*************************** Target system specification *************************** Prog8 targets the following hardware: - 8 bit MOS 6502/65c02/6510 CPU - 64 Kb addressable memory (RAM or ROM) - optional use of memory mapped I/O registers - optional use of system ROM routines Currently there are two machines that are supported as compiler target (selectable via the ``-target`` compiler argument): - 'c64': the well-known Commodore-64 - 'cx16': the `CommanderX16 `_ conceived by the 8-Bit Guy. This chapter explains the relevant system details of these machines. .. hint:: If you only use standard kernel and prog8 library routines, it is possible to compile the *exact same program* for both machines (just change the compiler target flag)! Memory Model ============ Physical address space layout ----------------------------- The 6502 CPU can address 64 kilobyte of memory. Most of the 64 kilobyte address space can be used by Prog8 programs. This is a hard limit: there is no built-in support for RAM expansions or bank switching. ====================== ================== ======== memory area type note ====================== ================== ======== ``$00``--``$ff`` ZeroPage contains many sensitive system variables ``$100``--``$1ff`` Hardware stack used by the CPU, normally not accessed directly ``$0200``--``$ffff`` Free RAM or ROM free to use memory area, often a mix of RAM and ROM ====================== ================== ======== A few of these memory addresses are reserved and cannot be used for arbitrary data. They have a special hardware function, or are reserved for internal use in the code generated by the compiler: ================== ======================= reserved address in use for ================== ======================= ``$00`` data direction (CPU hw) ``$01`` bank select (CPU hw) ``$02`` internal scratch variable ``$03`` internal scratch variable ``$fb - $fc`` internal scratch variable ``$fd - $fe`` internal scratch variable ``$fffa - $fffb`` NMI vector (CPU hw) ``$fffc - $fffd`` RESET vector (CPU hw) ``$fffe - $ffff`` IRQ vector (CPU hw) ================== ======================= The actual machine will often have many other special addresses as well, For example, the Commodore-64 has: - ROMs installed in the machine: BASIC, kernal and character roms. Occupying ``$a000``--``$bfff`` and ``$e000``--``$ffff``. - memory-mapped I/O registers, for the video and sound chips, and the CIA's. Occupying ``$d000``--``$dfff``. - RAM areas that are used for screen graphics and sprite data: usually at ``$0400``--``$07ff``. Prog8 programs can access all of those special memory locations but it will have a special meaning. .. _zeropage: ZeroPage ("ZP") --------------- The ZeroPage memory block ``$02``--``$ff`` can be regarded as 254 CPU 'registers', because they take less clock cycles to access and need fewer instruction bytes than accessing other memory locations outside of the ZP. Theoretically they can all be used in a program, with the follwoing limitations: - several addresses (``$02``, ``$03``, ``$fb - $fc``, ``$fd - $fe``) are reserved for internal use - most other addresses will already be in use by the machine's operating system or kernal, and overwriting them will probably crash the machine. It is possible to use all of these yourself, but only if the program takes over the entire system (and seizes control from the regular kernal). This means it can no longer use (most) BASIC and kernal routines from ROM. - it's more convenient and safe to let the compiler allocate these addresses for you and just use symbolic names in the program code. Prog8 knows what addresses are safe to use in the various ZP handling configurations. It will use the free ZP addresses to place its ZP variables in, until they're all used up. If instructed to output a program that takes over the entire machine, (almost) all of the ZP addresses are suddenly available and will be used. **ZeroPage handling is configurable:** There's a global program directive to specify the way the compiler treats the ZP for the program. The default is to be reasonably restrictive to use the part of the ZP that is not used by the C64's kernal routines. It's possible to claim the whole ZP as well (by disabling the operating system or kernal). If you want, it's also possible to be more restricive and stay clear of the addresses used by BASIC routines too. This allows the program to exit cleanly back to a BASIC ready prompt - something that is not possible in the other modes. IRQs and the ZeroPage ^^^^^^^^^^^^^^^^^^^^^ The normal IRQ routine in the C-64's kernal will read and write several addresses in the ZP (such as the system's software jiffy clock which sits in ``$a0 - $a2``): ``$a0 - $a2``; ``$91``; ``$c0``; ``$c5``; ``$cb``; ``$f5 - $f6`` These addresses will *never* be used by the compiler for ZP variables, so variables will not interfere with the IRQ routine and vice versa. This is true for the normal ZP mode but also for the mode where the whole system and ZP have been taken over. So the normal IRQ vector can still run and will be when the program is started! CPU === Directly Usable Registers ------------------------- The hardware CPU registers are not directly accessible from regular Prog8 code. If you need to mess with them, you'll have to use inline assembly. Be extra wary of the ``X`` register because it is used as an evaluation stack pointer and changing its value you will destroy the evaluation stack and likely crash the program. The status register (P) carry flag and interrupt disable flag can be written via a couple of special builtin functions (``set_carry()``, ``clear_carry()``, ``set_irqd()``, ``clear_irqd()``), and read via the ``read_flags()`` function. The 16 'virtual' 16-bit registers that are defined on the Commander X16 machine are not real hardware registers and are just 16 memory-mapped word values that you *can* access directly. IRQ Handling ============ Normally, the system's default IRQ handling is not interfered with. You can however install your own IRQ handler. This is possible ofcourse by doing it all using customized inline assembly, but there are a few library routines available to make setting up C-64 IRQs and raster IRQs a lot easier (no assembly code required). For the C64 these routines are:: c64.set_irqvec() c64.set_irqvec_excl() c64.set_rasterirq( ) c64.set_rasterirq_excl( ) c64.restore_irqvec() ; set it back to the systems default irq handler If you activate an IRQ handler with one of these, it expects the handler to be defined as a subroutine ``irq`` in the module ``irq`` so like this:: irq { sub irq() { ; ... irq handling here ... } }