Assembler Help with Procedures

UMBC CMSC 211

Assembler Help with Procedures

The Assembler can help with communicating with high-level lanaguages. Note the change in the .model statement: .MODEL SIZE, C ; or Pascal or STDCALL Note that there is no C++ version, use C instead. The default for .MODEL SMALL is NEAR PROCs.

This can also be applied to the procedure definition:

name PROC C; ; or PASCAL OR STDCALL

When you specify the language, it implies certain things for some instructions (PUBLIC, EXTRN, and PROC psuedo-instructions, and the RET instruction):

If the .MODEL statement or the PROC statement has specified a language, the PROC name is automatically declared PUBLIC.
If the C language is specified, all symbols declared PUBLIC or EXTRN (including PROC names) within range have a leading '_' added to them, which is the standard C convention.
Code to save and set up the bp register is added immediately after any PROC statement, and code to restore the bp register is added immediately before any ret statement.
The PROC statement can specify USES R1 R2 ... Rn where each Ri is one of the 80X86 machine registers. The USES clause causes the specified registers to be pushed on the stack immediately after the PROC statemnt and to be popped from the stack in reverse order just before each RET in the PROC. Note that there are no commas between the register names or before USES.
The PROC statement can specify the number of parameters that are used to call it after the optional USES clause. the assembler can then equate each parameter name to bp + n where n is the appropriate number. Account is taken as to whether or not the the model is large or small. For the Pascal language type, parameters are assumed to be pushed in the order encountered, and for C, they are assumed to be bushed in the reverse order.
When parameters are specified in the PROC statement and the language is Pascal or STDCALL, the ret instruction is automatically changed to ret m, where m is the number of bytes of parameters specified in the PROC statement.

The "lazy" Way

Notice where the commas are required or not!

Stuff      PROC      C  USES SI DI, A : WORD, B : DWORD, C : WORD
           ...
           RET
Stuff	   ENDP

The "hard" Way (or the equivalent way!)

           PUBLIC   _Stuff
_Stuff	   PROC
           push     bp
           mov      bp, sp
           push     si
           push     di
A          EQU      WORD PTR [bp + 4]
B          EQU      DWORD PTR [bp + 6]
C          EQU      WORD PTR [bp + 10 ]
           ...
           pop      di
           pop      si
           mov	    sp, bp
           pop      bp
           ret
_Stuff     ENDP

Oh, them local variables

One of the important things necessary for re-entrant or recursive procedures, is that we must have local variables. These are stored on the stack and accessed in a similar way as the parameters, except this are on the other side of the return address.

The "lazy" Way

aProc      PROC     USES SI DI
           LOCAL    L1 : WORD, L2 : DWORD

The "hard" Way (or the equivalent way!)

aProc      PROC
           push     bp
           mov      bp, sp
           sub	    sp, 6
L1	   EQU      WORD PTR [bp - 2]
L2         EQU      DWORD PTR [bp - 6]
           push     si
           push     di

Now the ret instruction becomes:

           pop      di
           pop      si
           mov      sp, bp
           pop      bp
           ret      whatever

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