Starts thinking out the mechanics of emulating a SCSI-1 bus.

Components/5380/SCSI.cpp

@@ -0,0 +1,34 @@
+//
+//  SCSI.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 12/08/2019.
+//  Copyright © 2019 Thomas Harte. All rights reserved.
+//
+
+#include "SCSI.hpp"
+
+using namespace SCSI;
+
+size_t Bus::add_device() {
+	const auto slot = device_states_.size();
+	device_states_.push_back(DefaultBusState);
+	return slot;
+}
+
+void Bus::set_device_output(size_t device, BusState output) {
+	device_states_[device] = output;
+	state_is_valid_ = false;
+}
+
+BusState Bus::get_state() {
+	if(!state_is_valid_) return state_;
+
+	state_is_valid_ = true;
+	state_ = DefaultBusState;
+	for(auto state: device_states_) {
+		state_ &= state;
+	}
+
+	return state_;
+}

Components/5380/SCSI.hpp

@@ -0,0 +1,79 @@
+//
+//  SCSI.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 12/08/2019.
+//  Copyright © 2019 Thomas Harte. All rights reserved.
+//
+
+#ifndef SCSI_hpp
+#define SCSI_hpp
+
+#include <limits>
+#include <vector>
+
+namespace SCSI {
+
+typedef int BusState;
+
+static const BusState DefaultBusState = std::numeric_limits<BusState>::max();
+
+/*!
+	SCSI bus state is encoded entirely within an int.
+	Bits correlate mostly but not exactly to the real SCSI bus.
+
+	TODO: validate levels below. The bus uses open collector logic,
+	so active low needs to be respected.
+*/
+enum Line: BusState {
+	/// Provides the value currently on the data lines.
+	Data 			= 0xff,
+	/// Parity of the data lines.
+	Parity		 	= 1 << 8,
+	/// Set if the SEL line is currently selecting a target.
+	/// Reset if it is selecting an initiator.
+	SelectTarget	= 1 << 9,
+	/// Reset to indicate an attention condition. Set otherwise.
+	Attention		= 1 << 10,
+	/// Set if control is on the bus. Reset if data is on the bus.
+	Control			= 1 << 11,
+	/// Reset if the bus is busy. Set otherwise.
+	Busy			= 1 << 12,
+	/// Reset if acknowledging a data transfer request. Set otherwise.
+	Acknowledge		= 1 << 13,
+	/// Reset if a bus reset is being requested. Set otherwise.
+	Reset			= 1 << 14,
+	/// Set if data is currently input. Reset if it is an output.
+	Input			= 1 << 15,
+	/// Set during the message phase. Reset otherwise.
+	MessagePhase	= 1 << 16
+};
+
+
+class Bus {
+	public:
+		/*!
+			Adds a device to the bus, returning the index it should use
+			to refer to itself in subsequent calls to set_device_output.
+		*/
+		size_t add_device();
+
+		/*!
+			Sets the current output for @c device.
+		*/
+		void set_device_output(size_t device, BusState output);
+
+		/*!
+			@returns the current state of the bus.
+		*/
+		BusState get_state();
+
+	private:
+		std::vector<BusState> device_states_;
+		BusState state_ = DefaultBusState;
+		bool state_is_valid_ = false;
+};
+
+}
+
+#endif /* SCSI_hpp */

Components/5380/ncr5380.cpp

@@ -12,19 +12,56 @@
 
 using namespace NCR::NCR5380;
 
+NCR5380::NCR5380() {
+	device_id_ = bus_.add_device();
+}
+
 void NCR5380::write(int address, uint8_t value) {
+	using Line = SCSI::Line;
 	switch(address & 7) {
 		case 0:
 			LOG("[SCSI 0] Set current SCSI bus state to " << PADHEX(2) << int(value));
+			data_bus_ = value;
 		break;
 
-		case 1:
+		case 1: {
 			LOG("[SCSI 1] Initiator command register set: " << PADHEX(2) << int(value));
-		break;
+			initiator_command_ = value;
+
+			SCSI::BusState mask = SCSI::DefaultBusState;
+			if(value & 0x80) mask &= ~Line::Reset;
+			test_mode_ = !!(value & 0x40);
+			/* bit 5 = differential enable if this were a 5381 */
+			if(value & 0x10) mask &= ~Line::Acknowledge;
+			if(value & 0x08) mask &= ~Line::Busy;
+			if(value & 0x04) mask &= ~Line::SelectTarget;
+			if(value & 0x02) mask &= ~Line::Attention;
+			assert_data_bus_ = (value & 0x01);
+			bus_output_ = (bus_output_ | Line::Reset | Line::Acknowledge | Line::Busy | Line::SelectTarget | Line::Attention) & mask;
+		} break;
 
