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align 4

proc init_mtrr

        cmp     [BOOT_VARS+BOOT_MTRR], byte 2

        je      .exit

        bt      [cpu_caps], CAPS_MTRR

        jnc     .exit

        mov     eax, cr0

        or      eax, 0x60000000 ;disable caching

        mov     cr0, eax

        wbinvd                  ;invalidate cache

        mov     ecx, 0x2FF

        rdmsr                   ;

; has BIOS already initialized MTRRs?

        test    ah, 8

        jnz     .skip_init

; rarely needed, so mainly placeholder

; main memory - cached

        push    eax

        mov     eax, [MEM_AMOUNT]

; round eax up to next power of 2

        dec     eax

        bsr     ecx, eax

        mov     ebx, 2

        shl     ebx, cl

        dec     ebx

; base of memory range = 0, type of memory range = MEM_WB

        xor     edx, edx

        mov     eax, MEM_WB

        mov     ecx, 0x200

        wrmsr

; mask of memory range = 0xFFFFFFFFF - (size - 1), ebx = size - 1

        mov     eax, 0xFFFFFFFF

        mov     edx, 0x0000000F

        sub     eax, ebx

        sbb     edx, 0

        or      eax, 0x800

        inc     ecx

        wrmsr

; clear unused MTRRs

        xor     eax, eax

        xor     edx, edx

@@:

        inc     ecx

        wrmsr

        cmp     ecx, 0x20F

        jb      @b

; enable MTRRs

        pop     eax

        or      ah, 8

        and     al, 0xF0; default memtype = UC

        mov     ecx, 0x2FF

        wrmsr

.skip_init:

        stdcall set_mtrr, [LFBAddress], [LFBSize], MEM_WC

        wbinvd                  ;again invalidate

        mov     eax, cr0

        and     eax, not 0x60000000

        mov     cr0, eax        ; enable caching

.exit:

        ret

endp

align 4

proc set_mtrr stdcall, base:dword,size:dword,mem_type:dword

; find unused register

        mov     ecx, 0x201

@@:

        rdmsr

        dec     ecx

        test    ah, 8

        jz      .found

        rdmsr

        mov     al, 0; clear memory type field

        cmp     eax, [base]

        jz      .ret

        add     ecx, 3

        cmp     ecx, 0x210

        jb      @b

; no free registers, ignore the call

.ret:

        ret

.found:

; found, write values

        xor     edx, edx

        mov     eax, [base]

        or      eax, [mem_type]

        wrmsr

        mov     ebx, [size]

        dec     ebx

        mov     eax, 0xFFFFFFFF

        mov     edx, 0x00000000

        sub     eax, ebx

        sbb     edx, 0

        or      eax, 0x800

        inc     ecx

        wrmsr

        ret

endp