Broken CPUID brand string? - c++

Question

I am printing some information about CPU in my OS using CPUID instruction.
Reading and printing vendor string(GenuineIntel) works well, but reading brand string gives me little strange string.
ok cpu-info <= Run command
CPU Vendor name: GenuineIntel <= Vendor string is good
CPU Brand: D: l(R) Core(TMD: CPU MD: <= What..?
ok
Vendor string supposed to be:
Intel(R) Core(TM) i5 CPU M 540
But what I got is:
D: l(R) Core(TMD: CPU MD:
C++ code:
char vendorString[13] = { 0, };
Dword eax, ebx, ecx, edx;
ACpuid(0, &eax, &ebx, &ecx, &edx);
*((Dword*)vendorString) = ebx;
*((Dword*)vendorString + 1) = edx;
*((Dword*)vendorString + 2) = ecx;
Console::Output.Write(L"CPU vendor name: ");
for (int i = 0; i < 13; i++) {
Console::Output.Write((wchar_t)(vendorString[i]));
}
Console::Output.WriteLine();
char brandString[48] = { 0, };
ACpuid(0x80000002, &eax, &ebx, &ecx, &edx);
*((Dword*)brandString) = eax;
*((Dword*)brandString + 1) = ebx;
*((Dword*)brandString + 2) = ecx;
*((Dword*)brandString + 3) = edx;
ACpuid(0x80000003, &eax, &ebx, &ecx, &edx);
*((Dword*)brandString + 4) = eax;
*((Dword*)brandString + 5) = ebx;
*((Dword*)brandString + 6) = ecx;
*((Dword*)brandString + 7) = edx;
ACpuid(0x80000004, &eax, &ebx, &ecx, &edx);
*((Dword*)brandString + 8) = eax;
*((Dword*)brandString + 9) = ebx;
*((Dword*)brandString + 10) = ecx;
*((Dword*)brandString + 11) = edx;
Console::Output.Write(L"CPU brand: ");
for (int i = 0; i < 48; i++) {
Console::Output.Write((wchar_t) brandString[i]);
}
Console::Output.WriteLine();
NOTE:
This program is UEFI application. No problem with permissions.
Console is an wrapper class for EFI console. Not C# stuff.
Dword = unsigned 32bit integer
Assembly code(MASM):
;Cpuid command
;ACpuid(Type, pEax, pEbx, pEcx, pEdx)
ACpuid Proc
;Type => Rcx
;pEax => Rdx
;pEbx => R8
;pEcx => R9
;pEdx => [ rbp + 48 ] ?
push rbp
mov rbp, rsp
push rax
push rsi
mov rax, rcx
cpuid
mov [ rdx ], eax
mov [ r8 ], ebx
mov [ r9 ], ecx
mov rsi, [ rbp + 48 ]
mov [ rsi ], rdx
pop rsi
pop rax
pop rbp
ret
ACpuid Endp

Answer 1

I agree with Ross Ridge that you should use the compiler intrinsic __cpuid. As for why your code likely doesn't work as is - there are some bugs that will cause problems.
CPUID destroys the contents of RAX, RBX, RCX, and RDX and yet you do this in your code:
cpuid
mov [ rdx ], eax
RDX has been destroyed by the time mov [ rdx ], eax is executed, rendering the pointer in RDX invalid. You'll need to move RDX to another register before using the CPUID instruction.
Per the Windows 64-bit Calling Convention these are the volatile registers that need to be preserved by the caller:
The registers RAX, RCX, RDX, R8, R9, R10, R11 are considered volatile and must be considered destroyed on function calls (unless otherwise safety-provable by analysis such as whole program optimization).
These are the non-volatile ones that need to be preserved by the callee:
The registers RBX, RBP, RDI, RSI, RSP, R12, R13, R14, and R15 are considered nonvolatile and must be saved and restored by a function that uses them.
We can use R10 (a volatile register) to store RDX temporarily. Rather than use RSI in the code we can reuse R10 for updating the value at pEdx. We won't need to preserve RSI if we don't use it. CPUID does destroy RBX, and RBX is non-volatile, so we need to preserve it. RAX is volatile so we don't need to preserve it.
In your code you have this line:
mov [ rsi ], rdx
RSI is a memory address (pEdx) provided by the caller to store the value in EDX. The code you have would move the contents of the 8-byte register RDX to a memory location that was expecting a 4-byte DWORD. This could potentially trash data in the caller. This really should have been:
mov [ rsi ], edx
With all of the above in mind we could code the ACpuid routine this way:
option casemap:none
.code
;Cpuid command
;ACpuid(Type, pEax, pEbx, pEcx, pEdx)
ACpuid Proc
;Type => Rcx
;pEax => Rdx
;pEbx => R8
;pEcx => R9
;pEdx => [ rbp + 48 ] ?
push rbp
mov rbp, rsp
push rbx ; Preserve RBX (destroyed by CPUID)
mov r10, rdx ; Save RDX before CPUID
mov rax, rcx
cpuid
mov [ r10 ], eax
mov [ r8 ], ebx
mov [ r9 ], ecx
mov r10, [ rbp + 48 ]
mov [ r10 ], edx ; Last parameter is pointer to 32-bit DWORD,
; Move EDX to the memory location, not RDX
pop rbx
pop rbp
ret
ACpuid Endp
end