C++ to Assembly

0x0 start

A simple code:

compile it and check its assembly:

two instructions:

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.text:004116D0                 push    ebp
.text:004116D1                 mov     ebp, esp

ebp saves current function (main function in this code) stack base address. When call another function, the process needs to use that function’s stack space. So it will save current stack base address and assign esp to ebp. use [ebp +??] to access function’s aguments and use [ebp - ??] to access function’s variable.

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.text:004116D3                 sub     esp, 0C0h
.text:004116D9                 push    ebx
.text:004116DA                 push    esi
.text:004116DB                 push    edi
.text:004116DC                 lea     edi, [ebp+var_C0]
.text:004116E2                 mov     ecx, 30h
.text:004116E7                 mov     eax, 0CCCCCCCCh
.text:004116EC                 rep stosd

sub esp, 0C0h will allocate a stack space,

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.text:004116D9                 push    ebx
.text:004116DA                 push    esi
.text:004116DB                 push    edi

save these values.

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.text:004116DC                 lea     edi, [ebp+var_C0]
.text:004116E2                 mov     ecx, 30h
.text:004116E7                 mov     eax, 0CCCCCCCCh
.text:004116EC                 rep stosd

rep is repeat these instructions. ECX is the time of these actions and assign these value as ccccc…, since this is debug version.

0x1 Data Type

Float

code:

Visual Studio 2015 assembly code:

movss: , Move Scalar Single-Precision Floating-Point Values.

cvtsi2ss: Convert one signed doubleword integer from r/m32 to one single-precision floating-point number in xmm.

cvtss2sd: Convert Scalar Single-Precision Floating-Point Value to Scalar Double-Precision Floating-Point Value

cvttss2si: Convert with Truncation Scalar Single-Precision Floating-Point Value to Integer

String

code:

VS 2015 assembly:

wchar_t is intended for representing text in fixed-width, multi-byte encodings; since wchar_t is usually 2 bytes in size it can be used to represent text in any 2-byte encoding. It can also be used for representing text in variable-width multi-byte encodings of which the most common is UTF-16.

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Pointer

code:

VS 2015 assembly:

Pointer is 32-bits, thats why its type is dword ptr. But in the above, since data type is different, it uses dword ptr, byte ptr and word ptr.

In C++, pointer can only use add and sub. (because it stands for address).

code:

different types of pointer add 1 depends on its type:

Reference

Code:

Here nVarType saves nVar’s address

check Add function:

eax saves nVar’s address.

Constant

Code:

compiler will replace NUMBER_ONE to number 10:

0x2 Operation

Add

code:

assembly code:

Looks like compiler did some work.

If we change the compiler options:

The final result will be much clear:

Multiplication

code:

compiler will try to use shift instruction if the argument is 2 4 8 etc. Otherwise it will use mul or imul

If there is a mixture of add and mul, compiler will use LEA instruction:

Div

code:

assembly:

cdq copies the sign (bit 31) of the value in the EAX register into every bit position in the EDX register.

reverse