From 2c95dea4db551bd84391af92744dd063a919a39f Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Tue, 26 Jul 2022 17:05:05 -0400
Subject: [PATCH] Introduce putative blitter sequencer.

---
 Machines/Amiga/Blitter.cpp | 28 +++++++++++++++
 Machines/Amiga/Blitter.hpp | 73 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)

diff --git a/Machines/Amiga/Blitter.cpp b/Machines/Amiga/Blitter.cpp
index a179d0527..775f9e9d4 100644
--- a/Machines/Amiga/Blitter.cpp
+++ b/Machines/Amiga/Blitter.cpp
@@ -191,6 +191,34 @@ uint16_t Blitter::get_status() {
 	return result;
 }
 
+// Due to the pipeline, writes are delayed by one slot — the first write will occur
+// after the second set of inputs has been fetched, and every sequence with writes enabled
+// will end with an additional write.
+//
+//    USE Code
+//       in        Active
+//    BLTCON0     Channels             Cycle Sequence
+//   ---------    --------             --------------
+//       F        A B C D     A0 B0 C0 -  A1 B1 C1 D0 A2 B2 C2 D1 D2
+//       E        A B C       A0 B0 C0 A1 B1 C1 A2 B2 C2
+//       D        A B   D     A0 B0 -  A1 B1 D0 A2 B2 D1 -  D2
+//       C        A B         A0 B0 -  A1 B1 -  A2 B2
+//       B        A   C D     A0 C0 -  A1 C1 D0 A2 C2 D1 -  D2
+//       A        A   C       A0 C0 A1 C1 A2 C2
+//       9        A     D     A0 -  A1 D0 A2 D1 -  D2
+//       8        A           A0 -  A1 -  A2
+//       7          B C D     B0 C0 -  -  B1 C1 D0 -  B2 C2 D1 -  D2
+//       6          B C       B0 C0 -  B1 C1 -  B2 C2
+//       5          B   D     B0 -  -  B1 D0 -  B2 D1 -  D2
+//       4          B         B0 -  -  B1 -  -  B2
+//       3            C D     C0 -  -  C1 D0 -  C2 D1 -  D2
+//       2            C       C0 -  C1 -  C2
+//       1              D     D0 -  D1 -  D2
+//       0         none       -  -  -  -
+//
+//
+//       Table 6-2: Typical Blitter Cycle Sequence
+
 bool Blitter::advance_dma() {
 	if(!height_) return false;
 
diff --git a/Machines/Amiga/Blitter.hpp b/Machines/Amiga/Blitter.hpp
index 7ca0b1da3..a2ce3a42c 100644
--- a/Machines/Amiga/Blitter.hpp
+++ b/Machines/Amiga/Blitter.hpp
@@ -17,6 +17,79 @@
 
 namespace Amiga {
 
+class BlitterSequencer {
+	public:
+		enum class Channel {
+			Write, C, B, A, None
+		};
+
+		void set_control(int control) {
+			control_ = control;
+		}
+
+		void complete() {
+			complete_ = true;
+		}
+
+		Channel next() {
+			if(complete_ && !index_) {
+				// TODO: this isn't quite right; some patterns leave a gap before
+				// the final write, some don't. Figure this out.
+				return Channel::Write;
+			}
+
+			Channel next = Channel::None;
+
+			switch(control_) {
+				default: break;
+
+				case 1: next = next_channel(pattern1); break;
+				case 2: next = next_channel(pattern2); break;
+				case 3: next = next_channel(pattern3); break;
+				case 4: next = next_channel(pattern4); break;
+				case 5: next = next_channel(pattern5); break;
+				case 6: next = next_channel(pattern6); break;
+				case 7: next = next_channel(pattern7); break;
+				case 8: next = next_channel(pattern8); break;
+				case 9: next = next_channel(pattern9); break;
+				case 10: next = next_channel(patternA); break;
+				case 11: next = next_channel(patternB); break;
+				case 12: next = next_channel(patternC); break;
+				case 13: next = next_channel(patternD); break;
+				case 14: next = next_channel(patternE); break;
+				case 15: next = next_channel(patternF); break;
+			}
+
+			return next;
+		}
+
+	private:
+		static constexpr std::array<Channel, 2> pattern1 = { Channel::Write, Channel::None };
+		static constexpr std::array<Channel, 2> pattern2 = { Channel::C, Channel::None };
+		static constexpr std::array<Channel, 3> pattern3 = { Channel::C, Channel::Write, Channel::None };
+		static constexpr std::array<Channel, 3> pattern4 = { Channel::B, Channel::None, Channel::None };
+		static constexpr std::array<Channel, 3> pattern5 = { Channel::B, Channel::Write, Channel::None };
+		static constexpr std::array<Channel, 3> pattern6 = { Channel::B, Channel::C, Channel::None };
+		static constexpr std::array<Channel, 4> pattern7 = { Channel::B, Channel::C, Channel::Write, Channel::None };
+		static constexpr std::array<Channel, 2> pattern8 = { Channel::A, Channel::None };
+		static constexpr std::array<Channel, 2> pattern9 = { Channel::A, Channel::Write };
+		static constexpr std::array<Channel, 2> patternA = { Channel::A, Channel::C };
+		static constexpr std::array<Channel, 3> patternB = { Channel::A, Channel::C, Channel::Write };
+		static constexpr std::array<Channel, 3> patternC = { Channel::A, Channel::B, Channel::None };
+		static constexpr std::array<Channel, 3> patternD = { Channel::A, Channel::B, Channel::Write };
+		static constexpr std::array<Channel, 3> patternE = { Channel::A, Channel::B, Channel::C };
+		static constexpr std::array<Channel, 4> patternF = { Channel::A, Channel::B, Channel::C, Channel::Write };
+		template <typename ArrayT> Channel next_channel(const ArrayT &list) {
+			const Channel result = list[index_];
+			index_ = (index_ + 1) % list.size();
+			return result;
+		}
+
+		int control_ = 0;
+		int index_ = 0;
+		bool complete_ = false;
+};
+
 class Blitter: public DMADevice<4, 4> {
 	public:
 		using DMADevice::DMADevice;