What are CWD, CDQ and CQO instructions in x86 and x64 assembly?

by | May 31, 2022 | Reverse Engineering

0000000000000000 <main>:
   0:	push   rbp
   1:	mov    rbp,rsp
   4:	mov    DWORD PTR [rbp-0xc],0x1
   b:	mov    DWORD PTR [rbp-0x8],0x2
  12:	mov    eax,DWORD PTR [rbp-0xc]
  15:	cdq    
  16:	idiv   DWORD PTR [rbp-0x8]
  19:	mov    DWORD PTR [rbp-0x4],eax
  1c:	mov    eax,0x0
  21:	pop    rbp
  22:	ret    

Here, the first two assembly instructions are part of the function prologue that is called every time a function is called. The RBP register points to the base of the stack and the local variables are referenced using the RBP register.

We are assigning 0x1 to the local variable a referenced by [RBP – 0xc] and 0x2 to the local variable b referenced by [RBP – 0x8].

Stack Frame CDQ Instruction In x86 And x64 Assembly

After that, we are moving the content of the variable referenced by [RBP – 0xc] to the EAX register. Now, we are using the CDQ instruction to sign-extend the content of the EAX register so that the value of the EAX register is stored in EDX:EAX register. Please note that we are using integers in the C code that are 32 bits in length and hence, we are using EDX and EAX registers.

Now, we are using the idiv instruction to divide the content of EDX:EAX registers by the local variable b referenced by [RBP – 0x8]. After the division operation, the quotient will be stored in the EAX register and the remainder will be stored in the EDX register.

So, we are now moving the quotient or the content of the EAX register to the local variable c referenced by [RBP – 0x4].

The function now returns 0x0 through the EAX register and then, the function epilogue instructions are executed.

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Amrita Mitra

Author

Ms. Amrita Mitra is an author, who has authored the books “Cryptography And Public Key Infrastructure“, “Web Application Vulnerabilities And Prevention“, “A Guide To Cyber Security” and “Phishing: Detection, Analysis And Prevention“. She is also the founder of Asigosec Technologies, the company that owns The Security Buddy.

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