TMS34010
C
Run-Time
Environment
necessity
for
a graphics
environment),
and
memory-mapped
I/O
is
much
more
straightforward.
5.2
Register
Conventions
Strict
conventions
are used
to
determine
which
registers are used
for
what
operations in the C environment. A
good
understanding
of
these
conventions
is
necessary
to
interface assembly language
to
a C program.
5.2.1
Dedicated
Registers
Three registers are used exclusively
by
the
C
environment
and
cannot
be
mo-
dified
by
a user in any
way
other
than
those
prescribed in Section 5.5,
Func-
tion
Call Conventions, page
5-7.
These registers
are:
SP
A14
(STK)
A13
(FP)
Pointer
to
the
top
of
the
system stack
Pointer
to
the
program
(user)
stack
Pointer
to
the
base
of
the
currently active frame
5.2.2
Using
Registers
5-4
Registers
AO
through
A
12
are generally available
for
use by a
function.
However,
two
conventions
apply:
1)
A
function
must
save
the
contents
of
each register used on entrance
to
the
function
and restore
those
contents
on exiting
the
function.
A8
is
the
only
exception; its
contents
do
not
have
to
be saved and restored.
2)
If
an integer value or a
pointer
is
to
be returned
from
a
function;
it
must
be placed in A8.
The codegen uses the registers in the
following
fashion:
Address generation
Expression analysis
Holding
constants
Return
value/Scratch
User register variabJes
Note:
AO,A2,A4,A6
A1,A3,A5,A7
A1,
A3,
A5,
A7,
A9
through
A12
A8
A9,
A10,
A11,
A12
Registers
BO
through
B14
are available for use
by
the
assembly-language
programmer and are
not
used
by
the
C compiler.
Address generation registers are allocated on a least-recently-used
(LRU)
basis,
with
the
low
registers allocated first. Expression analysis registers are
allocated from high
to
low
registers, based on availability and current use.
(Note
that
all integer expression analysis uses
32-bit
math.)
To
hold
con-
stants,
both
the
user registers and
the
expression analysis registers can be
used and are allocated
from
high
to
low
registers.
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