Community
cancel
Showing results for 
Search instead for 
Did you mean: 
DParks
Beginner
420 Views

Difference L2 size between CPUID(4) and CPUID(80000006h)

It is not obvious from the descriptions of the L2 cache sizes using CPUID(4) and CPUID(80000006h) in the software developer's manual volume 2 (325383-063US), why the two inquiry methods would return different sizes.

 

CPUID(80000006h) returns (eax, ebx, ecx, edx): 0 0 0x1006040 0. Documentation says that bits 31:16 of ecx is L2 cache size in K units, which translates to 256K.

 

The L2 cache information from CPUID(4) returns (eax, ebx, ecx, edx): 0x7c004143 0x3c0003f 0x3ff 0, which according to (heading page 3-216 Vol 2A.) "INPUT EAX = 04H: Returns Deterministic Cache Parameters for Each Level" size is computed as:

This Cache Size in Bytes

= (Ways + 1) * (Partitions + 1) * (Line_Size + 1) * (Sets + 1)

= (EBX[31:22] + 1) * (EBX[21:12] + 1) * (EBX[11:0] + 1) * (ECX + 1)

Which I believe given the ebx/ecx values comes to 1048576.

/proc/cpuinfo (Linux) shows:

processor    : 0

vendor_id    : GenuineIntel

cpu family   : 6

model      : 85

model name   : Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz

stepping    : 4

microcode    : 0x200004d

cpu MHz     : 1000.000

 

My question is: which method is correct? The 1MB L2 cache size as returned by CPUID(4) seems to match with all public information for this processor/model.

 

BTW, the discrepancy in L2 sizes only seems to happen with the Skylake Xeon processor that I have access to.

 

Dave

 

Here's my driver to the CPUID calls:

#include <stdio.h> #include <stdint.h> int cpuid_ecx(uint32_t eax, uint32_t *res, uint32_t ecx) { asm("cpuid" : "=a"(res[0]), "=b"(res[1]), "=c"(res[2]), "=d"(res[3]) : "a"(eax), "c"(ecx) :); }   int main() { uint32_t r[4]; uint32_t ecx = 0; uint32_t csz;   cpuid_ecx(0x80000006, r, ecx); printf("%u %#x %#x %#x %#x\n", ecx, r[0], r[1], r[2], r[3]); printf("0x80000006 L2 size=%u\n", r[2] >> 16);   while (1) { cpuid_ecx(0x4, r, ecx); printf("%u %#x %#x %#x %#x\n", ecx, r[0], r[1], r[2], r[3]); if (r[0] == 0) break; csz = (r[1] >> 22) + 1; csz *= ((r[1] >> 12) & 0x3ff) + 1; csz *= (r[1] & 0x3ff) + 1; csz *= r[2] + 1; printf("L%u size = %u\n", (r[0] >> 5) & 0x3, csz); ++ecx; } }

 

 

 

0 Kudos
1 Reply
Oscar_A_Intel1
Employee
55 Views

Hello DParks, Thank you for posting on the Intel Community. We provide basic troubleshooting for processors, mini PCs, etc.. For this type of inquiries please go to the Intel® Developer Zone for the appropriate support. https://software.intel.com/en-us/home Intel® Developer Zone forums: https://software.intel.com/en-us/forum Regards, Oscar A. Intel Customer Support Technician Under Contract to Intel Corporation
Reply