_sys() is supposed to be (primarily) intended to call ROM routines. Leveraging the "file overlay" mechanism of the cc65 build system allows to provide a Apple II specific _sys() implementation that temporarily switches in the ROM.
Make the same changes to the Apple II that were done with 0ee9b2e446 to the C64.
Notes:
- The startup code deliberately doesn't make use of symbols defined for the LC segment as that segment is optional.
- The <...>-asm.cfg configs move the segment BSS to an own memory area BSS although this doesn't seem necessary. However the benefit is that the size of the memeory area MAIN is identical to the number of bytes loaded from disk into RAM. To keep this an invariant for all Apple II configs allows to simplify the EXEHDR to just refer to the symbols defined for MAIN.
The constructors are _NOT_ allowed anymore to access the BSS. Rather they must use the DATA segment or the INIT segment. The latter isn't cleared at any point so the constructors may use it to expose values to the main program. However they must make sure to always write the values as they are not pre-initialized.
The BSS segment and the ONCE segment share the same start address. So they need to be placed in two different memory areas.
So far BSS was placed in the MAIN memory area and ONCE was placed in an additional memory area. Both memory areas were written to the output file. They just "happened" to be loadable and runnable at a stretch.
Now ONCE is placed in the MAIN memory area and BSS is placed in an additional memory area. Only MAIN is written to the output file. It becomes more obvious that BSS is "just" defined to share memory with ONCE.
The name RAM doesn't make much sense in general for a memeory area because i.e. the zero page is for sure RAM but is not part of the memory area named RAM.
For disk based targets it makes sense to put the disk file more into focus and here MAIN means the main part of the file - in contrast to some header.
Only for ROM based targets the name RAM is kept as it makes sense to focus on the difference between RAM and ROM.
The way we want to use the INITBSS segment - and especially the fact that it won't have the type bss on all ROM based targets - means that the name INITBSS is misleading. After all INIT is the best name from my perspective as it serves several purposes and therefore needs a rather generic name.
Unfortunately this means that the current INIT segment needs to be renamed too. Looking for a short (ideally 4 letter) name I came up with ONCE as it contains all code (and data) accessed only once during initialization.
Conceptually the INITBSS segment is not initialized in any way. Therefore it makes sense to not load it from disk. However the INIT segment has to be loaded from disk and therefore moved to its run location above the INITBSS segment. The necessary move routine increases runtime RAM usage :-(
Therefore we now "unnecessarily" load the INITBSS segment from disk too meaning that the INIT segment is loaded at its run location. Therefore there's no need for the move routine anymore.
After all we trade disk space for (runtime) RAM space - an easy decision ;-)
Notes:
- The code allowing to re-run a program without re-load present so far could not have worked as far as I can see as it only avoided to re-run the move routine but still tried to re-run the code in the INIT segment that was clobbered by zeroing the BSS. Therefore I removed the code in question altogether. I'm personally not into this "dirty re-run" but if someone wants to add an actually working solution I won't block that.
- INITBSS is intentionally not just merged with the DATA segment as ROM-based targets can't reuse the INIT segment for the BSS and therefore have no reason to place the INIT segment above INITBSS.
- Because ROM-based targets don't copy INITBSS from the ROM (like it is done with the DATA segment) all users of INITBSS _MUST_NOT_ presume INITBSS to be initialized with zeros!
A call to $FDA3 cannot be used because it re-enables the BASIC ROM. If a large program (such as Contiki's webbrowser80) has destructor code or data "behind" that ROM, then the program might crash when it tries to quit gracefully. Changing that code to set CIA2_PRA works well enough.
They are smaller and faster because they take advantage of the pce CPU's block-copy instructions.
Also, made a small improvement to the common memmove(), so that it is similar to the pce version.
Moving __cwd from BSS into INITBSS does of course ;-) not only impact the CBM targets but all targets with disk I/O support.
Note: Code using `__cwd-1` may trigger an ld65 range error because __cwd may end up at the very begining of a segment. As far as I see this is an ld65 bug which I'm not try to fix - at least here.
So far the INIT segment was run from the later heap+stack. Now the INIT segment is run from the later BSS. The background is that so far the INIT segment was pretty small (from $80 to $180 bytes). But upcoming changes will increase the INIT segment in certain scenarios up to ~ $1000 bytes. So programs with very limited heap+stack might just not been able to move the INIT segment to its run location. But moving the INIT segment to the later BSS allows it to occupy the later BSS+heap+stack.
In order to allow that the constructors are _NOT_ allowed anymore to access the BSS. Rather they must use the DATA segment or the new INITBSS segment. The latter isn't cleared at any point so the constructors may use it to expose values to the main program. However they must make sure to always write the values as they are not pre-initialized.
