X86-64: Canonical Addresses and Actual Available Range

Question

Intel and AMD documentation says that for 64-bit mode, only 48 bits are actually available for virtual addresses, and bits 48 to 63 should replicate bit 47 (character extension). As far as I know, all current CPUs are implemented in this way, but nothing (theoretically) allows us to expand the available space in future implementations (and this will not break binary compatibility).

Is there a standard way to programmatically determine the number of significant bits? (that is, some specific CPUID, as is the case for physical addresses).

I know that in practice, 48 bits are much more than enough for any reasonable application and OS; my question is theoretical.

Answer 1

Yes, you can use CPUID.80000008H:EAX[7:0]if supported.

The algorithm is as follows:

• Check the maximum extended value of E for cpuidwith CPUID.80000000h.EAX.
• If E> = 80000008h, use    CPUID.80000008H:EAX[7:0]for the number of bits of physical addresses.
    CPUID.80000008H:EAX[15:8]for the number of linear address bits.
• Otherwise, if CPUID.1:EDX.PAE(bit 6), then the CPU has 36 physical address bits and 32 linear address bits.
• Otherwise, the CPU has 32 physical and logical address bits.

---

Intel designation CPUID.X:R Bmeans

• X , eax cpuid.
• R , .
• B [: ] .bitname.

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AMD : 63:52.
48:40, .

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, NASM

BITS 64

GLOBAL max_phy_addr
GLOBAL max_lin_addr


SECTION .text


max_phy_addr:
 push rbx


 mov eax, 80000000h
 cpuid

 cmp eax, 80000008h
 jae .fromCpuid

 mov eax, 1
 cpuid

 mov eax, 32
 shr edx, 4
 and edx, 4

 add eax, edx

 pop rbx
 ret

.fromCpuid:

 mov eax, 80000008h
 cpuid

 movzx eax, al

 pop rbx
 ret


max_lin_addr:
 push rbx

 mov eax, 80000000h
 cpuid

 cmp eax, 80000008h
 jae .fromCpuid

 mov eax, 1
 cpuid

 mov eax, 32

 pop rbx
 ret

.fromCpuid:

 mov eax, 80000008h
 cpuid

 movzx eax, ah

 pop rbx
 ret

C,

#include <stdio.h>

long max_phy_addr();
long max_lin_addr();

int main()
{
   printf("Phy: %llu\nLin: %llu\n", max_phy_addr(), max_lin_addr());
   return 0;
}

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, - 48:39!