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156 lines
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156 lines
7.0 KiB
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***************************
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Target system specification
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***************************
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Prog8 targets the following hardware:
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- 8 bit MOS 6502/6510 CPU
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- 64 Kb addressable memory (RAM or ROM)
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- memory mapped I/O registers
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The main target machine is the Commodore-64, which is an example of this.
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This chapter explains the relevant system details of such a machine.
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Memory Model
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============
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Physical address space layout
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-----------------------------
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The 6502 CPU can address 64 kilobyte of memory.
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Most of the 64 kilobyte address space can be used by Prog8 programs.
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This is a hard limit: there is no built-in support for RAM expansions or bank switching.
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====================== ================== ========
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memory area type note
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====================== ================== ========
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``$00``--``$ff`` ZeroPage contains many sensitive system variables
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``$100``--``$1ff`` Hardware stack used by the CPU, normally not accessed directly
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``$0200``--``$ffff`` Free RAM or ROM free to use memory area, often a mix of RAM and ROM
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====================== ================== ========
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A few of these memory addresses are reserved and cannot be used for arbitrary data.
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They have a special hardware function, or are reserved for internal use in the
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code generated by the compiler:
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================== =======================
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reserved address in use for
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================== =======================
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``$00`` data direction (CPU hw)
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``$01`` bank select (CPU hw)
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``$02`` internal scratch variable
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``$03`` internal scratch variable
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``$fb - $fc`` internal scratch variable
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``$fd - $fe`` internal scratch variable
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``$fffa - $fffb`` NMI vector (CPU hw)
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``$fffc - $fffd`` RESET vector (CPU hw)
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``$fffe - $ffff`` IRQ vector (CPU hw)
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================== =======================
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The actual machine will often have many other special addresses as well,
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For example, the Commodore-64 has:
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- ROMs installed in the machine: BASIC, kernal and character roms. Occupying ``$a000``--``$bfff`` and ``$e000``--``$ffff``.
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- memory-mapped I/O registers, for the video and sound chips, and the CIA's. Occupying ``$d000``--``$dfff``.
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- RAM areas that are used for screen graphics and sprite data: usually at ``$0400``--``$07ff``.
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Prog8 programs can access all of those special memory locations but it will have a special meaning.
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.. _zeropage:
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ZeroPage ("ZP")
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---------------
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The ZeroPage memory block ``$02``--``$ff`` can be regarded as 254 CPU 'registers', because
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they take less clock cycles to access and need fewer instruction bytes than accessing other memory locations outside of the ZP.
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Theoretically they can all be used in a program, with the follwoing limitations:
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- several addresses (``$02``, ``$03``, ``$fb - $fc``, ``$fd - $fe``) are reserved for internal use
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- most other addresses will already be in use by the machine's operating system or kernal,
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and overwriting them will probably crash the machine. It is possible to use all of these
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yourself, but only if the program takes over the entire system (and seizes control from the regular kernal).
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This means it can no longer use (most) BASIC and kernal routines from ROM.
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- it's more convenient and safe to let the compiler allocate these addresses for you and just
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use symbolic names in the program code.
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.. todo::
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There must be a way to tell the compiler which variables you require to be in Zeropage:
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``zeropage`` modifier keyword on vardecl perhaps?
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Prog8 knows what addresses are safe to use in the various ZP handling configurations.
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It will use the free ZP addresses to place its ZP variables in,
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until they're all used up. If instructed to output a program that takes over the entire
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machine, (almost) all of the ZP addresses are suddenly available and will be used.
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**ZeroPage handling is configurable:**
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There's a global program directive to specify the way the compiler
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treats the ZP for the program. The default is to be reasonably restrictive to use the
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part of the ZP that is not used by the C64's kernal routines.
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It's possible to claim the whole ZP as well (by disabling the operating system or kernal).
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If you want, it's also possible to be more restricive and stay clear of the addresses used by BASIC routines too.
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IRQs and the ZeroPage
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^^^^^^^^^^^^^^^^^^^^^
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The normal IRQ routine in the C-64's kernal will read and write several addresses in the ZP
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(such as the system's software jiffy clock which sits in ``$a0 - $a2``):
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``$a0 - $a2``; ``$91``; ``$c0``; ``$c5``; ``$cb``; ``$f5 - $f6``
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These addresses will *never* be used by the compiler for ZP variables, so variables will
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not interfere with the IRQ routine and vice versa. This is true for the normal ZP mode but also
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for the mode where the whole system and ZP have been taken over.
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So the normal IRQ vector can still run and will be when the program is started!
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CPU
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===
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Directly Usable Registers
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-------------------------
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The following 6502 CPU hardware registers are directly usable in program code (and are reserved symbols):
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- ``A``, ``X``, ``Y`` the three main cpu registers (8 bits)
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- ``AX``, ``AY``, ``XY`` surrogate 16-bit registers: LSB-order (lo/hi) combined register pairs
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- the status register (P) carry flag and interrupt disable flag can be written via the ``P_carry`` and ``P_irqd`` builtin functions.
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Subroutine Calling Conventions
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------------------------------
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Subroutine arguments and results are passed via registers.
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Sometimes the status register's Carry flag is used as well (as a boolean flag).
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Additional arguments can be passed via memory locations as well ofcourse.
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But you'll have to be careful when dealing with chained or even recursive calls then,
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because there's a big risk of overwriting those memory locations.
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In Prog8 the "caller saves" principle applies to calling subroutines.
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This means the code that calls a subroutine that clobbers certain
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registers (``A``, ``X`` or ``Y``), is responsible for storing and restoring the original values if
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those values are needed by the rest of the code.
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Normally, registers are *not* preserved when calling a subroutine or when a certian
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operations are performed. Most calls will be simply a few instructions to load the
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values in the registers and then a ``JSR`` or ``JMP``.
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By using the ``%saveregisters`` directive in a block, you can tell the
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compiler to preserve all registers. This does generate a lot of extra code that puts
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original values on the stack and gets them off the stack again once the subroutine is done.
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In this case however you don't have to worry about ``A``, ``X`` and ``Y`` losing their original values
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and you can essentially treat them as three local variables instead of scratch data.
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You can also use a ``!`` on a single subroutine call to preserve register values, instead of
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setting this behavior for the entire block.
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.. important::
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Basically, you should assume that the 3 hardware registers ``A``, ``X`` and ``Y``
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are volatile. Their values cannot be depended upon, unless you explicitly make sure otherwise.
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