mirror of
https://github.com/michaelcmartin/Ophis.git
synced 2024-12-30 10:30:47 +00:00
271 lines
10 KiB
Plaintext
271 lines
10 KiB
Plaintext
<chapter>
|
|
<title>Functionals</title>
|
|
|
|
<para>
|
|
This essay deals with indirect calls. These are the core of an
|
|
enormous number of high level languages: LISP's closures, C's
|
|
function pointers, C++ and Java's virtual method calls, and some
|
|
implementations of the <literal>switch</literal> statement.
|
|
</para>
|
|
|
|
<para>
|
|
These techniques vary in complexity, and most will not be
|
|
appropriate for large-scale assembler projects. Of them, however,
|
|
the Data-Directed approach is the most likely to lead to organized
|
|
and maintainable code.
|
|
</para>
|
|
|
|
<section>
|
|
<title>Function Pointers</title>
|
|
|
|
<para>
|
|
Because assembly language is totally untyped, function pointers
|
|
are the same as any other sixteen-bit integer. This makes
|
|
representing them really quite easy; most assemblers should permit
|
|
routines to be declared simply by naming the routine as
|
|
a <literal>.word</literal> directly.
|
|
</para>
|
|
|
|
<para>
|
|
To actually invoke these methods, copy them to some sixteen-bit
|
|
location (say, <literal>target</literal>) and then invoking the
|
|
method is a simple matter of the using an indirect jump:
|
|
the <literal>JMP (target)</literal> instruction.
|
|
</para>
|
|
|
|
<para>
|
|
There's really only one subtlety here, and it's that the indirect
|
|
jump is an indirect <emphasis>jump</emphasis>, not an
|
|
indirect <emphasis>function call</emphasis>. Thus, if some
|
|
function <literal>A</literal> makes in indirect jump to some
|
|
routine, when that routine returns, it returns to whoever
|
|
called <literal>A</literal>, not <literal>A</literal>
|
|
itself.
|
|
</para>
|
|
|
|
<para>
|
|
There are several ways of dealing with this, but only one correct
|
|
way, which is to structure your procedures so that any call
|
|
to <literal>JMP (xxxx)</literal> occurs at the very
|
|
end.
|
|
</para>
|
|
</section>
|
|
<section>
|
|
<title>A quick digression on how subroutines work</title>
|
|
<para>
|
|
Ordinarily, subroutines are called with <literal>JSR</literal> and
|
|
finished with <literal>RTS</literal>. The <literal>JSR</literal>
|
|
instruction takes its own address, adds 2 to it, and pushes this
|
|
16-bit value on the stack, high byte first, then low byte (so that
|
|
the low byte will be popped off first).
|
|
</para>
|
|
|
|
<para>
|
|
But wait, you may object. All <literal>JSR</literal> instructions
|
|
are three bytes long. This <quote>return address</quote> is in
|
|
the middle of the instruction. And you would be quite right;
|
|
the <literal>RTS</literal> instruction pops off the 16-bit
|
|
address, adds one to it, and <emphasis>then</emphasis> sets the
|
|
program counter to that value.
|
|
</para>
|
|
|
|
<para>
|
|
So it <emphasis>is</emphasis> possible to set up
|
|
a <quote><literal>JSR</literal> indirect</quote> kind of operation
|
|
by adding two to the indirect jump's address and then pushing that
|
|
value onto the stack before making the jump; however, you wouldn't
|
|
want to do this. It takes six bytes and trashes your accumulator,
|
|
and you can get the same functionality with half the space and
|
|
with no register corruption by simply defining the indirect jump
|
|
to be a one-instruction routine and <literal>JSR</literal>-ing to
|
|
it directly. As an added bonus, that way if you have multiple
|
|
indirect jumps through the same pointer, you don't need to
|
|
duplicate the jump instruction.
|
|
</para>
|
|
|
|
<para>
|
|
Does this mean that abusing <literal>JSR</literal>
|
|
and <literal>RTS</literal> is a dead-end, though? Not at all...
|
|
</para>
|
|
</section>
|
|
<section>
|
|
<title>Dispatch-on-type and Data-Directed Assembler</title>
|
|
|
|
<para>
|
|
Most of the time, you care about function pointers because you've
|
|
arranged them in some kind of table. You hand it an index
|
|
representing the type of your argument, or which method it is
|
|
you're calling, or some other determinator, and then you index
|
|
into an array of routines and execute the right one.
|
|
</para>
|
|
|
|
<para>
|
|
Writing a generic routine to do this is kind of a pain. First you
|
|
have to pass a 16-bit pointer in, then you have to dereference it
|
|
to figure out where your table is, then you have to do an indexed
|
|
dereference on <emphasis>that</emphasis> to get the routine you
|
|
want to run, then you need to copy it out to somewhere fixed so
|
|
that you can write your jump instruction. And making this
|
|
non-generic doesn't help a whole lot, since that only saves you
|
|
the first two steps, but now you have to write them out in every
|
|
single indexed jump instruction. If only there were some way to
|
|
easily and quickly pass in a local pointer directly...
|
|
</para>
|
|
|
|
<para>
|
|
Something, say, like the <literal>JSR</literal> instruction, only not for
|
|
program code.
