; 2pm.asm

format binary

   use32
   org   0x100 ; DOS COM application

   ; clear screen
   mov   ax, 0x3
   int   0x10

   ; open A20-line
   in    al, 0x92
   or    al, 0x2
   out   0x92, al

   ; calculate linear address of protected mode entry point
   xor   eax, eax
   mov   ax, cs
   shl   eax, 0x4
   mov   ebx, eax
   add   eax, pm_entry_point
   mov   [pmep], eax

   ; calculate linear address of global descriptor table
   mov   eax, ebx
   add   eax, gdt
   mov   [gdtr_address], eax

   ; load global descriptor table
   lgdt  fword [gdtr]

   ; disable maskable interrupts
   cli

   ; disable non maskable interrupts
   in    al, 0x70
   or    al, 0x80
   out   0x70, al

   ; switch to protected mode
   mov   eax, cr0
   or    al, 0x1
   mov   cr0, eax

   ; load new selector to CS reister
   db    0x66  ; prefix to change operand capacity
   db    0xEA  ; opcod of JMP FAR command
   pmep  rd 1  ; 32-bit offset
   dw    00001000b ; selector of the first descriptor

; Global Descriptor Table
gdt: times 8 db 0x0

; Descriptors

; code segment descriptor: base = 0x0, size = 0xFFFFFFFF, 32bit, DPL=3
dCode  db 0xFF, 0xFF, 0x0, 0x0, 0x0, 10011100b, 11001111b, 0x0

; data segment descriptor: base = 0x0, size = 0xFFFFFFFF, 32bit, DPL=3
dData  db 0xFF, 0xFF, 0x0, 0x0, 0x0, 10010100b, 11001111b, 0x0

; video segment descripor: base =  0x0B80, size = 0xFFFF, 32bit, DPL=3
dVideo db 0FFh, 0FFh, 00h, 80h, 0Bh, 10010100b, 00000010b, 00h

; Global Descriptor Table Size
szgdt  db $ - gdt

; Data to be loaded into gdtr
gdtr:
gdtr_size dw szgdt - 1 ; size of global descriptor table
gdtr_address rd 1      ; linear address of global desriptor table

; message which will be printed with in protected mode
; (hello from pm, every symbol with attribute 0x7)
msg_hello_from_pm db 'h',7,'e',7,'l',7,'l',7,'o',7,' ',7,'f',7,'r',7,'o',7,'m',7,' ',7,'p',7,'m',7
szmsg_hello_from_pm dd  $ - msg_hello_from_pm ; size of string (include attributes)

align 0x4
pm_entry_point:

   ; linear address of message to ESI
   xor   esi, esi
   mov   si, ds
   shl   esi, 0x4
   add   esi, msg_hello_from_pm

   ; define symbolic names for segment selectors
   SDATA equ 00010000b ; dData segment selector
   SVIDEO equ 00011000b ; dVideo segment selector

   ; selector for dData segment to DS
   mov   ax, SDATA
   mov   ds, ax

   ; selector for a dVideo segment to ES
   mov   ax, SVIDEO
   mov   es, ax

   ; print 'hello from pm'
   xor   edi, edi ; position on the screen (relativy from base of dVideo segment 0B8000h)
   mov   ecx, [szmsg_hello_from_pm]
   rep   movsb

   jmp   $
    

; print 'hello from pm' 
   xor   edi, edi 
   mov   ecx, [szmsg_hello_from_pm] 
   rep   movsb
    

   xor   edi, edi 
   xor   eax, eax
   mov   ax, ds
   shl   eax, 0x4
   add   eax, szmsg_hello_from_pm
   mov   ecx, [ds:eax]
   rep   movsb