Calling Assembly from C++

• Nearly everything from the last set of notes applies here as well.
• Most importantly follow the calling convention.
• For today:
  • cCaller.cpp
  • myASMlib.asm
• A no parameter/no return value call

  • ; asm
    global PRINT_HELLO
    ...
    section .data
       HELLO_STRING db `Hello from Assembly!\n`,0
       helloLen equ $ - HELLO_STRING
    ...
    PRINT_HELLO:
        push rbp
        mov rbp, rsp
    
        mov rdi, HELLO_STRING
        mov rsi, helloLen
        call PRINT_STRING
    
        pop rbp
        ret
    
    // c++ 
    extern "C" void PRINT_HELLO();
    ...
    PRINT_HELLO(); 

• Passing a string, and string length
  • This uses PRINT_STRING from my MY_IO library

  • // c++
    extern "C" void PRINT_STRING(const char  phrase[], int size);
    ...
    string message{"Hello from C++/from Assembly!\n"}; 
    ...
    PRINT_STRING(message.c_str(), message.size());

• Printing a number is the same as print string.
• Returning an integer

  • ; nasm
    global DOUBLE_NUMBER
    ...
    ; rdi holds the number
    ; return value in rax
    DOUBLE_NUMBER:
    
    mov rax, rdi
    add rax, rax
    ret
    // c++
    int number{100};
    ...
    number = DOUBLE_NUMBER(number); 

• A pass by pointer (reference?)

  • ; nasm
    ; rdi holds the address of the number;
    ; at first I was using r8, but crashed on return
    ; I was over writing memory that was not part of the variable.
    DOUBLE_IN_PLACE:
        mov r8d, DWORD [rdi]
        add r8d, r8d
        mov DWORD [rdi], r8d
        ret
    // c++
    extern "C" int DOUBLE_IN_PLACE(int & number);
    ...
    DOUBLE_IN_PLACE(number);

• Passing an array:

  • ; rdi holds the address of the array
    ; rsi holds the size of the array
    ; rdx holds the fill value
    FILL_ARRAY:
        push rbp
        mov rbp, rsp
        push r15 ; lcv
        push r14 ; offset
    
        mov r15, 0
        mov r14, 0
    .top:
        cmp r15, rsi
        je  .done
    
        mov [rdi+r14], rdx
    
        add r14, 4
        inc r15
        jmp .top
    .done:
    
        pop r14
        pop r15
        pop rbp
        ret
    // c++
    extern "C" void FILL_ARRAY(int array[], int size, int fillvalue);
    ...
    iint values[SIZE];
    ...
    FILL_ARRAY(values, SIZE, 100);