Chapter
1
I
System Calls
Calling MS-DOS Functions
Most
of
the MS-DOS function calls require input
to
be
passed
to
them in registers. After setting the proper reg-
ister values, the function may be invoked in one
of
the
following ways:
1.
2.
3.
Place the function number in AH and execute
a
long
call
to
offset 50H in your Program Segment Prefix.
Note that programs using this method will not oper-
ate correctly on earlier versions
of
MS-DOS.
Place the function number in AH and issue Interrupt
21H. All
of
the examples in this chapter use this
method.
An additional method exists
for
programs that were
written with different calling conventions. This method
should be avoided
for
all new programs. The function
number is placed in the CL register and other regis-
ters are set according to the function specification.
Then an intrasegment call is made to location 5 in
the current code segment. That location contains a
long call to the MS-DOS function dispatcher. Register
AX
is always destroyed if this method is used; other-
wise, it is the same
as
normal function calls. Note
that this method is valid only for Function Requests
OOH through 024H.
CP/M
”’
-Compatible Calling Sequence
A
different sequence can be used
for
programs that must
conform
to
CPiM calling conventions:
1. Move any required data into the appropriate regis-
ters (just as in the standard sequence).
2.
Move the function number into the
CL
register.
3.
Execute an intrasegment call to location 5 in the
current code segment.
This method can be used only with the functions OOH
through 24H that do not pass a parameter in AL. Regis-
ter
AX
is
always destroyed when
a
function is called in
this manner.
27
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