Function calls in NASM

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Function calls in NASM
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• Caller side
• The caller pushes the function's parameters on
  the stack, one after another, in reverse order.
• The caller executes a CALL instruction to pass
  control to the callee. This CALL is either near
  or far depending on the memory model.

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• Stack structure

BP
word
BP
word
Ret add (offset)
word
Ret addr (offset)
word
Ret addr (segment)
word

Param 1

Param 2
Param 1
Param 2


Near call
Far call
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• Function call example
• STR DB Printing two ints ld, ld,10,0
• MOV EAX, 15
• PUSH EAX 3rd parameter
• PUSH DWORD 20 2nd parameter
• PUSH DWORD STR 1st parameter
• CALL PRINTF
• ADD SP, 12 3 dwords 12 bytes

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• Stack contents

how to get parameters MOV BP, SP MOV EAX,
BP4 1st parameter MOV EBX, BP8 2nd
parameter MOV ECX, BP12 3rd
parameter MOV BP-2, AX can use the
stack
BP
word
Ret add (offset)
word

STR
dword
20
dword
15
dword
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• Returning from function
• Use RETN or RETF for near and far calls.
• BP and return address are removed from
• stack, parameters are not.
• Use
• ADD SP, params size
• to clear the stack
• (faster than POP)

Param 1
Param 2