Assembly Attribute

An Attribute which declares a method to emit literal assembly code.

Attribute

[AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
public class AssemblyAttribute : Attribute>
{
    public readonly bool CalliHeader;
    public AssemblyAttribute(bool requiredCallingHeader = false) { ... }
}

• CalliHeader

• Tells Compiler to emit calli assembly stubs.
• Follows __stdcall calling convention.
• Default Value: false

Usage

• Declaration

using Atomixilc.Machine;
using Atomixilc.Attributes;
using Atomixilc.Machine.x86;
...
[Assembly]
internal static int Random()
{
   // Do Something
}

• To Enable __stdcall calli stubs use [Assembly(true)].
• Method Body

• Assembly Methods are directly invoked by the Compiler.
• Directly invocation causes no direct link to non-Assembly stuffs.
• Like It is impossible to refer parameters/fields/functions directly.
• Assembly Instruction can be added by creating an instance of required instruction.

• Example - (CalliHeader = false)

• C# code void Test()

 [Assembly]
 internal static int Test()
 {
     new Cli(); // clear interrupts
     // push ESP register onto the stack
     new Push { DestinationReg = Register.ESP };
     new Sti(); // store interrupts
 }

• Generated code

 cli
 push dword ESP
 sti

• Example - (CalliHeader = true)

• C# code

 [Assembly(true)]
 internal static int Test()
 {
     new Cli(); // clear interrupts
     // push ESP register onto the stack
     new Push { DestinationReg = Register.ESP };
     new Sti(); // store interrupts
 }

• Generated code

 push dword EBP
 mov dword EBP, ESP
 cli
 push dword ESP
 sti
 xor dword ECX, ECX
 leave
 ret 0x0

Behavior

• CalliHeader = false

• Return value should be saved in EAX register.
• Make sure you clear ECX register before exiting. (Read: Exceptions)
• You can use NoException Attribute if does not want to clear ECX register.
• Last Argument: [ESP + 0x4] (ESP: Initial Stack Position)
• Second Last Argument: [ESP + 0x4 + sizeof(Last Argument)] and so on

• CalliHeader = true

• Return value should be pushed onto the stack.
• Last Argument: [EBP + 0x8]
• Second Last Argument: [EBP + 0x8 + sizeof(Last Argument)] and so on

Instructions

• DestinationSourceSize

• Mnemonic SIZE DES, SRC
• SIZE (bytes) = {BYTE (8),WORD (16),DWORD (32 default), QWORD (64)}
• DES/SRC = {Register Reference, Memory Reference} + {Displacement, Indirect}

• DestinationSource

• Mnemonic DES, SRC

• DestinationOnly

• Mnemonic DES

• OnlySize

• Mnemonic SIZE

Example

• Return Sum of Two Numbers

  [Assembly(true)]
  internal static int Sum(int A, int B)
  {
      // mov eax, [ebp+0x8] ; mov eax, B
      new Mov
      {
          DestinationReg = Register.EAX,
          SourceReg = Register.EBP,
          SourceDisplacement = 0x8,
          SourceIndirect = true
      };
      // mov ebx, [ebp+0xc] ; mov ebx, A
      new Mov
      {
          DestinationReg = Register.EBX,
          SourceReg = Register.EBP,
          SourceDisplacement = 0xC,
          SourceIndirect = true
      };
      // add eax, ebx
      new Add { DestinationReg = Register.EAX, SourceReg = Register.EBX };
      new Push { DestinationReg = Register.EAX };
  }