ID Number: Q70671
5.10 5.10a 6.00 | 5.10 5.10a 6.00
MS-DOS | OS/2
Summary:
With the Macro Assembler (MASM), if a CALL is made to a label within a
procedure, a near call is generated by default. Depending on the
memory model being used, you may need to explicitly specify the
correct return.
More Information:
The standard RET instruction pops off the return address depending on
the memory model. In a small memory model program, the RET instruction
is converted to a RETN, which only pops off the offset to return. In
medium and large memory models, the RET is converted to RETF, which
pops off both the segment and offset to return from the far call.
Because a call to a label within a procedure always results in a near
call, the RETF instruction that is generated in medium and large
models will pop a segment value for the return address that was never
pushed.
To ensure that the right size address is popped off the stack relative
to that pushed on the stack, one of the following methods may be used:
1. Use the RET return, but push CS before the call so that the segment
and offset will both be on the stack for the far return:
PUSH CS
CALL label1
- or -
2. Use the FAR PTR override on the call along with a RET to return, so
that both segment and offset are pushed and popped off the stack:
CALL FAR PTR label1
- or -
3. Use the LABEL FAR directive along with a RET return so that the
assembler will generate a far return to match the far label call:
label1 LABEL FAR
- or -
4. If the label is called ONLY from within the procedure, use RETN
explicitly (instead of RET) to indicate the NEAR return, so that
only the offset is popped off the stack:
label1:
RETN
The sample code below demonstrates the need for the proper return for
a call to a label. When assembled and run as is, the code will hang
the computer. Adding any of the solutions described above will correct
the code. Comments are included at the locations where each correction
should go.
Sample Code
-----------
; Assemble options needed : none
.MODEL large
.STACK
.CODE
start: call proc1
mov ah, 4ch
mov al, 0
int 21h
proc1 PROC
; #1 - add PUSH CS instruction here
CALL label1 ; #2 - change call to CALL FAR PTR label1
RET
label1: ; #3 - change label to label1 LABEL FAR
RET ; #4 - change return to RETN
proc1 ENDP
END start