From d989825216ee3d57eba094a335a9c22537771943 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Tue, 2 Nov 2021 03:47:39 -0700
Subject: [PATCH] Add bonus notes on VPOSR.

---
 Machines/Amiga/Chipset.cpp | 11 ++++++++++-
 Machines/Amiga/Chipset.hpp |  1 +
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/Machines/Amiga/Chipset.cpp b/Machines/Amiga/Chipset.cpp
index 51d339edc..be3590d55 100644
--- a/Machines/Amiga/Chipset.cpp
+++ b/Machines/Amiga/Chipset.cpp
@@ -612,7 +612,16 @@ void Chipset::perform(const CPU::MC68000::Microcycle &cycle) {
 		// Raster position.
 		case Read(0x004): {		// VPOSR; b15 = LOF, b0 = b8 of y position.
 			const uint16_t position = uint16_t(y_ >> 8);
-			cycle.set_value16(position);
+			cycle.set_value16(
+				position |
+				(is_long_field_ ? 0x8000 : 0x0000)
+			);
+
+			// b8–b14 should be:
+			//	00 for PAL Agnus or fat Agnus
+			//	10 for NTSC Agnus or fat Agnus
+			//	20 for PAL high-res
+			//	30 for NTSC high-res
 		} break;
 		case Read(0x006): {		// VHPOSR; b0–b7 = horizontal; b8–b15 = low bits of vertical position.
 			const uint16_t position = uint16_t(((line_cycle_ >> 1) & 0x00ff) | (y_ << 8));
diff --git a/Machines/Amiga/Chipset.hpp b/Machines/Amiga/Chipset.hpp
index 2521110c2..8ef5cf47a 100644
--- a/Machines/Amiga/Chipset.hpp
+++ b/Machines/Amiga/Chipset.hpp
@@ -253,6 +253,7 @@ class Chipset: private ClockingHint::Observer {
 		bool hold_and_modify_ = false;
 		bool dual_playfields_ = false;
 		bool interlace_ = false;
+		bool is_long_field_ = false;
 
 		class BitplaneShifter {
 			public: