From 56d88f23ef53c800772fcf3c19d71935f2a8bf0d Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Mon, 23 Apr 2018 22:29:36 -0700
Subject: [PATCH] Teeters closer and closer to trying actually to run the Disk
 II state machine.

---
 Components/DiskII/DiskII.cpp    | 41 +++++++++++++++++++++++++++++----
 Components/DiskII/DiskII.hpp    |  9 +++++++-
 Machines/AppleII/DiskIICard.cpp |  9 ++++----
 3 files changed, 50 insertions(+), 9 deletions(-)

diff --git a/Components/DiskII/DiskII.cpp b/Components/DiskII/DiskII.cpp
index b93d12097..cab110b8c 100644
--- a/Components/DiskII/DiskII.cpp
+++ b/Components/DiskII/DiskII.cpp
@@ -18,20 +18,53 @@ void DiskII::set_control(Control control, bool on) {
 
 void DiskII::set_mode(Mode mode) {
 	printf("Set mode %d\n", mode);
+	state_ = (state_ & ~0x08) | ((mode == Mode::Write) ? 0x8 : 0x0);
 }
 
 void DiskII::select_drive(int drive) {
 	printf("Select drive %d\n", drive);
 }
 
-void DiskII::set_shift_register(uint8_t value) {
-	printf("Set shift register\n");
+void DiskII::set_data_register(uint8_t value) {
+	printf("Set data register (?)\n");
+	state_ |= 0x4;
+	data_register_ = value;
 }
 
 uint8_t DiskII::get_shift_register() {
-	printf("Get shift register\n");
-	return 0xff;
+	printf("Get shift register (?)\n");
+	state_ &= ~0x4;
+	return shift_register_;
 }
 
 void DiskII::run_for(const Cycles cycles) {
+/*
+... address the P6 ROM with an index byte built up as:
++-------+-------+-------+-------+-------+-------+-------+-------+
+| STATE | STATE | STATE | PULSE |  Q7   |  Q6   |  SR   | STATE |
+| bit 0 | bit 2 | bit 3 |       |       |       |  MSB  | bit 1 |
++-------+-------+-------+-------+-------+-------+-------+-------+
+    7       6       5       4       3       2       1       0
+
+...
+
+The bytes in the P6 ROM has the high four bits reversed compared to the BAPD charts, so you will have to reverse them after fetching the byte.
+
+*/
+
+	uint8_t command = 0;
+	switch(command) {
+		case 0x0:	shift_register_ = 0;	break;	// clear
+		case 0x9:	shift_register_ = static_cast<uint8_t>(shift_register_ << 1);			break;	// shift left, bringing in a zero
+		case 0xd:	shift_register_ = static_cast<uint8_t>((shift_register_ << 1) | 1);		break;	// shift left, bringing in a one
+		case 0xa:
+			shift_register_ = (shift_register_ >> 1) | (is_write_protected() ? 0x80 : 0x00);
+		break;	// shift right, bringing in write protected status
+		case 0xb:	shift_register_ = data_register_;				break;	// load
+		default: break;
+	}
+}
+
+bool DiskII::is_write_protected() {
+	return true;
 }
diff --git a/Components/DiskII/DiskII.hpp b/Components/DiskII/DiskII.hpp
index 49006bf3e..45155918f 100644
--- a/Components/DiskII/DiskII.hpp
+++ b/Components/DiskII/DiskII.hpp
@@ -29,10 +29,17 @@ class DiskII {
 		void set_control(Control control, bool on);
 		void set_mode(Mode mode);
 		void select_drive(int drive);
-		void set_shift_register(uint8_t value);
+		void set_data_register(uint8_t value);
 		uint8_t get_shift_register();
 
 		void run_for(const Cycles cycles);
+
+	private:
+		uint8_t state_ = 0;
+		uint8_t shift_register_ = 0;
+		uint8_t data_register_ = 0;
+
+		bool is_write_protected();
 };
 
 }
diff --git a/Machines/AppleII/DiskIICard.cpp b/Machines/AppleII/DiskIICard.cpp
index d536dfe63..38bbb203d 100644
--- a/Machines/AppleII/DiskIICard.cpp
+++ b/Machines/AppleII/DiskIICard.cpp
@@ -43,14 +43,15 @@ void DiskIICard::perform_bus_operation(CPU::MOS6502::BusOperation operation, uin
 			case 0xa:	diskii_.select_drive(0);					break;
 			case 0xb:	diskii_.select_drive(1);					break;
 
-			case 0xc:
+			case 0xc: {
 				/* shift register? */
+				const uint8_t shift_value = diskii_.get_shift_register();
 				if(isReadOperation(operation))
-					*value = diskii_.get_shift_register();
-			break;
+					*value = shift_value;
+			} break;
 			case 0xd:
 				/* data register? */
-				diskii_.set_shift_register(*value);
+				diskii_.set_data_register(*value);
 			break;
 
 			case 0xe:	diskii_.set_mode(Mode::Read);				break;