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310 lines
8.4 KiB
C++
310 lines
8.4 KiB
C++
//
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// IWM.cpp
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// Clock Signal
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//
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// Created by Thomas Harte on 05/05/2019.
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// Copyright © 2019 Thomas Harte. All rights reserved.
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//
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#include "IWM.hpp"
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#include <cstdio>
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using namespace Apple;
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namespace {
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const int CA0 = 1 << 0;
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const int CA1 = 1 << 1;
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const int CA2 = 1 << 2;
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const int LSTRB = 1 << 3;
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const int ENABLE = 1 << 4;
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const int DRIVESEL = 1 << 5; /* This means drive select, like on the original Disk II. */
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const int Q6 = 1 << 6;
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const int Q7 = 1 << 7;
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const int SEL = 1 << 8; /* This is an additional input, not available on a Disk II, with a confusingly-similar name to SELECT but a distinct purpose. */
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}
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IWM::IWM(int clock_rate) :
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clock_rate_(clock_rate) {}
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// MARK: - Bus accessors
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uint8_t IWM::read(int address) {
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access(address);
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// Per Inside Macintosh:
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//
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// "Before you can read from any of the disk registers you must set up the state of the IWM so that it
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// can pass the data through to the MC68000's address space where you'll be able to read it. To do that,
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// you must first turn off Q7 by reading or writing dBase+q7L. Then turn on Q6 by accessing dBase+q6H.
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// After that, the IWM will be able to pass data from the disk's RD/SENSE line through to you."
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//
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// My understanding:
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//
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// Q6 = 1, Q7 = 0 reads the status register. The meaning of the top 'SENSE' bit is then determined by
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// the CA0,1,2 and SEL switches as described in Inside Macintosh, summarised above as RD/SENSE.
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if(address&1) {
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return 0xff;
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}
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switch(state_ & (Q6 | Q7 | ENABLE)) {
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default:
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printf("Invalid read\n");
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return 0xff;
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// "Read all 1s".
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case 0:
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printf("Reading all 1s\n");
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return 0xff;
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case ENABLE: /* Read data register. */
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if(data_register_ & 0x80) {
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printf("[%02x] ", data_register_);
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data_register_ = 0;
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}
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printf("Reading data register\n");
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return data_register_;
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case Q6: case Q6|ENABLE: {
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/*
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[If A = 0], Read status register:
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bits 0-4: same as mode register.
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bit 5: 1 = either /ENBL1 or /ENBL2 is currently low.
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bit 6: 1 = MZ (reserved for future compatibility; should always be read as 0).
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bit 7: 1 = SENSE input high; 0 = SENSE input low.
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(/ENBL1 is low when the first drive's motor is on; /ENBL2 is low when the second drive's motor is on.
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If the 1-second timer is enabled, motors remain on for one second after being programmatically disabled.)
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*/
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printf("Reading status (including [%d] ", active_drive_);
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// Determine the SENSE input.
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uint8_t sense = 0x00;
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switch(state_ & (CA2 | CA1 | CA0 | SEL)) {
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default:
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printf("unknown [%c%c%c%c])\n", (state_ & CA2) ? '2' : '-', (state_ & CA1) ? '1' : '-', (state_ & CA0) ? '0' : '-', (state_ & SEL) ? 'S' : '-');
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break;
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case 0: // Head step direction.
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printf("head step direction)\n");
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break;
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case SEL: // Disk in place.
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printf("disk in place)\n");
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sense = drives_[active_drive_] && drives_[active_drive_]->has_disk() ? 0x00 : 0x80;
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break;
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case CA0: // Disk head stepping.
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printf("head stepping)\n");
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break;
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case CA0|SEL: // Disk locked (i.e. write-protect tab).
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printf("disk locked)\n");
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break;
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case CA1: // Disk motor running.
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printf("disk motor running)\n");
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sense = drives_[active_drive_] && drives_[active_drive_]->get_motor_on() ? 0x00 : 0x80;
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break;
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case CA1|SEL: // Head at track 0.
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printf("head at track 0)\n");
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sense = drives_[active_drive_] && drives_[active_drive_]->get_is_track_zero() ? 0x00 : 0x80;
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break;
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case CA1|CA0|SEL: // Tachometer (?)
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printf("tachometer)\n");
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break;
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case CA2: // Read data, lower head.
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printf("data, lower head)\n");
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break;
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case CA2|SEL: // Read data, upper head.
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printf("data, upper head)\n");
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break;
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case CA2|CA1: // Single- or double-sided drive.
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printf("single- or double-sided drive)\n");
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break;
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case CA2|CA1|CA0|SEL: // Drive installed.
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printf("drive installed)\n");
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sense = drives_[active_drive_] ? 0x00 : 0x80;
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break;
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}
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return
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(mode_&0x1f) |
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(drive_motor_on_ ? 0x20 : 0x00) |
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sense;
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} break;
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case Q7: case Q7|ENABLE:
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/*
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Read write-handshake register:
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bits 0-5: reserved for future use (currently read as 1).
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bit 6: 1 = write state (cleared to 0 if a write underrun occurs).
