cc65/doc/internal.txt

	  		    Internals doc for CC65



Stacks:
-------

The program stack used by programs compiled with CC65 is located in high
memory.  The stack starts there and grows down.  Arguments to functions, local
data etc are allocated on this stack, and deallocated when functions exit.

The program code and data is located in low memory. The heap is located
between the program code and the stack. The default size for the parameter
stack is 2K, you may change this for most platforms in the linker
configuration.

Note: The size of the stack is only needed if you use the heap, or if you
call the stack checking routine (_stkcheck) from somewhere in your program.

When calling other functions, the return address goes on the normal 6502
stack, *not* on the parameter stack.



Registers:
----------

Since CC65 is a member of the Small-C family of compilers, it uses the notion
of a 'primary register'.  In the CC65 implementation, I used the AX register
pair as the primary register.  Just about everything interesting that the
library code does is done by somehow getting a value into AX, and then calling
some routine or other.  In places where Small-C would use a secondary
register, top-of-stack is used, so for instance two argument function like
integer-multiply work by loading AX, pushing it on the stack, loading the
second value, and calling the internal function.  The stack is popped, and the
result comes back in AX.



Calling sequences:
------------------

C functions are called by pushing their args on the stack, and JSR'ing to the
entry point.  (See ex 1, below) If the function returns a value, it comes back
in AX.  NOTE!!!  A potentially significant difference between the CC65
environment and other C environments is that the CALLEE pops arguments, not
the CALLER.  (This is done so as to generate more compact code) In normal use,
this doesn't cause any problems, as the normal function entry/exit conventions
take care of popping the right number of things off the stack, but you may
have to worry about it when doing things like writing hand-coded assembly
language routines that take variable numbers of arguments.  More about that
later.

Ex 1:  Function call:  Assuming 'i' declared int and 'c' declared
       char, the following C code

       	i = baz(i, c);

       in absence of a prototype generates this assembler code.  I've added
       the comments.

       	lda	_i 		; get 'i', low byte
       	ldx	_i+1		; get 'i', hi byte
       	jsr	pushax		; push it
       	lda	_c 		; get 'c'
       	ldx	#0 		; fill hi byte with 0
       	jsr	pushax		; push it
       	ldy	#4     	       	; arg size
       	jsr	_baz		; call the function
       	sta	_i  		; store the result
	stx	_i+1

       In presence of a prototype, the picture changes slightly, since the
       compiler is able to do some optimizations:

       	lda	_i  	      	; get 'i', low byte
       	ldx	_i+1	      	; get 'i', hi byte
       	jsr	pushax	      	; push it
       	lda	_c  	      	; get 'c'
       	jsr    	pusha 	      	; push it
       	jsr	_baz	      	; call the function
       	sta	_i  	      	; store the result
       	stx	_i+1


Note that the two words of arguments to baz were popped before it exitted.
The way baz could tell how much to pop was by the argument count in Y at call
time.  Thus, even if baz had been called with 3 args instead of the 2 it was
expecting, that would not cause stack corruption.

There's another tricky part about all this, though.  Note that the args to baz
are pushed in FORWARD order, ie the order they appear in the C statement.
That means that if you call a function with a different number of args than it
was expecting, they wont end up in the right places, ie if you call baz, as
above, with 3 args, it'll operate on the LAST two, not the first two.



Symbols:
--------

CC65 does the usual trick of prepending an underbar ('_') to symbol names when
compiling them into assembler.  Therefore if you have a C function named
'bar', CC65 will define and refer to it as '_bar'.



Systems:
--------

Supported systems at this time are: C64, C128, Plus/4, CBM 500, CBM 600/700, 
the newer PET machines (not 2001), Atari 8bit, and the Apple ][ (thanks to 
Kevin Ruland, who did the port).

C64:  	The program runs in a memory configuration, where only the kernal ROM
      	is enabled. The text screen is expected at the usual place ($400), so
       	50K of memory are available to the program.

C128: 	The startup code will reprogram the MMU, so that only the kernal ROM
      	is enabled. This means, there are 41K of memory available to the
      	program.

Plus/4:	Unfortunately, the Plus/4 is not able to disable only part of it's
      	ROM, it's an all or nothing approach. So, on the Plus/4, the program
       	has only 28K available (16K machines are detected and the amount of
      	free memory is reduced to 12K).

CBM 500:
    	The C program runs in bank #0 and has about 61K memory available. The
	memory available on the CBM5x0 is slightly less than that available
	on its bigger brothers (CBM 600/700) because the video ram is also
	placed into bank #0 to allow sprites.

CBM 600/700:
      	The C program runs in a separate segment and has almost full 64K of
      	memory available.

PET:  	The startup code will adjust the upper memory limit to the installed
      	memory. However, only linear memory is used, this limits the top to
      	$8000, so on a 8032 or similar machine, 31K of memory are available to
      	the program.

APPLE2:	The program starts at $800, end of RAM is $8E00, so 33.5K of memory
	(including stack) are available.

Atari:  The startup code will adjust the upper memory limit to the installed
        memory, considering future graphics memory usage (which is allocated
        at top of RAM). The programmer can specify which graphics mode is
        about to be used by defining a variable _graphmode_used, unsigned
        char, to the mode value (mode values like Atari DOS, 0-31).
	(Please note that graphics mode selection isn't supported in the
	Atari runtime lib yet!)
	In the default case the upper memory limit will be $8035 (with Basic
        cartridge) and $A035 (without cartridge). This is the default which
        leaves room for the biggest possible graphics mode. If only standard
        text mode is used (_graphmode_used = 0), the values are $9C1F (with
        Basic) and $BC1F (no cartridge).
	The program starts at $1F00 (to leave room for DOS), and the free
        memory values are $6135 (24K, Basic, default mode), $8135 (32K, no
        Basic, default mode), $7D1F (31K, Basic, mode 0) and $9D1F (39K,
        no Basic, mode 0).
	These values are for a 48K or 64K machine.

Note: The above numbers do not mean that the remaining memory is unusable.
However, it is not linear memory and must be accessed by other, nonportable
methods. I'm thinking about a library extension that allows access to the
additional memory as a far heap, but these routines do not exist until now.



Inline Assembly:
----------------

CC65 allows inline assembly by a special keyword named "asm". Inline assembly
looks like a function call. The string in parenthesis is output in the
assembler file.

Example, insert a break instruction into the code:

       	asm ("\t.byte\t$00")

Note: The \t in the string is replaced by the tab character, as in all other
strings.

Beware: Be careful when inserting inline code since this may collide with
the work of the optimizer.



Pseudo variables:
-----------------

There are two special variables available named __AX__ and __EAX__. These
variables must never be declared (this gives an error), but may be used as any
other variable. However, accessing these variables will access the primary
register that is used by the compiler to evaluate expressions, return
functions results and pass parameters.

This feature is useful with inline assembly and macros. For example, a macro
that reads a CRTC register may be written like this:

#define wr(idx) (__AX__=(idx),					\
       	       	asm("\tsta\t$2000\n\tlda\t$2000\n\tldx\t#$00"),	\
		__AX__)