bits 64

global start_user_mode

section .text

extern syscall_encode_color

encode_color_syscall:
  call syscall_encode_color
  mov edi, eax
  xor rax, rax
  mov eax, edi
  xor rdi, rdi
  xor rsi, rsi
  xor rdx, rdx
  jmp syscall_return

extern syscall_get_fb_vaddr
extern syscall_get_fb_dims
extern syscall_get_fb_pitch

get_framebuffer_syscall:
  call syscall_get_fb_vaddr
  push rax
  call syscall_get_fb_dims
  push rax
  call syscall_get_fb_pitch
  xor rsi, rsi
  mov esi, eax
  pop rdi
  pop rax
  xor rdx, rdx
  jmp syscall_return

extern syscall_copy_framebuffer

draw_framebuffer_syscall:
  call syscall_copy_framebuffer
  xor rax, rax
  xor rdi, rdi
  xor rsi, rsi
  xor rdx, rdx
  jmp syscall_return

bad_syscall:
  xor rax, rax
  xor rdi, rdi
  xor rsi, rsi
  xor rdx, rdx
  jmp syscall_return

syscall_entry:
  mov r11, rsp
  mov rsp, 0xfffffffffffff000
  push r11
  push rcx

  cmp rax, 0
  je encode_color_syscall
  cmp rax, 1
  je get_framebuffer_syscall
  cmp rax, 2
  je draw_framebuffer_syscall
  jmp bad_syscall

syscall_return:
  xor r8, r8
  xor r9, r9
  xor r10, r10
  xor r11, r11
  or r11, 0x200
  pop rcx
  pop rsp

  o64 sysret

start_user_mode:
;intended rip in rdi
;intended rsp in rsi
;intended p4_paddr in rdx

  mov rax, rdx
  mov cr3, rax

  ;efer <- efer | 0x1
  mov rcx, 0xc0000080
  rdmsr
  or al, 1
  wrmsr

  ;lstar <- syscall_entry
  mov rdx, syscall_entry
  mov eax, edx
  shr rdx, 32
  mov ecx, 0xc0000082
  wrmsr

  ;star <- 0x0030.0028.0000.0000
  mov edx, 0x00300028
  xor eax, eax
  mov ecx, 0xc0000081
  wrmsr

  mov rcx, rdi
  mov rsp, rsi
  xor r11, r11
  or r11, 0x200

  xor rax, rax
  xor rbx, rbx
  xor rdx, rdx
  xor rdi, rdi
  xor rsi, rsi
  xor rbp, rbp
  xor r8, r8
  xor r9, r9
  xor r10, r10
  xor r12, r12
  xor r13, r13
  xor r14, r14
  xor r15, r15

  o64 sysret