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120 lines
5.4 KiB
ReStructuredText
120 lines
5.4 KiB
ReStructuredText
=================
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Technical details
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=================
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All variables are static in memory
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----------------------------------
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All variables are allocated statically, there is no concept of dynamic heap or stack frames.
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Essentially all variables are global (but scoped) and can be accessed and modified anywhere,
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but care should be taken ofcourse to avoid unexpected side effects.
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Especially when you're dealing with interrupts or re-entrant routines: don't modify variables
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that you not own or else you will break stuff.
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Software stack for expression evaluation
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----------------------------------------
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Prog8 uses a software stack to evaluate complex expressions that it can't calculate in-place or
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directly into the target variable, register, or memory location.
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'software stack' means: seperated and not using the processor's hardware stack.
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The software stack is implemented as follows:
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- 2 pages of memory are allocated for this, exact locations vary per machine target.
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For the C-64 they are set at $ce00 and $cf00 (so $ce00-$cfff is reserved).
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For the Commander X16 they are set at $0400 and $0500 (so $0400-$05ff are reserved).
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- these are the high and low bytes of the values on the stack (it's a 'split 16 bit word stack')
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- for byte values just the lsb page is used, for word values both pages
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- float values (5 bytes) are chopped up into 2 words and 1 byte on this stack.
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- the X register is permanently allocated to be the stack pointer in the software stack.
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- you can use the X register as long as you're not using the software stack.
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But you *must* make sure it is saved and restored after the code that modifies it,
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otherwise the evaluation stack gets corrupted.
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Subroutine Calling Convention
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-----------------------------
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Calling a subroutine requires three steps:
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#. preparing the arguments (if any) and passing them to the routine
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#. calling the routine
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#. preparig the return value (if any) and returning that from the call.
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Calling the routine is just a simple JSR instruction, but the other two work like this:
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``asmsub`` routines
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^^^^^^^^^^^^^^^^^^^
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These are usually declarations of kernal (ROM) routines or low-level assembly only routines,
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that have their arguments solely passed into specific registers.
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Sometimes even via a processor status flag such as the Carry flag.
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Return values also via designated registers.
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The processor status flag is preserved on returning so you can immediately act on that for instance
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via a special branch instruction such as ``if_z`` or ``if_cs`` etc.
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regular subroutines
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^^^^^^^^^^^^^^^^^^^
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- subroutine parameters are just variables scoped to the subroutine.
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- the arguments passed in a call are evaluated (using the eval-stack if needed) and then
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copied into those variables.
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This sometimes can seem inefficient but it's required to allow subroutines to work locally
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with their parameters and allow them to modify them as required, without changing the
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variables used in the call's arguments. If you want to get rid of this overhead you'll
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have to make an ``asmsub`` routine in assembly instead.
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- the return value is passed back to the caller via cpu register(s):
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Byte values will be put in ``A`` .
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Word values will be put in ``A`` + ``Y`` register pair.
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Float values will be put in the ``FAC1`` float 'register' (Basic allocated this somewhere in ram).
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Calls to builtin functions are treated in a special way:
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Generally if they have a single argument it's passed in a register or register pair.
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Multiple arguments are passed like a normal subroutine, into variables.
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Some builtin functions have a fully custom implementation.
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The compiler will warn about routines that are called and that return a value, if you're not
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doing something with that returnvalue. This can be on purpuse if you're simply not interested in it.
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Use the ``void`` keyword in front of the subroutine call to get rid of the warning in that case.
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The 6502 CPU's X-register: off-limits
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-------------------------------------
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Prog8 uses the cpu's X-register as a pointer in its internal expression evaluation stack.
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When only writing code in Prog8, this is taken care of behind the scenes for you by the compiler.
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However when you are including or linking with assembly routines or kernal/ROM calls that *do*
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use the X register (either clobbering it internally, or using it as a parameter, or return value register),
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those calls will destroy Prog8's stack pointer and this will result in invalid calculations.
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You should avoid using the X register in your assembly code, or take preparations.
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If you make sure that the value of the X register is preserved before calling a routine
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that uses it, and restored when the routine is done, you'll be ok.
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Routines that return a value in the X register can be called from Prog8 but the return value is
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inaccessible unless you write a short piece of inline assembly code to deal with it yourself, such as::
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ubyte returnvalue
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%asm {{
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stx P8ZP_SCRATCH_REG ; use 'phx/plx' if using 65c02 cpu
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ldx #10
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jsr routine_using_x
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stx returnvalue
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ldx P8ZP_SCRATCH_REG
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}}
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; now use 'returnvalue' variable
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Prog8 also provides some help to deal with this:
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- you should use a ``clobbers(X)`` specification for asmsub routines that modify the X register; the compiler will preserve it for you automatically when such a routine is called
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- the ``sys.rsave()`` and ``sys.rrestore()`` routines can preserve and restore *all* registers (but this is very slow and overkill if you only need to save X)
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