 		case 2:
 			LOG("[SCSI 2] Set mode: " << PADHEX(2) << int(value));
 			mode_ = value;
+
+			// bit 7: 1 = use block mode DMA mode (if DMA mode is also enabled)
+			// bit 6: 1 = be a SCSI target; 0 = be an initiator
+			// bit 5: 1 = check parity
+			// bit 4: 1 = generate an interrupt if parity checking is enabled and an error is found
+			// bit 3: 1 = generate an interrupt when an EOP is received from the DMA controller
+			// bit 2: 1 = generate an interrupt and reset low 6 bits of register 1 if an unexpected loss of Line::Busy occurs
+			// bit 1: 1 = use DMA mode
+			// bit 0: 1 = begin arbitration mode (device ID should be in register 0)
+
+			/*
+				Arbitration is accomplished using a bus-free filter to continuously monitor BSY.
+				If BSY remains inactive for at least 400 nsec then the SCSI bus is considered free
+				and arbitration may begin. Arbitration will begin if the bus is free, SEL is inactive
+				and the ARBITRATION bit (port 2, bit 0) is active. Once arbitration has begun
+				(BSY asserted), an arbitration delay of 2.2 /Lsec must elapse before the data bus
+				can be examined to deter- mine if arbitration has been won. This delay must be
+				implemented in the controlling software driver.
+			*/
 		break;
 
 		case 3:
@@ -47,17 +84,32 @@ void NCR5380::write(int address, uint8_t value) {
 			LOG("[SCSI 7] Start DMA initiator receive: " << PADHEX(2) << int(value));
 		break;
 	}
+
+	// Data is output only if the data bus is asserted.
+	if(assert_data_bus_) {
+		bus_output_ &= data_bus_;
+	} else {
+		bus_output_ |= SCSI::Line::Data;
+	}
+
+	// In test mode, still nothing is output. Otherwise throw out
+	// the current value of bus_output_.
+	if(test_mode_) {
+		bus_.set_device_output(device_id_, SCSI::DefaultBusState);
+	} else {
+		bus_.set_device_output(device_id_, bus_output_);
+	}
 }
 
 uint8_t NCR5380::read(int address) {
 	switch(address & 7) {
 		case 0:
 			LOG("[SCSI 0] Get current SCSI bus state");
-		return 0xff;
+		return uint8_t(bus_.get_state());
 
 		case 1:
 			LOG("[SCSI 1] Initiator command register get");
-		return 0xff;
+		return initiator_command_;
 
 		case 2:
 			LOG("[SCSI 2] Get mode");

Components/5380/ncr5380.hpp

@@ -11,6 +11,10 @@
 
 #include <cstdint>
 
+#include "SCSI.hpp"
+#include "../../ClockReceiver/ClockReceiver.hpp"
+
+
 namespace NCR {
 namespace NCR5380 {
 
@@ -19,14 +23,40 @@ namespace NCR5380 {
 */
 class NCR5380 {
 	public:
+		NCR5380();
+
 		/*! Writes @c value to @c address.  */
 		void write(int address, uint8_t value);
 
 		/*! Reads from @c address. */
 		uint8_t read(int address);
 
+		/*!
+			As per its design manual:
+
+				"The NCR 5380 is a clockless device. Delays such as bus
+				free delay, bus set delay and bus settle delay are
+				implemented using gate delays."
+
+			Therefore this fictitious implementation of an NCR5380 needs
+			a way to track time even though the real one doesn't take in
+			a clock. This is provided by `run_for`.
+
+			Nevertheless, the clocking doesn't need to be very precise.
+			Please provide a clock that is close to 200,000 Hz.
+		*/
+		void run_for(Cycles);
+
 	private:
+		SCSI::Bus bus_;
+		size_t device_id_;
+
+		SCSI::BusState bus_output_ = SCSI::DefaultBusState;
 		uint8_t mode_ = 0xff;
+		uint8_t initiator_command_ = 0xff;
+		uint8_t data_bus_ = 0xff;
+		bool test_mode_ = false;
+		bool assert_data_bus_ = false;
 };
 
 }

OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj

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 		4B894540201967D6007DE474 /* Machines.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = Machines.hpp; sourceTree = "<group>"; };
+		4B89BCFA23024BB500EA0782 /* SCSI.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = SCSI.cpp; sourceTree = "<group>"; };
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@@ -3327,6 +3331,8 @@
 			children = (
 				4BDACBEA22FFA5D20045EF7E /* ncr5380.cpp */,
 				4BDACBEB22FFA5D20045EF7E /* ncr5380.hpp */,
+				4B89BCFA23024BB500EA0782 /* SCSI.cpp */,
+				4B89BCFB23024BB500EA0782 /* SCSI.hpp */,
 			);
 			path = 5380;
 			sourceTree = "<group>";
@@ -4037,6 +4043,7 @@
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 				4B0333B02094081A0050B93D /* AppleDSK.cpp in Sources */,
 				4B894535201967B4007DE474 /* AddressMapper.cpp in Sources */,
+				4B89BCFD23024BB500EA0782 /* SCSI.cpp in Sources */,
 				4B055AD41FAE9B0B0060FFFF /* Oric.cpp in Sources */,
 				4B055A921FAE85B50060FFFF /* PRG.cpp in Sources */,
 				4B055AAF1FAE85FD0060FFFF /* UnformattedTrack.cpp in Sources */,
@@ -4194,6 +4201,7 @@
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 				4BBB70A4202011C2002FE009 /* MultiMediaTarget.cpp in Sources */,
 				4B89453A201967B4007DE474 /* StaticAnalyser.cpp in Sources */,
+				4B89BCFC23024BB500EA0782 /* SCSI.cpp in Sources */,
 				4BB697CB1D4B6D3E00248BDF /* TimedEventLoop.cpp in Sources */,
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