/* opencoreC - Ring 3 User Mode Transition (IRETQ) */
.global enter_user_mode
.extern set_kernel_stack

.section .text
/*
 * void enter_user_mode(uint64_t entry_rip, uint64_t user_rsp, uint64_t pml4_paddr, uint64_t kernel_stack_top);
 * RDI = entry_rip
 * RSI = user_rsp
 * RDX = pml4_paddr
 * RCX = kernel_stack_top
 */
enter_user_mode:
    /* 1. Save kernel stack in TSS (RSP0) for future interrupts/syscalls */
    push rdi
    push rsi
    push rdx
    mov rdi, rcx
    call set_kernel_stack
    pop rdx
    pop rsi
    pop rdi

    /* 2. Switch to User Address Space (PML4) */
    mov cr3, rdx

    /* 3. Build IRETQ Stack Frame:
     * [RSP + 32] = SS     (User Data: 0x18 | 3 = 0x1B)
     * [RSP + 24] = RSP    (User Stack Pointer)
     * [RSP + 16] = RFLAGS (Interrupt Flag Enabled: 0x202)
     * [RSP + 8]  = CS     (User Code: 0x20 | 3 = 0x23)
     * [RSP + 0]  = RIP    (User Entry Point)
     */
    mov ax, 0x1B
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    push 0x1B          /* User SS */
    push rsi           /* User RSP */
    push 0x0202        /* RFLAGS (IF enabled) */
    push 0x23          /* User CS */
    push rdi           /* User RIP */

    /* 4. Clear general-purpose registers */
    xor rax, rax
    xor rbx, rbx
    xor rcx, rcx
    xor rdx, rdx
    xor rsi, rsi
    xor rdi, rdi
    xor rbp, rbp
    xor r8,  r8
    xor r9,  r9
    xor r10, r10
    xor r11, r11
    xor r12, r12
    xor r13, r13
    xor r14, r14
    xor r15, r15

    /* 5. Jump to Ring 3 */
    iretq