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bit 7: 1 = write data buffer ready for data.
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*/
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printf("Reading write handshake\n");
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return 0x1f;
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}
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return 0xff;
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}
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void IWM::write(int address, uint8_t input) {
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access(address);
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switch(state_ & (Q6 | Q7 | ENABLE)) {
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default: break;
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case Q7|Q6:
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/*
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Write mode register:
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bit 0: 1 = latch mode (should be set in asynchronous mode).
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bit 1: 0 = synchronous handshake protocol; 1 = asynchronous.
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bit 2: 0 = 1-second on-board timer enable; 1 = timer disable.
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bit 3: 0 = slow mode; 1 = fast mode.
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bit 4: 0 = 7Mhz; 1 = 8Mhz (7 or 8 mHz clock descriptor).
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bit 5: 1 = test mode; 0 = normal operation.
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bit 6: 1 = MZ-reset.
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bit 7: reserved for future expansion.
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*/
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mode_ = input;
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switch(mode_ & 0x18) {
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case 0x00: bit_length_ = Cycles(24); break; // slow mode, 7Mhz
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case 0x08: bit_length_ = Cycles(12); break; // fast mode, 7Mhz
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case 0x10: bit_length_ = Cycles(32); break; // slow mode, 8Mhz
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case 0x18: bit_length_ = Cycles(16); break; // fast mode, 8Mhz
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}
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printf("IWM mode is now %02x\n", mode_);
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break;
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case Q7|Q6|ENABLE: // Write data register.
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printf("Data register write\n");
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break;
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}
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}
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// MARK: - Switch access
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void IWM::access(int address) {
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// Keep a record of switch state; bits in state_
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// should correlate with the anonymous namespace constants
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// defined at the top of this file — CA0, CA1, etc.
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address &= 0xf;
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const auto mask = 1 << (address >> 1);
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if(address & 1) {
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state_ |= mask;
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} else {
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state_ &= ~mask;
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}
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// React appropriately to motor requests.
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switch(address >> 1) {
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default: break;
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case 4:
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if(address & 1) {
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drive_motor_on_ = true;
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if(drives_[active_drive_]) drives_[active_drive_]->set_motor_on(true);
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} else {
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// If the 1-second delay is enabled, set up a timer for that.
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if(!(mode_ & 4)) {
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cycles_until_motor_off_ = Cycles(clock_rate_);
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} else {
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drive_motor_on_ = false;
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if(drives_[active_drive_]) drives_[active_drive_]->set_motor_on(false);
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}
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}
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break;
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case 5: {
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const int new_drive = address & 1;
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if(new_drive != active_drive_) {
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if(drives_[active_drive_]) drives_[active_drive_]->set_motor_on(false);
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active_drive_ = new_drive;
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if(drives_[active_drive_]) drives_[active_drive_]->set_motor_on(drive_motor_on_);
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}
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} break;
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}
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}
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void IWM::set_select(bool enabled) {
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// Augment switch state with the value of the SEL line;
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// it's active low, which is implicitly inverted here for
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// consistency in the meaning of state_ bits.
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if(!enabled) state_ |= 0x100;
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else state_ &= ~0x100;
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}
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// MARK: - Active logic
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void IWM::run_for(const Cycles cycles) {
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// Check for a timeout of the motor-off timer.
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if(cycles_until_motor_off_ > Cycles(0)) {
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cycles_until_motor_off_ -= cycles;
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if(cycles_until_motor_off_ <= Cycles(0)) {
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drive_motor_on_ = false;
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if(drives_[active_drive_])
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drives_[active_drive_]->set_motor_on(false);
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}
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}
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// Activity otherwise depends on mode and motor state.
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const bool run_disk = drive_motor_on_ && drives_[active_drive_];
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int integer_cycles = cycles.as_int();
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switch(state_ & (Q6 | Q7 | ENABLE)) {
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case ENABLE: // i.e. read mode.
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while(integer_cycles--) {
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if(run_disk) {
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drives_[active_drive_]->run_for(Cycles(1));
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}
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++cycles_since_shift_;
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if(cycles_since_shift_ == bit_length_ + Cycles(2)) {
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propose_shift(0);
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}
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}
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break;
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default:
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if(run_disk) drives_[active_drive_]->run_for(cycles);
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break;
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}
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}
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void IWM::process_event(const Storage::Disk::Track::Event &event) {
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switch(event.type) {
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case Storage::Disk::Track::Event::IndexHole: return;
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case Storage::Disk::Track::Event::FluxTransition:
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propose_shift(1);
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break;
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}
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}
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void IWM::propose_shift(uint8_t bit) {
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// TODO: synchronous mode.
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shift_register_ = uint8_t((shift_register_ << 1) | bit);
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if(shift_register_ & 0x80) {
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printf("%02x -> data\n", shift_register_);
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data_register_ = shift_register_;
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shift_register_ = 0;
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}
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cycles_since_shift_ = Cycles(0);
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}
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void IWM::set_drive(int slot, Storage::Disk::Drive *drive) {
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drives_[slot] = drive;
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drive->set_event_delegate(this);
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}
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