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SYSTEM DESCRIPTION
24.8 -- System Function Calls
the value oT the Tunction number (with the sign
bit
set
to
zero) to facilitate indexing into the user's function table.
Since
MDOS
assumes
control of the SWI
vecto~
which is
normally used
by
EXbug, certain precautions must be
observed
when debugging programs using the debug monitor.
1.
MDOS
mu s
t
1i 0
t
b
e
in i
t
i
ali zed
v
i
a
the
deb u 9
monito~
command
"E800iG"
or "MDOS" without
first
having depressed the ABORT or
RESTART
pushbuttons
on
the
EXORciseT"s
front
panel.
These
two
pushbuttons will restore EXbug's SWI vector.
2.
The
normal breakpoints can be used while testing
a
program,
regardless
or
whetheT'
MOOS
system
functions
are
used
01'
notj
noweveT",
breakpoints
set
by
simply placing
an
SWI
instruction
into
memor~
via the
memor~
change function will cause
a system runction to be executed
rather
than
a
breakpoint
to
occur.
Breakpoints must only be
set or cleared via the debug monitor commands.
3.
Breakpoints can be set on an SWI instruction that
i
5
an MDDS
$
Y
S
t emf u net
i.o
n call;
howe v e r , b e
r
0
r e
continuing from that particular
breakpoint
with
the
"
i
P"
0
r
I l ;
Nil comma n d s '" the b rea k poi n t s h
0 U
1 d
be cleared (this
is
only
true
for
the
newer
versions
or
the
M6800
MPU
which
do not give
control to the IRG
vector
when
an
NMI
occurs
while an SWI is executing).
4.
MDOS
sy stem
Tunc ti ons
cannot
be
trac ed
OT"
single-stepped through with
the
EXbug
commands
uiN"
or
"iT".
Since
these
debug
monitor
functions utilize the stack, parts of
MDOS
will
be
over~ritten
due
to
the internal use of the
stack pointer within the system function handleT'o
MDOS system
function
calls
or
user-defined
function
call.
are
programmed
by
using the SWI instruction mnemonic
and the
FeB
assembler directive.
If programs
are
assembled
wit h
the MDOS e
ct
u ate
f i
1 e ( n ext sec t ion )
I
the p
1" 0
vi d e d ma c
l' 0
definitions with the names SCALL and UCALL
can
be
used
to
generate
the code for MDOS system functions and user-defined
functions, respectively.
The macros require an
argument
to
be
passed.
This
argument
is
the
name
or
value of the
function to be executed.
The names
aT
MDOS
functions
are
assigned
symbols
in the MDOS equate rile so that the use
Or
absolute numbers is not necessary.
Use oT the SCALL or UCALL
macro
makes
the program a bit easier to read,
especially if
.names are used
fo~
the macro arguments.
MOOS system Tunctions receive their
parameters
in
the
registers
O~
in tables that are pointed to
by
the registers.
/
Page
24-24
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