Linking multiple .asm files in Visual Studio - visual-studio

Question

I use visual studio 2017
Lets say I one assembly file named "Factorial.asm" and I break it into two .asm files named "one.asm" and "two.asm":
Factiorial.asm works just fine.
Factorial.asm contains
.386
.model flat, stdcall
option casemap :none
includelib \masm32\lib\msvcrt.lib
sprintf proto C :vararg
includelib \masm32\lib\user32.lib
MessageBoxA proto :ptr,:ptr,:ptr,:DWORD
includelib \masm32\lib\kernel32.lib
ExitProcess proto :dword
.data
format db "%llu", 13, 10, 0
_title db "Result",13,10,0
.code
main PROC
LOCAL szBuf[9]:byte
mov eax, 15 ; initial value (low-order bits)
xor edx, edx ; initial value's high-order bits are 0
mov ecx, eax ; loop counter
Factorial:
dec ecx ; decrement counter
jz Finished ; when counter == 0, we're done
mov ebx, ecx ; make copy of counter
imul ebx, edx ; high-order bits * multiplier
mul ecx ; low-order bits * multiplier
add edx, ebx ; add high-order product to high-order bits of low-order product
cmp ecx, 1
jg Factorial ; keep looping as long as counter > 1
Finished:
invoke sprintf, addr szBuf, offset format, eax, edx
invoke MessageBoxA, 0, addr szBuf, offset _title, 0
invoke ExitProcess, 0
main ENDP
one.asm contains
.386
.model flat, stdcall
option casemap :none
includelib \masm32\lib\msvcrt.lib
sprintf proto C :vararg
includelib \masm32\lib\user32.lib
MessageBoxA proto :ptr,:ptr,:ptr,:DWORD
includelib \masm32\lib\kernel32.lib
ExitProcess proto :dword
.data
format db "%llu", 13, 10, 0
_title db "Result",13,10,0
.code
main PROC
LOCAL szBuf[9]:byte
mov eax, 15 ; initial value (low-order bits)
xor edx, edx ; initial value's high-order bits are 0
mov ecx, eax ; loop counter
Factorial:
dec ecx ; decrement counter
jz Finished ; when counter == 0, we're done
mov ebx, ecx ; make copy of counter
imul ebx, edx ; high-order bits * multiplier
mul ecx ; low-order bits * multiplier
add edx, ebx ; add high-order product to high-order bits of low-order product
cmp ecx, 1
jg Factorial ; keep looping as long as counter > 1
main ENDP
two.asm contains
Finished:
invoke sprintf, addr szBuf, offset format, eax, edx
invoke MessageBoxA, 0, addr szBuf, offset _title, 0
invoke ExitProcess, 0
How would I link "one.asm" and "two.asm" using Visual Studio 2017. Or in other words, call labels from separate .asm files?

Answer 1

Use the directive extrn to declare functions outside of the current source file:
extrn foo:proc
You can optionally use public to declare local functions as public, but I think functions are public by default.
Since VS2015, printf is now inlined with C / C++ code, at least in the case of 64 bit builds. One way to deal with this is to have a C / C++ source file that makes a reference to printf, in which case the assembly code can then access printf. I don't know if this applies to sprintf also. You'll get a link error if this is an issue.

Answer 2

There are usually three types of objects that you need to transfer information across files to.
Proc defined in source file called for in user file : By default, in masm, all procs are global. So, the source file needs no declaration, the user file needs a declaration :
extern <proc_name> : proc
Variable defined in source file and called by a procedure in user file : In this situation, the source file needs a declaration
public <var_name>
and the user file must contain the declaration :
extern <varname> : var_size ; where var_size is word, qword ymmword...
It is NOT mandatory that the declaration size must match the extern size. However, consequences of not ensuring this match are usually expensive. Also, if a variable has been declared as an extern, and subsequently, not used in that file, it still must match a corresponding public declaration in some file, otherwise linker will fail.
Label defined in source file and jumped to from a user file : identical treatment as a variable (case 2) is treated
Trust this helps.