CLK/Components/8272/i8272.cpp

//
//  i8272.cpp
//  Clock Signal
//
//  Created by Thomas Harte on 05/08/2017.
//  Copyright © 2017 Thomas Harte. All rights reserved.
//

#include "i8272.hpp"
#include "../../Storage/Disk/Encodings/MFM.hpp"

#include <cstdio>

using namespace Intel;

#define SetDataRequest()				(main_status_ |= 0x80)
#define ResetDataRequest()				(main_status_ &= ~0x80)
#define DataRequest()					(main_status_ & 0x80)

#define SetDataDirectionToProcessor()	(main_status_ |= 0x40)
#define SetDataDirectionFromProcessor()	(main_status_ &= ~0x40)
#define DataDirectionToProcessor()		(main_status_ & 0x40)

#define SetNonDMAExecution()			(main_status_ |= 0x20)
#define ResetNonDMAExecution()			(main_status_ &= ~0x20)

#define SetBusy()						(main_status_ |= 0x10)
#define ResetBusy()						(main_status_ &= ~0x10)
#define Busy()							(main_status_ & 0x10)

#define SetAbnormalTermination()		(status_[0] |= 0x40)
#define SetInvalidCommand()				(status_[0] |= 0x80)
#define SetReadyChanged()				(status_[0] |= 0xc0)
#define SetSeekEnd()					(status_[0] |= 0x20)
#define SetEquipmentCheck()				(status_[0] |= 0x10)
#define SetNotReady()					(status_[0] |= 0x08)

#define SetEndOfCylinder()				(status_[1] |= 0x80)
#define SetDataError()					(status_[1] |= 0x20)
#define SetOverrun()					(status_[1] |= 0x10)
#define SetNoData()						(status_[1] |= 0x04)
#define SetNotWriteable()				(status_[1] |= 0x02)
#define SetMissingAddressMark()			(status_[1] |= 0x01)

#define SetControlMark()				(status_[2] |= 0x40)
#define ControlMark()					(status_[2] & 0x40)

#define SetDataFieldDataError()			(status_[2] |= 0x20)
#define SetWrongCyinder()				(status_[2] |= 0x10)
#define SetScanEqualHit()				(status_[2] |= 0x08)
#define SetScanNotSatisfied()			(status_[2] |= 0x04)
#define SetBadCylinder()				(status_[2] |= 0x02)
#define SetMissingDataAddressMark()		(status_[2] |= 0x01)

i8272::i8272(Cycles clock_rate, int clock_rate_multiplier, int revolutions_per_minute) :
	Storage::Disk::MFMController(clock_rate, clock_rate_multiplier, revolutions_per_minute),
	main_status_(0),
	interesting_event_mask_((int)Event8272::CommandByte),
	resume_point_(0),
	delay_time_(0),
	head_timers_running_(0),
	expects_input_(false) {
	posit_event((int)Event8272::CommandByte);
}

void i8272::run_for(Cycles cycles) {
	Storage::Disk::MFMController::run_for(cycles);

	// check for an expired timer
	if(delay_time_ > 0) {
		if(cycles.as_int() >= delay_time_) {
			delay_time_ = 0;
			posit_event((int)Event8272::Timer);
		} else {
			delay_time_ -= cycles.as_int();
		}
	}

	// update seek status of any drives presently seeking
	if(main_status_ & 0xf) {
		for(int c = 0; c < 4; c++) {
			if(drives_[c].phase == Drive::Seeking) {
				drives_[c].step_rate_counter += cycles.as_int();
				int steps = drives_[c].step_rate_counter / (8000 * step_rate_time_);
				drives_[c].step_rate_counter %= (8000 * step_rate_time_);
				while(steps--) {
					// Perform a step.
					int direction = (drives_[c].target_head_position < drives_[c].head_position) ? -1 : 1;
					drives_[c].drive->step(direction);
					drives_[c].head_position += direction;

					// Check for completion.
					if(drives_[c].seek_is_satisfied()) {
						drives_[c].phase = Drive::CompletedSeeking;
						if(drives_[c].target_head_position == -1) drives_[c].head_position = 0;
						break;
					}
				}
			}
		}
	}

