Chapter 3.6

Introducing 64-bit Programming

Simple AddTwo example for 64-bit MASM

ExitProcess PROTO

.data
	sum DWORD 0
.code
main PROC
	mov eax, 5
	add eax, 6
	mov sum, eax
	
	mov ecx, 0
	call ExitProcess
main ENDP
END

The differences from the 32-bit version:

• .386 .model and .stack directives are not used
• Statements using PROTO do not have parameters
• No INVOKE directive, ExitProcess called using call
• mov ecx, 0 before calling ExitProcess. INVOKE did it automatically.
• END directive does not specify entry point

Using 64-bit registers

To use 64-bit registers, we change EAX into RAX, and also have to define sum as QWORD to be able to move data into it using simple mov

.data
	sum QWORD 0
.code
main PROC
	mov rax, 5
	add rax, 6
	mov sum, rax
	...