As described e.g. in the Apple IIe Technote #6: 'The Apple II Paddle Circuits' it doesn't work to call PREAD several times in immediate succession. However, so far the Apple II joystick driver did just that in order to read the two joystick axis.
Therefore the driver now uses a custom routine that reads both paddles _at_the_same_time_. The code doing so requires nearly twice the cycles meaning that the overall time for a joy_read() stays roughly the same. However, twice the cycles in the read loop means half the resolution. But for the cc65 joystick driver use case that doesn't hurt at all as the driver is supposed to only detect neutral vs. left/right and up/down.
CPU accelerators are supposed to detect access to $C070 and slow down for some time automatically. However, the IIgs rather comes with a modified ROM routine. Therefore it is necessary to manually slow down the IIgs when replacing the ROM routine.
For some reason or another both the author of the PR in question and its reviewers didn't notice that the two functions in question were totally overlooked.
Do not check $fbbe when detecting the Apple //e card. This byte is a version number for the Apple //e card according to misc technote #7 and it appears that the last version of the software that I am aware of has a 3 at this location.
Prior to this change, Apple //e cards which we not version 0 would be detected as an Apple //e enhanced.
tmp1 was used at two places resulting in the bug that VDC_CSET was set to garbage on 16k VDC.
pagecount and curpage were not reset on INSTALL resulting in non-reentrant code on static linkage of emd driver.
To do this we add a constructor call to UPDCRAMPTR, which is the ROM
routine that fixes up CRAM_PTR to match the screen location pointed to
by SCREEN_PTR.
This adds two additional bytes to programs using cputc() or other
routines that call it. These are in theory recoverable, but the VIC-20
does not yet free space used by constructors after the constructors
have been called.
Thanks to <greg.king5@verizon.net> (GitHub: greg-king5) for
investigating the difference in the VIC-20 KERNAL from the C64 and
proposing this solution to the problem.[1]
[1]: https://github.com/cc65/cc65/issues/946#issuecomment-538502820
On C64, VIC-20 and Plus/4, the conio library PLOT routine uses direct
calls into the Kernal, including the Kernal PLOT routine that we're
replacing. These were previously hardcoded addresses; we change these
to use the symbols for those routines defined in cbm_kernal.inc. (This
changes no functionality.)
To do this, we need to import cbm_kernal.inc in a namespace so we
don't have a collision between the PLOT that we're defining and the
Kernal definition.
We also add a UPDCRAMPTR symbol (used by kplot for VIC-20 and C64) to
the direct entry kernal routines in in cbm_kernal.inc, and expand the
comments describing what the "direct entry" Kernal routines are.
<greg.king5@verizon.net> (GitHub: greg-king5) came up with this idea
and did initial testing of it.
This has been tested on VICE xvic, x64 and xplus4 emulators with a
program that does a cputs() call (github.com/0cjs/vic20cc65) to
confirm that it works the same way after as it did before.
Restructured according to review in pull request #567.
The "inputc" function was moved in slightly modified form
to kbhit.s and replaces the old keyboard scanner.
cgetc() uses the new kbhit() to read a character.
* The sprite-types for black and transparent backgrounds were swapped.
* A filler-byte for text output isn't printed. (A hardware bug might not need that work-around in most cases.)