	// check for any head unloads
	if(head_timers_running_) {
		for(int c = 0; c < 4; c++) {
			for(int h = 0; h < 2; h++) {
				if(drives_[c].head_unload_delay[c] > 0) {
					if(cycles.as_int() >= drives_[c].head_unload_delay[c]) {
						drives_[c].head_unload_delay[c] = 0;
						drives_[c].head_is_loaded[c] = false;
						head_timers_running_--;
						if(!head_timers_running_) return;
					} else {
						drives_[c].head_unload_delay[c] -= cycles.as_int();
					}
				}
			}
		}
	}
}

void i8272::set_register(int address, uint8_t value) {
	// don't consider attempted sets to the status register
	if(!address) return;

	// if not ready for commands, do nothing
	if(!DataRequest() || DataDirectionToProcessor()) return;

	if(expects_input_) {
		input_ = value;
		has_input_ = true;
		ResetDataRequest();
	} else {
		// accumulate latest byte in the command byte sequence
		command_.push_back(value);
		posit_event((int)Event8272::CommandByte);
	}
}

uint8_t i8272::get_register(int address) {
	if(address) {
		if(result_stack_.empty()) return 0xff;
		uint8_t result = result_stack_.back();
		result_stack_.pop_back();
		if(result_stack_.empty()) posit_event((int)Event8272::ResultEmpty);

		return result;
	} else {
		return main_status_;
	}
}

void i8272::set_disk(std::shared_ptr<Storage::Disk::Disk> disk, int drive) {
	if(drive < 4 && drive >= 0) {
		drives_[drive].drive->set_disk(disk);
	}
}

#define BEGIN_SECTION()	switch(resume_point_) { default:
#define END_SECTION()	}

#define MS_TO_CYCLES(x)			x * 8000
#define WAIT_FOR_EVENT(mask)	resume_point_ = __LINE__; interesting_event_mask_ = (int)mask; return; case __LINE__:
#define WAIT_FOR_TIME(ms)		resume_point_ = __LINE__; interesting_event_mask_ = (int)Event8272::Timer; delay_time_ = MS_TO_CYCLES(ms); case __LINE__: if(delay_time_) return;

#define PASTE(x, y) x##y
#define CONCAT(x, y) PASTE(x, y)

#define FIND_HEADER()	\
	CONCAT(find_header, __LINE__): WAIT_FOR_EVENT((int)Event::Token | (int)Event::IndexHole); \
	if(event_type == (int)Event::IndexHole) index_hole_limit_--;	\
	else if(get_latest_token().type == Token::ID) goto CONCAT(header_found, __LINE__);	\
	\
	if(index_hole_limit_) goto CONCAT(find_header, __LINE__);	\
	CONCAT(header_found, __LINE__):	0;\

#define FIND_DATA()	\
	CONCAT(find_data, __LINE__): WAIT_FOR_EVENT((int)Event::Token | (int)Event::IndexHole); \
	if(event_type == (int)Event::Token && get_latest_token().type != Token::Data && get_latest_token().type != Token::DeletedData) goto CONCAT(find_data, __LINE__);

#define READ_HEADER()	\
	distance_into_section_ = 0;	\
	set_data_mode(Reading);	\
	CONCAT(read_header, __LINE__): WAIT_FOR_EVENT(Event::Token); \
	header_[distance_into_section_] = get_latest_token().byte_value;	\
	distance_into_section_++; \
	if(distance_into_section_ < 6) goto CONCAT(read_header, __LINE__);	\
	set_data_mode(Scanning);

#define CLEAR_STATUS()	\
	status_[0] = status_[1] = status_[2] = 0;

#define SET_DRIVE_HEAD_MFM()	\
	active_drive_ = command_[1]&3;	\
	active_head_ = (command_[1] >> 2)&1;	\
	set_drive(drives_[active_drive_].drive);	\
	drives_[active_drive_].drive->set_head((unsigned int)active_head_);	\
	set_is_double_density(command_[0] & 0x40);	\
	invalidate_track();

#define LOAD_HEAD()	\
	if(!drives_[active_drive_].head_is_loaded[active_head_]) {	\
		drives_[active_drive_].head_is_loaded[active_head_] = true;	\
		WAIT_FOR_TIME(head_load_time_);	\
	} else {	\
		if(drives_[active_drive_].head_unload_delay[active_head_] > 0) {	\
			drives_[active_drive_].head_unload_delay[active_head_] = 0;	\
			head_timers_running_--;	\
		}	\
	}

#define SCHEDULE_HEAD_UNLOAD()	\
	if(drives_[active_drive_].head_is_loaded[active_head_]) {\
		if(drives_[active_drive_].head_unload_delay[active_head_] == 0) {	\
			head_timers_running_++;	\
		}	\
		drives_[active_drive_].head_unload_delay[active_head_] = MS_TO_CYCLES(head_unload_time_);\
	}

void i8272::posit_event(int event_type) {
	if(!(interesting_event_mask_ & event_type)) return;
	interesting_event_mask_ &= ~event_type;

