From a1e4389f63de06c38542de04c7c8e8f5c9d7475e Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Tue, 15 Jun 2021 17:43:13 -0400
Subject: [PATCH] Give Nick some RAM to inspect and just enough sense to know
 when it should reload its line parameter table.

---
 Machines/Enterprise/Enterprise.cpp | 13 +++++++------
 Machines/Enterprise/Nick.cpp       | 23 ++++++++++++++++++++---
 Machines/Enterprise/Nick.hpp       |  7 ++++++-
 3 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/Machines/Enterprise/Enterprise.cpp b/Machines/Enterprise/Enterprise.cpp
index 3964c5425..5896c881b 100644
--- a/Machines/Enterprise/Enterprise.cpp
+++ b/Machines/Enterprise/Enterprise.cpp
@@ -63,7 +63,8 @@ class ConcreteMachine:
 	public MachineTypes::TimedMachine {
 	public:
 		ConcreteMachine([[maybe_unused]] const Analyser::Static::Enterprise::Target &target, const ROMMachine::ROMFetcher &rom_fetcher) :
-			z80_(*this) {
+			z80_(*this),
+			nick_(ram_.end() - 65536) {
 			// Request a clock of 4Mhz; this'll be mapped upwards for Nick and Dave elsewhere.
 			set_clock_rate(4'000'000);
 
@@ -189,11 +190,9 @@ class ConcreteMachine:
 		}
 
 	private:
-		CPU::Z80::Processor<ConcreteMachine, false, false> z80_;
-
 		// MARK: - Memory layout
-		std::array<uint8_t, 32 * 1024> exos_;
 		std::array<uint8_t, 256 * 1024> ram_;
+		std::array<uint8_t, 32 * 1024> exos_;
 		const uint8_t min_ram_slot_ = 0xff - 3;
 
 		const uint8_t *read_pointers_[4] = {nullptr, nullptr, nullptr, nullptr};
@@ -209,7 +208,8 @@ class ConcreteMachine:
 			}
 
 			if(offset >= min_ram_slot_) {
-				const size_t address = (offset - min_ram_slot_) * 0x4000;
+				const auto ram_floor = 4194304 - ram_.size();
+				const size_t address = offset * 0x4000 - ram_floor;
 				page<slot>(&ram_[address], &ram_[address]);
 				return;
 			}
@@ -236,7 +236,8 @@ class ConcreteMachine:
 			z80_.run_for(cycles);
 		}
 
-		// MARK: - Video.
+		// MARK: - Chips.
+		CPU::Z80::Processor<ConcreteMachine, false, false> z80_;
 		JustInTimeActor<Nick, HalfCycles, 444923, 125000> nick_;
 		// Cf. timing guesses above.
 };
diff --git a/Machines/Enterprise/Nick.cpp b/Machines/Enterprise/Nick.cpp
index 4cf9268e1..3cd39de86 100644
--- a/Machines/Enterprise/Nick.cpp
+++ b/Machines/Enterprise/Nick.cpp
@@ -12,17 +12,34 @@
 
 using namespace Enterprise;
 
-Nick::Nick() :
-	crt_(16 * 57, 16, Outputs::Display::Type::PAL50, Outputs::Display::InputDataType::Red4Green4Blue4) {
+Nick::Nick(const uint8_t *ram) :
+	crt_(16 * 57, 16, Outputs::Display::Type::PAL50, Outputs::Display::InputDataType::Red4Green4Blue4),
+	ram_(ram) {
 
 	// Just use RGB for now.
 	crt_.set_display_type(Outputs::Display::DisplayType::RGB);
 }
 
 void Nick::write(uint16_t address, uint8_t value) {
+	printf("Nick write: %02x -> %d\n", value, address & 3);
 	switch(address & 3) {
 		default:
-			printf("Unhandled Nick write: %02x -> %d\n", value, address & 3);
+			printf("Unhandled\n");
+		break;
+
+		case 2:
+			line_parameter_base_ = uint16_t((line_parameter_base_ & 0xf000) | (value << 4));
+		break;
+		case 3:
+			line_parameter_base_ = uint16_t((line_parameter_base_ & 0x0ff0) | (value << 12));
+
+			// Still a mystery to me: the exact meaning of the top two bits here. For now
+			// just treat a 0 -> 1 transition of the MSB as a forced frame restart.
+			if((value^line_parameter_control_) & value & 0x80) {
+				// TODO: restart frame.
+				printf("Should restart frame from %04x\n", line_parameter_base_);
+			}
+			line_parameter_control_ = value & 0xc0;
 		break;
 	}
 }
diff --git a/Machines/Enterprise/Nick.hpp b/Machines/Enterprise/Nick.hpp
index 69a5bcb12..d38aa59a3 100644
--- a/Machines/Enterprise/Nick.hpp
+++ b/Machines/Enterprise/Nick.hpp
@@ -17,7 +17,7 @@ namespace Enterprise {
 
 class Nick {
 	public:
-		Nick();
+		Nick(const uint8_t *ram);
 
 		void write(uint16_t address, uint8_t value);
 		uint8_t read(uint16_t address);
@@ -29,6 +29,11 @@ class Nick {
 
 	private:
 		Outputs::CRT::CRT crt_;
+		const uint8_t *const ram_;
+
+		uint8_t line_parameter_control_ = 0xc0;
+		uint16_t line_parameter_base_ = 0x0000;
+		int horizontal_counter_ = 0;
 };