apple2gs: Improve IRQ emulation (#14277)

* g65816: fix IRQ to be level triggered

Followup b3f2535: stop clearing LINE_IRQ when jumping to the IRQ vector.
Properly written IRQ handlers won't notice any difference, but this fixes
the behavior for software which (possibly intentionally) fails to clear
an IRQ source; in that case the IRQ should continue to trigger (probably
leading to a system hang.)

LINE_SO is generally not hooked up, but if it is, it doesn't raise IRQ.

Also delete unused G65816_INT_* macros.

* apple2gs: improve VGC/MegaII IRQ emulation

All of the VGC and MegaII IRQ sources set their status bits even when
the corresponding IRQ is not enabled (this was only implemented for SCB).

Clearing enable bits works differently between VGC and MegaII: the MegaII
can lower the IRQ line (when all sources are clear), the VGC does not.
Neither clears their corresponding status bits.

VGCINTCLEAR reads the floating bus.  Status bits (and IRQs) are only
cleared when writing a zero.

ADB reset needs to clear (at least) m_irqmask.

Also cleanup CLRVBLINT into do_io().

* apple2gs: implement undocumented MegaII interaction

Reading the C02E/C02F counters resets the SCB status bit in C023
(the same as writing to C032).

This behavior is not documented by Apple, but was verified on hardware.

Move the relevant logic into a helper, to avoid code duplication.

* apple2gs: improve C019 VBL behavior

Ensure that VBL is cleared on scanline 0 and set on scanline 192, even in SHR.

* apple2gs: remove IOUDIS, fix AN3 and paddle

Followup 6a4854b: IOUDIS only exists on the IIc and IIc+.

AN2/3 are set by ROM during boot, and AN3 is readable via C046.

Also followup 23904ff: paddle only triggers on r/w of C070, and
it reads the floating bus.

* apple2gs: reduce do_io() log spam

16-bit writes in demos were tripping "unknown switch".

No functional change.

* apple2gs: update stale link

Author: arekkusu42

src/devices/cpu/g65816/g65816.cpp

@@ -725,7 +725,6 @@ void g65816_device::g65816i_check_maskable_interrupt()
 	{
 		g65816i_interrupt_hardware((FLAG_E) ? VECTOR_IRQ_E : VECTOR_IRQ_N);
 		CPU_STOPPED &= ~STOP_LEVEL_WAI;
-		LINE_IRQ=0; // FIXME: IRQ is level triggered, this makes it act as a HOLD_LINE
 	}
 }
 

src/devices/cpu/g65816/g65816.h

@@ -27,10 +27,9 @@ Copyright Karl Stenerud
 /* =============================== DEFINES ================================ */
 /* ======================================================================== */
 
-/* Interrupt lines - used with g65816_set_irq_line() */
+/* Interrupt lines - used with g65816_set_line() */
 enum
 {
-	G65816_LINE_NONE, // FIXME: there is no such pin
 	G65816_LINE_IRQ,
 	G65816_LINE_NMI,
 	G65816_LINE_ABORT,
@@ -39,10 +38,6 @@ enum
 	G65816_LINE_RESET
 };
 
-#define G65816_INT_NONE G65816_LINE_NONE
-#define G65816_INT_IRQ G65816_LINE_IRQ
-#define G65816_INT_NMI G65816_LINE_NMI
-
 
 class g65816_device : public cpu_device, public g65816_disassembler::config
 {

src/devices/cpu/g65816/g65816op.ipp

@@ -1830,7 +1830,6 @@ TABLE_FUNCTION(void, set_line, (int line, int state))
 				{
 					CPU_STOPPED &= ~STOP_LEVEL_WAI;
 				}
-				return;
 			}
 			return;
 		case G65816_LINE_NMI:
@@ -1855,8 +1854,6 @@ TABLE_FUNCTION(void, set_line, (int line, int state))
 		case G65816_LINE_RDY:
 			return;
 	}
-
-	LINE_IRQ=1; // FIXME: this can't be right!
 }