Persistent Realities » Home

Site
News
Files

Visual Basic
Strings
Math
General
Properties
Memory
Methods

Inline ASM-VB
Strings
Math
General
Memory

Example: Using ASM and a UDT (user defined type)

This simple example will demonstrate how you can use an UDT in ASM. Right now I couldn't find a good example yet, so this is a very simple addition to the second item in the udt.

It took me a while to figure out that I shouldn't add more to [EBP+8], which is already the pointer. I had to add to the actual pointer location, which the code below demonstrates.

Public Type myUDT
myLong1 As Long
myLong2 As Long
myLong3 As Long
End Type

'//Usage example:
Dim I As Long
Dim myType As myUDT
myType.myLong1 = 1
myType.myLong2 = 2
myType.myLong3 = 4

I = mdlASM.asmUDT(myType)

MsgBox "Return: " & I & " UDT myLong2, was 2 is now:" & myType.myLong2

Public Function asmUDT(udt As myUDT) As Long
'#ASM_START
' push ebp
' mov ebp, esp

' ;This will get the first argument. UDT's are byref, so this is a POINTER to the actual UDT.
' ;Now, don't make the mistake that ebp+12 is the next item in the Type. It isn't!

' mov eax, [ebp+8] ; First argument. This is also the first long, myLong1!

' ;If we add 4.. we get to our second item in the Type.. myLong2!
' add eax, 4

' ;Move the "pointer to value" to eax, so we get the actual value instead of our pointer!
' mov eax, [eax]

' ;We'll put 4 in ecx register, and multiply it with eax.
' mov ecx, 4
' mul ecx

' ;The result is now in eax, which is returned by this function. But we also want to change
' ;The actual item in the udt!

' ;Move the eax contents backto our saved pointer ([ebp+8] + 4)
' mov edx, [ebp+8] ; First argument. This is also the first long, myLong1!
' add edx, 4 ;If we add 4.. we get to our second item in the Type.. myLong2!
' mov [edx], eax

' mov esp, ebp ;MOV/POP is much faster
' pop ebp ;on 486 and Pentium than Leave
' ret 8
'#ASM_END
End Function

User contributed notes: