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OTHER SYSTEM FUNCTIONS
27.1 -- Register Functions
significant
byte
first,
most
significant
byte
last,
the
register
pail'
liB,
AU
was
chosen
instead
of
itA, B".
The
relationship
or
the
B and A registers on the stack is then
identical to the other sixteen bit registers
saved
in
this
r
a
$
h ion
( for
the
M6800
0
n
1
y ) .
For
the
M6809}
i
t i s not
anticipated that
the
user
will
use
the
register
system
Functions
as there are instructions to perform most of these
functions.
However,
the
system
Tunction
calls
remain
identical
between
MDOS
and
MDOS09 to allow
po~tability
of
program written initially
ror
the 6800.
27.
1. 1 TT'ans reT' X
to
B, A -- . TXBA
The .
TXBA
function
transfers
the
contents
or
the
X
register into the register
pair
a,A.
ENTRY PARAMETERS:
EXIT CONDITIONS:
No.ne.
A
contains
XL.
B
contains
XH.
X
is unchanged.
CC is indeteT'minate.
27.
1.
2
TT'ans,reT' BJ A
to
X -- . TBAX
The
.
TBAX
function
transrers
the
contents
of
the
register pail' B,A into the X
l'egist~r.
ENTRY PARAMETERS:
EXIT CONDITIONS:
None.
A
is unchanged.
B
is unchanged.
XH
c onta ins
B.
XL c
onta ins
A.
CC is indeterminate.
27.
1.
3 Ex c
han 9
e
B
I
A
UJ
i
t
h X -- . XBAX
The
.
XBAX
function
exchanges
the
contents
of
the
register pail'
a,A
with the contents of the
X
register.
ENTRY PARAMETERS:
EXIT CONDITIONS:
None.
A
contains entry value
Or
XL.
B
contains entry value
Or
XH.
XH
contains entry value
Or
B.
XL
contains entry value of
A ..
CC
is unchanged.
27.
1.
4 Add B to X -- . ADBX
)
Page
27-02
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