Parameter Passing

1
Parameter Passing

• Assembly Language Programming
• University of Akron
• Dr. Tim Margush

2
Arguments and Parameters

• Arguments are information passed to a procedure
  when it is called
• Parameters receive the information passed by an
  argument
• Passing mechanisms
• Call by Value
• Call by Reference

3
Call by Value

• The parameter is a copy of the data value
  represented by the argument
• Commonly used for "small" data values or values
  that must be protected from change
• Since the original data is copied and the
  procedure accesses the copy only, the original
  data is protected

4
Call by Reference

• The parameter is the address of the data value
  represented by the argument
• The address is passed using call by value
• Used when the original data is to be changed by
  the procedure, or it is inefficient to make a
  copy of the argument due to size
• Since the procedure receives the address of the
  argument it cannot be protected against change

5
Register Parameters

• One or more registers may be designated as
  parameter locations
• Caller places arguments (values or addresses) in
  designated registers and then calls the procedure

• When the procedure starts, the parameters are
  found in the designated registers
• Values may be used directly
• Base or indexed addressing is used to access
  reference parameters

6
Parameter Block

• Copy arguments (values or addresses) into
  components of a prearranged record structure
  devoted to communicating with the procedure
• Call the procedure

• The address of the parameter block may be passed
  to the procedure, or it may access it by name
• The procedure accesses parameters in the
  prearranged storage locations

7
Inline Parameters

• Arguments (values or addresses) are placed in
  storage immediately after the call statement
• Call the procedure
• Procedure must alter return address or a JMP
  instruction is used to skip the args

• Procedure expects its return address to be on the
  top of stack
• This is used to locate the parameters
• The return address may be modified to point past
  the parameter area

8
Stack Frame

• Arguments (values or addresses) are placed on the
  stack
• Call the procedure
• Arguments may need to be removed from the stack
  if the procedure does not do this

• A standard protocol is followed to access the
  parameters using based addressing
• The parameters may be removed by specifying the
  size of the parameter block in the RET statement

9
Return Values

• If a procedure is to return a value, it is
  customary to place it in a register
• AX is most common, but any register may be used
• Return values are limited to byte or word data
• Flags are sometimes used for Boolean return
  values (especially the Carry flag)
• Use call by Reference for more complex data

10
Saving Registers

• It is usually the procedure's responsibility to
  save and restore any registers it needs to use
• Registers used for return values are of course
  exempt from this convention
• The 286 and higher processor has a PUSHA and POPA
  to put all registers on the stack and restore
  them
• This is local storage for the procedure

11
Conventional Stack Frames

• Procedures can use the stack
• for parameters
• saved registers
• local storage
• This allows recursion, reduces dependency on a
  data segment, and standardizes procedure
  interfaces

12
Standard Entrance/Exit Code

• Save current BP on stack
• Record old BP's location in BP
• Allocate local storage
• push BP
• mov BP,SP
• sub SP,ls_size

• Deallocate local storage by setting SP to old BP
  location
• Restore old BP
• Return, removing parameter block
• mov SP,BP
• pop BP
• ret pb_size

13
A Typical Stack Frame
BP-2
BP4
BP-3
BP6
BP-5
BP8
LW
LW
LB
BP
RA
P1
P2
P3
Parameters
Locals
SP
BP
14
Stack Frame Access

• sample equates
• arry_ptr equ word ptr BP4
• loc_char equ byte ptr BP-3
• loc_BX equ word ptr BP-5
• sample access
• mov loc_char,'A' initialize
• mov loc_BX,bx save BX
• mov BX,arry_ptr addr of array