	BEGIN_SECTION();

	// Resets busy and non-DMA execution, clears the command buffer, sets the data mode to scanning and flows
	// into wait_for_complete_command_sequence.
	wait_for_command:
			expects_input_ = false;
			set_data_mode(Storage::Disk::MFMController::DataMode::Scanning);
			ResetBusy();
			ResetNonDMAExecution();
			command_.clear();

	// Sets the data request bit, and waits for a byte. Then sets the busy bit. Continues accepting bytes
	// until it has a quantity that make up an entire command, then resets the data request bit and
	// branches to that command.
	wait_for_complete_command_sequence:
			SetDataRequest();
			SetDataDirectionFromProcessor();
			WAIT_FOR_EVENT(Event8272::CommandByte)
			SetBusy();

			switch(command_[0] & 0x1f) {
				case 0x06:	// read data
				case 0x0b:	// read deleted data
					if(command_.size() < 9) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto read_data;

				case 0x05:	// write data
				case 0x09:	// write deleted data
					if(command_.size() < 9) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto write_data;

				case 0x02:	// read track
					if(command_.size() < 9) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto read_track;

				case 0x0a:	// read ID
					if(command_.size() < 2) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto read_id;

				case 0x0d:	// format track
					if(command_.size() < 6) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto format_track;

				case 0x11:	// scan low
					if(command_.size() < 9) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto scan_low;

				case 0x19:	// scan low or equal
					if(command_.size() < 9) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto scan_low_or_equal;

				case 0x1d:	// scan high or equal
					if(command_.size() < 9) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto scan_high_or_equal;

				case 0x07:	// recalibrate
					if(command_.size() < 2) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto recalibrate;

				case 0x08:	// sense interrupt status
					ResetDataRequest();
					goto sense_interrupt_status;

				case 0x03:	// specify
					if(command_.size() < 3) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto specify;

				case 0x04:	// sense drive status
					if(command_.size() < 2) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto sense_drive_status;

				case 0x0f:	// seek
					if(command_.size() < 3) goto wait_for_complete_command_sequence;
					ResetDataRequest();
					goto seek;

				default:	// invalid
					ResetDataRequest();
					goto invalid;
			}

	// Decodes drive, head and density, loads the head, loads the internal cylinder, head, sector and size registers,
	// and searches for a sector that meets those criteria. If one is found, inspects the instruction in use and
	// jumps to an appropriate handler.
	read_write_find_header:
		// Establishes the drive and head being addressed, and whether in double density mode; populates the internal
		// cylinder, head, sector and size registers from the command stream.
			if(!dma_mode_) SetNonDMAExecution();
			SET_DRIVE_HEAD_MFM();
			LOAD_HEAD();
			CLEAR_STATUS();
			cylinder_ = command_[2];
			head_ = command_[3];
			sector_ = command_[4];
			size_ = command_[5];

		// Sets a maximum index hole limit of 2 then performs a find header/read header loop, continuing either until
		// the index hole limit is breached or a sector is found with a cylinder, head, sector and size equal to the
		// values in the internal registers.
			index_hole_limit_ = 2;
		find_next_sector:
			FIND_HEADER();
			if(!index_hole_limit_) {
				// Two index holes have passed wihout finding the header sought.
				SetNoData();
				goto abort_read_write;
			}
			READ_HEADER();
			if(get_crc_generator().get_value()) {
				// This implies a CRC error in the header; mark as such but continue.
				SetDataError();
			}
			if(header_[0] != cylinder_ || header_[1] != head_ || header_[2] != sector_ || header_[3] != size_) goto find_next_sector;

			// Branch to whatever is supposed to happen next
			switch(command_[0] & 0x1f) {
				case 0x06:	// read data
				case 0x0b:	// read deleted data
				goto read_data_found_header;

				case 0x05:	// write data
				case 0x09:	// write deleted data
				goto write_data_found_header;
			}

	// Sets abnormal termination of the current command and proceeds to an ST0, ST1, ST2, C, H, R, N result phase.
	abort_read_write:
		SetAbnormalTermination();
		goto post_st012chrn;

	// Performs the read data or read deleted data command.
	read_data:
		read_next_data:
			goto read_write_find_header;

