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77 lines
3.1 KiB
ReStructuredText
77 lines
3.1 KiB
ReStructuredText
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===============
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Technical stuff
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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 kernel (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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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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- 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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