|
|
</para>
|
|
|
|
<para>
|
|
Or we could just use the <literal>JSR</literal> statement itself,
|
|
but only call this routine at the ends of other routines, much
|
|
like we were organizing for indirect jumps to begin with. This
|
|
lets us set up routines that look like this:
|
|
</para>
|
|
|
|
<programlisting>
|
|
jump'table'alpha:
|
|
jsr do'jump'table
|
|
.word alpha'0, alpha'1, alpha'2
|
|
</programlisting>
|
|
|
|
<para>
|
|
Where the <literal>alpha'x</literal> routines are the ones to be
|
|
called when the index has that value. This leaves the
|
|
implementation of do'jump'table, which in this case uses the Y
|
|
register to hold the index:
|
|
</para>
|
|
|
|
<programlisting>
|
|
do'jump'table:
|
|
sta _scratch
|
|
pla
|
|
sta _jmpptr
|
|
pla
|
|
sta _jmpptr+1
|
|
tya
|
|
asl
|
|
tay
|
|
iny
|
|
lda (_jmpptr), y
|
|
sta _target
|
|
iny
|
|
lda (_jmpptr), y
|
|
sta _target+1
|
|
lda _scratch
|
|
jmp (_target)
|
|
</programlisting>
|
|
|
|
<para>
|
|
The <literal>TYA:ASL:TAY:INY</literal> sequence can actually be
|
|
omitted if you don't mind having your Y indices be 1, 3, 5, 7, 9,
|
|
etc., instead of 0, 1, 2, 3, 4, etc. Likewise, the instructions
|
|
dealing with <literal>_scratch</literal> can be omitted if you
|
|
don't mind trashing the accumulator. Keeping the accumulator and
|
|
X register pristine for the target call comes in handy, though,
|
|
because it means we can pass in a pointer argument purely in
|
|
registers. This will come in handy soon...
|
|
</para>
|
|
</section>
|
|
<section>
|
|
<title>VTables and Object-Oriented Assembler</title>
|
|
|
|
<para>
|
|
The usual technique for getting something that looks
|
|
object-oriented in non-object-oriented languages is to fill a
|
|
structure with function pointers, and have those functions take
|
|
the structure itself as an argument. This works just fine in
|
|
assembler, of course (and doesn't really require anything more
|
|
than your traditional jump-indirects), but it's also possible to
|
|
use a lot of the standard optimizations that languages such as C++
|
|
provide.
|
|
</para>
|
|
|
|
<para>
|
|
The most important of these is the <emphasis>vtable</emphasis>.
|
|
Each object type has its own vtable, and it's a list of function
|
|
pointers for all the methods that type provides. This is a space
|
|
savings over the traditional structs-with-function-pointers
|
|
approach because when you have many objects of the same class, you
|
|
only have to represent the vtable once. So that all objects may
|
|
be treated identically, the vtable location is traditionally fixed
|
|
as being the first entry in the corresponding structure.
|
|
</para>
|
|
|
|
<para>
|
|
Virtual method invocation takes an object pointer (traditionally
|
|
called <literal>self</literal> or <literal>this</literal>) and a
|
|
method index and invokes the approprate method on that object.
|
|
Gee, where have we seen that before?
|
|
</para>
|
|
|
|
<programlisting>
|
|
sprite'vtable:
|
|
jsr do'jump'table
|
|
.word sprite'init, sprite'update, sprite'render
|
|
</programlisting>
|
|
|
|
<para>
|
|
We mentioned before that vtables are generally the first entries
|
|
in objects. We can play another nasty trick here, paying an
|
|
additional byte per object to have the vtable be not merely a
|
|
pointer to its vtable routine, but an actual jump instruction to
|
|
it. (That is, if an object is at location X, then location X is
|
|
the byte value <literal>$4C</literal>,
|
|
representing <literal>JMP</literal>, location X+1 is the low byte
|
|
of the vtable, and location X+2 is the high byte of the vtable.)
|
|
Given that, our <literal>invokevirtual</literal> function becomes
|
|
very simple indeed:
|
|
</para>
|
|
|
|
<programlisting>
|
|
invokevirtual:
|
|
sta this
|
|
stx this+1
|
|
jmp (this)
|
|
</programlisting>
|
|
|
|
<para>
|
|
Which, combined with all our previous work here, takes
|
|
the <literal>this</literal> pointer in <literal>.AX</literal> and
|
|
a method identifier in <literal>.Y</literal> and invokes that
|
|
method on that object. Arguments besides <literal>this</literal>
|
|
need to be set up before the call
|
|
to <literal>invokevirtual</literal>, probably in some global
|
|
argument array somewhere as discussed back in <xref linkend="hll2">.
|
|
</para>
|
|
</section>
|
|
<section>
|
|
<title>A final reminder</title>
|
|
|
|
<para>
|
|
We've been talking about all these routines as if they could be
|
|
copy-pasted or hand-compiled from C++ or Java code. This isn't
|
|
really the case, primarily because <quote>local variables</quote>
|
|
in your average assembler routines aren't really local, so
|
|
multiple calls to the same method will tend to trash the program
|
|
state. And since a lot of the machinery described here shares a
|
|
lot of memory (in particular, every single method invocation
|
|
everywhere shares a <literal>this</literal>), attempting to shift
|
|
over standard OO code into this format is likely to fail
|
|
miserably.
|
|
</para>
|
|
|
|
<para>
|
|
You can get an awful lot of flexibility out of even just one layer
|
|
of method-calls, though, given a thoughtful
|
|
design. The <literal>do'jump'table</literal> routine, or one very
|
|
like it, was extremely common in NES games in the mid-1980s and
|
|
later, usually as the beginning of the frame-update loop.
|
|
</para>
|
|
|
|
<para>
|
|
If you find you really need multiple layers of method calls,
|
|
though, then you really are going to need a full-on program stack,
|
|
and that's going to be several kinds of mess. That's the topic
|
|
for the final chapter.
|
|
</para>
|
|
</section>
|
|
</chapter>
|