		// Finds the next data block and sets data mode to reading, setting an error flag if the on-disk deleted
		// flag doesn't match the sort the command was looking for.
		read_data_found_header:
			FIND_DATA();
			distance_into_section_ = 0;
			if((get_latest_token().type == Token::Data) != ((command_[0]&0xf) == 0x6)) {
				if(!(command_[0]&0x20)) {
					// SK is not set; set the error flag but read this sector before finishing.
					SetControlMark();
				} else {
					// SK is set; skip this sector.
					goto read_next_data;
				}
			}
			set_data_mode(Reading);

		// Waits for the next token, then supplies it to the CPU by: (i) setting data request and direction; and (ii) resetting
		// data request once the byte has been taken. Continues until all bytes have been read.
		//
		// TODO: signal if the CPU is too slow and missed a byte; at the minute it'll just silently miss. Also allow for other
		// ways that sector size might have been specified.
		get_byte:
			WAIT_FOR_EVENT(Event::Token);
			result_stack_.push_back(get_latest_token().byte_value);
			distance_into_section_++;
			SetDataRequest();
			SetDataDirectionToProcessor();
			WAIT_FOR_EVENT((int)Event8272::ResultEmpty | (int)Event::Token | (int)Event::IndexHole);
			switch(event_type) {
				case (int)Event8272::ResultEmpty:	// The caller read the byte in time; proceed as normal.
					ResetDataRequest();
					if(distance_into_section_ < (128 << size_)) goto get_byte;
				break;
				case (int)Event::Token:				// The caller hasn't read the old byte yet and a new one has arrived
					SetOverrun();
					goto abort_read_write;
				break;
				case (int)Event::IndexHole:
				break;
			}

		// read CRC, without transferring it, then check it
			WAIT_FOR_EVENT(Event::Token);
			WAIT_FOR_EVENT(Event::Token);
			if(get_crc_generator().get_value()) {
				// This implies a CRC error in the sector; mark as such and temrinate.
				SetDataError();
				SetDataFieldDataError();
				goto abort_read_write;
			}

		// check whether that's it: either the final requested sector has been read, or because
		// a sector that was [/wasn't] marked as deleted when it shouldn't [/should] have been
			if(sector_ != command_[6] && !ControlMark()) {
				sector_++;
				goto read_next_data;
			}

		// For a final result phase, post the standard ST0, ST1, ST2, C, H, R, N
			goto post_st012chrn;

	write_data:
		printf("Write [deleted] data\n");

		write_next_data:
			goto read_write_find_header;

		write_data_found_header:
			begin_writing();

			// Write out the requested gap between ID and data.
			for(int c = 0; c < command_[7]; c++) {
				write_byte(0x4e);
			}
			WAIT_FOR_EVENT(Event::DataWritten);

			{
				bool is_deleted = (command_[0] & 0x1f) == 0x09;
				if(get_is_double_density()) {
					get_crc_generator().set_value(Storage::Encodings::MFM::MFMPostSyncCRCValue);
					write_raw_short(Storage::Encodings::MFM::MFMSync);
					write_byte(is_deleted ? Storage::Encodings::MFM::DeletedDataAddressByte : Storage::Encodings::MFM::DataAddressByte);
				} else {
					get_crc_generator().reset();
					get_crc_generator().add(is_deleted ? Storage::Encodings::MFM::DeletedDataAddressByte : Storage::Encodings::MFM::DataAddressByte);
					write_raw_short(is_deleted ? Storage::Encodings::MFM::FMDeletedDataAddressMark : Storage::Encodings::MFM::FMDataAddressMark);
				}
			}

			SetDataDirectionFromProcessor();
			SetDataRequest();
			expects_input_ = true;
			distance_into_section_ = 0;

		write_loop:
			WAIT_FOR_EVENT(Event::DataWritten);
			if(!has_input_) {
				SetOverrun();
				end_writing();
				goto abort_read_write;
			}
			write_byte(input_);
			has_input_ = false;
			distance_into_section_++;
			if(distance_into_section_ < (128 << size_)) {
				SetDataRequest();
				goto write_loop;
			}

			write_crc();
			expects_input_ = false;
			WAIT_FOR_EVENT(Event::DataWritten);
			end_writing();

		goto post_st012chrn;

	read_track:
		printf("Read track unimplemented!!\n");
		goto wait_for_command;

	// Performs the read ID command.
	read_id:
		// Establishes the drive and head being addressed, and whether in double density mode.
			printf("Read ID\n");
			SET_DRIVE_HEAD_MFM();
			LOAD_HEAD();

