EDX, ESI, EDI
system_call_dispatcher:
    pushad                  ; Save all general-purpose registers
    push ds
    push es
    push fs
    push gs

    mov ax, KERNEL_DS       ; Load kernel data segment
    mov ds, ax
    mov es, ax

    cmp eax, SYSCALL_MAX    ; Check if syscall number is valid
    jae invalid_syscall

    ; Use syscall number as index into the syscall table
    lea esi, [syscall_table]
    mov ebx, eax            ; Copy syscall number
    shl ebx, 2              ; Multiply by 4 (size of dword pointer)
    add esi, ebx            ; Add offset to table base

    ; Call the appropriate handler
    call [esi]

    ; Restore segment registers and general registers
    pop gs
    pop fs
    pop es
    pop ds
    popad
    iret                    ; Return from interrupt (syscall)

invalid_syscall:
    mov eax, -1             ; Return error code (example)
    jmp syscall_exit        ; Go to common exit path

syscall_exit:
    pop gs
    pop fs
    pop es
    pop ds
    popad
    iret

; --- Interrupt Handlers (Example: Timer) ---
irq0_handler:
    pushad
    ; Acknowledge interrupt to PIC (Programmable Interrupt Controller)
    out 0x20, 0x20

    call timer_interrupt_handler ; Call specific C or ASM handler
    call schedule_next_process   ; Trigger scheduler on timer tick

    popad
    iret                     ; Return from interrupt

; --- Placeholder Subsystem Initialization ---
initialize_gdt:
    ; ... complex GDT setup code ...
    lgdt [gdt_descriptor]
    ret

initialize_idt:
    ; ... complex IDT setup code ...
    lidt [idt_descriptor]
    ret

initialize_memory_manager:
    ; ... code to map physical memory, setup paging ...
    mov eax, cr0
    or eax, 0x80000001      ; Enable paging and protection
    mov cr0, eax
    ret

initialize_scheduler:
    ; ... code to setup process queues, timer frequency ...
    ret

initialize_drivers:
    ; ... code to detect and init hardware drivers ...
    ret

; ============================================================================
; Data Section
; ============================================================================
section .data
    welcome_message db "OS Kernel Initialized.", 0x0A, 0
    gdt_descriptor dw gdt_end - gdt_start - 1 ; Limit
                   dd gdt_start               ; Base
    idt_descriptor dw 256 * 8 - 1             ; Limit (256 entries * 8 bytes)
                   dd idt_start               ; Base

    ; Syscall jump table (array of function pointers)
    syscall_table:
        dd handle_syscall_0 ; Example: Exit
        dd handle_syscall_1 ; Example: Fork
        dd handle_syscall_2 ; Example: Read
        dd handle_syscall_3 ; Example: Write
        ; ... add pointers for all supported syscalls ...
        times SYSCALL_MAX - 4 dd invalid_syscall ; Pad with invalid handler

; ============================================================================
; BSS Section (Uninitialized Data)
; ============================================================================
section .bss
    KERNEL_STACK_BOTTOM resb KERNEL_STACK_SIZE ; Reserve stack space
    KERNEL_STACK_TOP:

    gdt_start resb 256 * 8 ; Reserve space for GDT
    gdt_end:

    idt_start resb 256 * 8 ; Reserve space for IDT
    idt_end:

; ============================================================================
; External Syscall Handlers (Placeholders)
; ============================================================================
%macro DEFINE_SYSCALL_HANDLER 1
    global handle_syscall_%1
    handle_syscall_%1:
        ; Placeholder logic for syscall %1
        ; Result typically returned in EAX
        mov eax, %1 ; Return syscall number for demo
        jmp syscall_exit
%endmacro

DEFINE_SYSCALL_HANDLER 0
DEFINE_SYSCALL_HANDLER 1
DEFINE_SYSCALL_HANDLER 2
DEFINE_SYSCALL_HANDLER 3
; ... define more syscall handlers ...