Windows MASM / CPU Counting

Author __Thanat0s__
Platform Windows
Language MASM
Technique CPU Counting

Description:

This code snippet is written in assembly language and is used to detect the hyperthreading capacity and number of CPUs in a system.

For detecting hyperthreading capacity, the code sets EAX to 1 to access the leaf processor info and feature bits. It then calls CPUID instruction, shifts the 28th bit (which is the hyperthread bit on Intel processors), and clears other bits with an AND instruction. If the result in EDX is 0, it means that the system does not have hyperthreading capacity.

For detecting the number of CPUs, the code retrieves the Processor Environment Block (PEB) and gets the CPU count information stored at an offset. It then decrements the value and checks if it is not equal to zero. If it is not equal to zero, it means that there is more than one CPU in the system.

Code

Detect hypertreading capacity through CPUID
   mov eax, 1 ; Set EAX to 1 in order to access to the leaf processor info and feature bits
   cpuid ; call cpuid
   shr edx,28 ; shift the bit 28 ( which is hypertread bit on intel )
   and edx,1 ; cleanup
   ; 0 in edx means no hyperthearding capacity.

Detect unique cpu through PEB
   mov   eax, [fs:0x30]    ; Get PEB
   mov   eax,[eax+0x64]    ; Get Cpu Count
   dec   eax
   jnz   _isnot_pebuniq    ; If eax = 0 , we have only one CPU
   ret
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Created

January 30, 2023

Last Revised

April 22, 2024