		// Sets a maximum index hole limit of 2 then waits either until it finds a header mark or sees too many index holes.
		// If a header mark is found, reads in the following bytes that produce a header. Otherwise branches to data not found.
			index_hole_limit_ = 2;
		read_id_find_next_sector:
			FIND_HEADER();
			if(!index_hole_limit_) {
				SetNoData();
				goto abort_read_write;
			}
			READ_HEADER();

		// Sets internal registers from the discovered header and posts the standard ST0, ST1, ST2, C, H, R, N.
			cylinder_ = header_[0];
			head_ = header_[1];
			sector_ = header_[2];
			size_ = header_[3];

			goto post_st012chrn;

	format_track:
			printf("Format track\n");
			if(!dma_mode_) SetNonDMAExecution();
			SET_DRIVE_HEAD_MFM();
			LOAD_HEAD();
			CLEAR_STATUS();

			// Wait for the index hole.
			WAIT_FOR_EVENT(Event::IndexHole);
			begin_writing();

			// Write start-of-track.
			// TODO: single density.
			for(int c = 0; c < 80; c++) {
				write_byte(0x4e);
			}
			for(int c = 0; c < 12; c++) {
				write_byte(0x00);
			}
			for(int c = 0; c < 3; c++) {
				write_raw_short(Storage::Encodings::MFM::MFMIndexSync);
			}
			write_byte(Storage::Encodings::MFM::IndexAddressByte);
			for(int c = 0; c < 50; c++) {
				write_byte(0x4e);
			}
			WAIT_FOR_EVENT(Event::DataWritten);
			sector_ = 0;

		format_track_write_sector:
			for(int c = 0; c < 12; c++) {
				write_byte(0x00);
			}
			for(int c = 0; c < 3; c++) {
				write_raw_short(Storage::Encodings::MFM::MFMSync);
			}
			get_crc_generator().set_value(Storage::Encodings::MFM::MFMPostSyncCRCValue);
			write_byte(Storage::Encodings::MFM::IDAddressByte);

			// Write the sector header, obtaining its contents
			// from the processor.
			SetDataDirectionFromProcessor();
			SetDataRequest();
			expects_input_ = true;
			distance_into_section_ = 0;
		format_track_write_header:
			WAIT_FOR_EVENT(Event::DataWritten);
			// TODO: overrun?
			header_[distance_into_section_] = input_;
			write_byte(input_);
			has_input_ = false;
			distance_into_section_++;
			if(distance_into_section_ < 4) {
				SetDataRequest();
				goto format_track_write_header;
			}
			write_crc();

			// Write the sector body.
			for(int c = 0; c < 22; c++) {
				write_byte(0x4e);
			}
			for(int c = 0; c < 12; c++) {
				write_byte(0x00);
			}
			for(int c = 0; c < 3; c++) {
				write_raw_short(Storage::Encodings::MFM::MFMSync);
			}
			get_crc_generator().set_value(Storage::Encodings::MFM::MFMPostSyncCRCValue);
			write_byte(Storage::Encodings::MFM::DataAddressByte);
			for(int c = 0; c < (128 << command_[2]); c++) {
				write_byte(command_[5]);
			}
			write_crc();

			// Write the prescribed gap.
			for(int c = 0; c < command_[4]; c++) {
				write_byte(0x4e);
			}

			// Consider repeating.
			sector_++;
			if(sector_ < command_[3]) {
				goto format_track_write_sector;
			}

			// Otherwise, pad out to the index hole.
		format_track_pad:
			write_byte(0x4e);
			WAIT_FOR_EVENT((int)Event::DataWritten | (int)Event::IndexHole);
			if(event_type != (int)Event::IndexHole) goto format_track_pad;

			end_writing();

			cylinder_ = header_[0];
			head_ = header_[1];
			sector_ = header_[2] + 1;
			size_ = header_[3];

		goto post_st012chrn;

	scan_low:
		printf("Scan low unimplemented!!\n");
		goto wait_for_command;

	scan_low_or_equal:
		printf("Scan low or equal unimplemented!!\n");
		goto wait_for_command;

	scan_high_or_equal:
		printf("Scan high or equal unimplemented!!\n");
		goto wait_for_command;

	// Performs both recalibrate and seek commands. These commands occur asynchronously, so the actual work
	// occurs in ::run_for; this merely establishes that seeking should be ongoing.
	recalibrate:
	seek:
			printf((command_.size() > 2) ? "Seek\n" : "Recalibrate\n");

