Macros and procedures |
Procedure
Definition of procedure
A D V E R T I S E M E N T
A procedure is a collection of instructions to which we can direct the flow
of our program, and once the execution of these instructions is over
control is given back to the next line to process of the code which called
on the procedure.
Procedures help us to create legible and easy to modify programs.
At the time of invoking a procedure the address of the next instruction of
the program is kept on the stack so that, once the flow of the program has
been transferred and the procedure is done, one can return to the next line
of the original program, the one which called the procedure.
Syntax of a Procedure
There are two types of procedures, the intrasegments, which are found on
the same segment of instructions, and the inter-segments which can be
stored on different memory segments.
When the intrasegment procedures are used, the value of IP is stored on the
stack and when the intrasegments are used the value of CS:IP is stored.
To divert the flow of a procedure (calling it), the following directive is
used:
CALL NameOfTheProcedure
The part which make a procedure are:
Declaration of the procedure
Code of the procedure
Return directive
Termination of the procedure
For example, if we want a routine which adds two bytes stored in AH and AL
each one, and keep the addition in the BX register:
Adding Proc Near ; Declaration of the procedure
Mov Bx, 0 ; Content of the procedure
Mov B1, Ah
Mov Ah, 00
Add Bx, Ax
Ret ; Return directive
Add Endp ; End of procedure declaration
On the declaration the first word, Adding, corresponds to the name of out
procedure, Proc declares it as such and the word Near indicates to the MASM
that the procedure is intrasegment.
The Ret directive loads the IP address stored on the stack to return to the
original program, lastly, the Add Endp directive indicates the end of the
procedure.
To declare an inter segment procedure we substitute the word Near for the
word FAR.
The calling of this procedure is done the following way:
Call Adding
Macros offer a greater flexibility in programming compared to the
procedures, nonetheless, these last ones will still be used.
Macros
Definition of the macro
A macro is a gro of repetitive instructions in a program which are
codified only once and can be used as many times as necessary.
The main difference between a macro and a procedure is that in the macro
the passage of parameters is possible and in the procedure it is not, this
is only applicable for the TASM - there are other programming languages
which do allow it. At the moment the macro is executed each parameter is
substituted by the name or value specified at the time of the call.
We can say then that a procedure is an extension of a determined program,
while the macro is a module with specific functions which can be used by
different programs.
Another difference between a macro and a procedure is the way of calling
each one, to call a procedure the use of a directive is required, on the
other hand the call of macros is done as if it were an assembler
instruction.
Syntax of a Macro
The parts which make a macro are:
Declaration of the macro
Code of the macro
Macro termination directive
The declaration of the macro is done the following way:
NameMacro MACRO [parameter1, parameter2...]
Even though we have the functionality of the parameters it is possible to
create a macro which does not need them.
The directive for the termination of the macro is: ENDM
An example of a macro, to place the cursor on a determined position on the
screen is:
Position MACRO Row, Column
PUSH AX
PUSH BX
PUSH DX
MOV AH, 02H
MOV DH, Row
MOV DL, Column
MOV BH, 0
INT 10H
POP DX
POP BX
POP AX
ENDM
To use a macro it is only necessary to call it by its name, as if it were
another assembler instruction, since directives are no longer necessary as
in the case of the procedures. Example:
Position 8, 6
Macro Libraries
One of the facilities that the use of macros offers is the creation of
libraries, which are groups of macros which can be included in a program
from a different file.
The creation of these libraries is very simple, we only have to write a
file with all the macros which will be needed and save it as a text file.
To call these macros it is only necessary to use the following instruction
Include NameOfTheFile, on the part of our program where we would normally
write the macros, this is, at the beginning of our program, before the
declaration of the memory model.
The macros file was saved with the name of MACROS.TXT, the
instruction Include would be used the following way:
;Beginning of the program
Include MACROS.TXT
.MODEL SMALL
.DATA
;The data goes here
.CODE
Beginning:
;The code of the program is inserted here
.STACK
;The stack is defined
End beginning
;Our program ends
