From b34403164ecb151c5eda7fbae42410c9c8462ef6 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 18 Aug 2023 14:30:40 -0400
Subject: [PATCH] Abstract out VGC interrupt register; fix clearing bug.

---
 Machines/Apple/AppleIIgs/Video.cpp | 18 ++++-------
 Machines/Apple/AppleIIgs/Video.hpp | 52 ++++++++++++++++++++++++++++--
 2 files changed, 56 insertions(+), 14 deletions(-)

diff --git a/Machines/Apple/AppleIIgs/Video.cpp b/Machines/Apple/AppleIIgs/Video.cpp
index e78e6ff4b..3f6269193 100644
--- a/Machines/Apple/AppleIIgs/Video.cpp
+++ b/Machines/Apple/AppleIIgs/Video.cpp
@@ -285,7 +285,7 @@ void Video::output_row(int row, int start, int end) {
 
 				// Post an interrupt if requested.
 				if(line_control_ & 0x40) {
-					set_interrupts(0x20);
+					interrupts_.add(0x20);
 				}
 
 				// Set up appropriately for fill mode (or not).
@@ -498,19 +498,19 @@ uint8_t Video::get_new_video() {
 }
 
 void Video::clear_interrupts(uint8_t mask) {
-	set_interrupts(interrupts_ & ~(mask & 0x60));
+	interrupts_.clear(mask);
 }
 
 void Video::set_interrupt_register(uint8_t mask) {
-	set_interrupts(interrupts_ | (mask & 0x6));
+	interrupts_.set_control(mask);
 }
 
 uint8_t Video::get_interrupt_register() {
-	return interrupts_;
+	return interrupts_.status();
 }
 
 bool Video::get_interrupt_line() {
-	return (interrupts_&0x80) || (megaii_interrupt_mask_&megaii_interrupt_state_);
+	return interrupts_.active() || (megaii_interrupt_mask_ & megaii_interrupt_state_);
 }
 
 void Video::set_megaii_interrupts_enabled(uint8_t mask) {
@@ -526,13 +526,7 @@ void Video::clear_megaii_interrupts() {
 }
 
 void Video::notify_clock_tick() {
-	set_interrupts(interrupts_ | 0x40);
-}
-
-void Video::set_interrupts(uint8_t new_value) {
-	interrupts_ = new_value & 0x7f;
-	if((interrupts_ >> 4) & interrupts_ & 0x6)
-		interrupts_ |= 0x80;
+	interrupts_.add(0x40);
 }
 
 void Video::set_border_colour(uint8_t colour) {
diff --git a/Machines/Apple/AppleIIgs/Video.hpp b/Machines/Apple/AppleIIgs/Video.hpp
index a6f5c9788..9d30f2dff 100644
--- a/Machines/Apple/AppleIIgs/Video.hpp
+++ b/Machines/Apple/AppleIIgs/Video.hpp
@@ -123,8 +123,56 @@ class Video: public Apple::II::VideoSwitches<Cycles> {
 		void advance(Cycles);
 
 		uint8_t new_video_ = 0x01;
-		uint8_t interrupts_ = 0x00;
-		void set_interrupts(uint8_t);
+
+		class Interrupts {
+			public:
+				void add(uint8_t value) {
+					// Taken literally, status accumulates regardless of being enabled,
+					// potentially to be polled, it simply doesn't trigger an interrupt.
+					value_ |= value;
+					test();
+				}
+
+				void clear(uint8_t value) {
+					// Zeroes in bits 5 or 6 clear the respective interrupts.
+					value_ &= value | ~0x60;
+					test();
+				}
+
+				void set_control(uint8_t value) {
+					// Ones in bits 1 or 2 enable the respective interrupts.
+					value_ = (value_ & ~0x6) | (value & 0x6);
+					test();
+				}
+
+				uint8_t status() const {
+					return value_;
+				}
+
+				bool active() const {
+					return value_ & 0x80;
+				}
+
+			private:
+				void test() {
+					value_ &= 0x7f;
+					if((value_ >> 4) & value_ & 0x6) {
+						value_ |= 0x80;
+					}
+				}
+
+				// Overall meaning of value is as per the VGC interrupt register, i.e.
+				//
+				//	b7: interrupt status;
+				//	b6: 1-second interrupt status;
+				//	b5: scan-line interrupt status;
+				//	b4: reserved;
+				//	b3: reserved;
+				//	b2: 1-second interrupt enable;
+				//	b1: scan-line interrupt enable;
+				//	b0: reserved.
+				uint8_t value_ = 0x00;
+		} interrupts_;
 
 		int cycles_into_frame_ = 0;
 		const uint8_t *ram_ = nullptr;