		// Declines to act if a seek is already ongoing; otherwise resets all status registers, sets the drive
		// into seeking mode, sets the drive's main status seeking bit, and sets the target head position: for
		// a recalibrate the target is -1 and ::run_for knows that -1 means the terminal condition is the drive
		// returning that its at track zero, and that it should reset the drive's current position once reached.
			if(drives_[command_[1]&3].phase != Drive::Seeking) {
				int drive = command_[1]&3;
				drives_[drive].phase = Drive::Seeking;
				drives_[drive].steps_taken = 0;
				drives_[drive].target_head_position = (command_.size() > 2) ? command_[2] : -1;
				drives_[drive].step_rate_counter = 0;
				drives_[drive].seek_failed = false;

				// Check whether any steps are even needed.
				if(drives_[drive].seek_is_satisfied()) {
					drives_[drive].phase = Drive::CompletedSeeking;
				} else {
					main_status_ |= 1 << (command_[1]&3);
				}
			}
			goto wait_for_command;

	// Performs sense interrupt status.
	sense_interrupt_status:
			printf("Sense interrupt status\n");
			{
				// Find the first drive that is in the CompletedSeeking state.
				int found_drive = -1;
				for(int c = 0; c < 4; c++) {
					if(drives_[c].phase == Drive::CompletedSeeking) {
						found_drive = c;
						break;
					}
				}

				// If a drive was found, return its results. Otherwise return a single 0x80.
				if(found_drive != -1) {
					drives_[found_drive].phase = Drive::NotSeeking;
					status_[0] = (uint8_t)found_drive;
					SetSeekEnd();
					main_status_ &= ~(1 << found_drive);

					result_stack_.push_back(drives_[found_drive].head_position);
					result_stack_.push_back(status_[0]);
				} else {
					result_stack_.push_back(0x80);
				}
			}
			goto post_result;

	// Performs specify.
	specify:
		// Just store the values, and terminate the command.
			step_rate_time_ = command_[1] &0xf0;		// i.e. 16 to 240m
			head_unload_time_ = command_[1] & 0x0f;		// i.e. 1 to 16ms
			head_load_time_ = command_[2] & ~1;			// i.e. 2 to 254 ms in increments of 2ms
			dma_mode_ = !(command_[2] & 1);
			goto wait_for_command;

	sense_drive_status:
			{
				int drive = command_[1] & 3;
				result_stack_.push_back(
					(command_[1] & 7) |	// drive and head number
					0x08 |				// single sided
					(drives_[drive].drive->get_is_track_zero() ? 0x10 : 0x00)	|
					(drives_[drive].drive->get_is_ready() ? 0x20 : 0x00)		|
					(drives_[drive].drive->get_is_read_only() ? 0x40 : 0x00)
				);
			}
			goto post_result;

	// Performs any invalid command.
	invalid:
			// A no-op, but posts ST0 (but which ST0?)
			result_stack_.push_back(0x80);
			goto post_result;

	// Posts ST0, ST1, ST2, C, H, R and N as a result phase.
	post_st012chrn:
			SCHEDULE_HEAD_UNLOAD();

			result_stack_.push_back(size_);
			result_stack_.push_back(sector_);
			result_stack_.push_back(head_);
			result_stack_.push_back(cylinder_);

			result_stack_.push_back(status_[2]);
			result_stack_.push_back(status_[1]);
			result_stack_.push_back(status_[0]);

			goto post_result;

	// Posts whatever is in result_stack_ as a result phase. Be aware that it is a stack — the
	// last thing in it will be returned first.
	post_result:
			printf("Result: ");
			for(size_t c = 0; c < result_stack_.size(); c++) {
				printf("%02x ", result_stack_[result_stack_.size() - 1 - c]);
			}
			printf("\n");

			// Set ready to send data to the processor, no longer in non-DMA execution phase.
			ResetNonDMAExecution();
			SetDataRequest();
			SetDataDirectionToProcessor();

			// The actual stuff of unwinding result_stack_ is handled by ::get_register; wait
			// until the processor has read all result bytes.
			WAIT_FOR_EVENT(Event8272::ResultEmpty);

			// Reset data direction and end the command.
			goto wait_for_command;

	END_SECTION()
}

bool i8272::Drive::seek_is_satisfied() {
	return	(target_head_position == head_position) ||
			(target_head_position == -1 && drive->get_is_track_zero());
}