mulx.asm

use16

org 0x7c00

cli
jmp far 0x0000:_start

gdtd:
dw gdt_end - gdt - 1
dd gdt

gdt:
dq 0                    ; 0x00: NULL
dq 0x00cf9a000000ffff   ; 0x08: 32 bit CS
dq 0x00cf92000000ffff   ; 0x10: 32 bit DS
dq 0x00af9a000000ffff   ; 0x18: 64 bit CS

gdt_end:

_start:
push    cs
pop     ds

lgdt    [ds:gdtd]

mov     eax,cr0
or      eax,0x00000001
mov     cr0,eax

jmp     0x08:pm

use32

pm:

mov     ax,0x10
mov     ds,ax
mov     es,ax
mov     ss,ax
mov     esp,0xa0000

mov     eax,cr4
or      eax,0x20
mov     cr4,eax

mov     edi,0x300000

mov     eax,0x30100f
stosd
mov     ecx,0x03ff
xor     eax,eax
rep     stosd

mov     eax,0x30200f
stosd
mov     ecx,0x03ff
xor     eax,eax
rep     stosd

mov     ecx,0x0200
mov     esi,0x008b

map_loop:
mov     eax,esi
stosd
xor     eax,eax
stosd
add     esi,0x200000
sub     ecx,1
jnz     map_loop

mov     eax,0x300000
mov     cr3,eax

mov     ecx,0xc0000080
rdmsr
or      eax,0x00000100
wrmsr

mov     eax,cr0
or      eax,0x80000000
mov     cr0,eax

jmp     0x18:lm

use64

lm:

mov     rdx,0x0000111111111111
mulx    rsi,rdi,rdx
mulx    rax,rax,rdx

hlt


times 510-($-$$) db 0
dw 0xaa55