From e14b26f8763747f535ae4c9da5701f692a84b340 Mon Sep 17 00:00:00 2001
From: Ivan Izaguirre <ivanizag@gmail.com>
Date: Mon, 9 Dec 2019 20:40:00 +0100
Subject: [PATCH] Time based disk reads

---
 cardDisk2.go        |  9 ++++++---
 diskette16sector.go | 47 ++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 46 insertions(+), 10 deletions(-)

diff --git a/cardDisk2.go b/cardDisk2.go
index ccd217d..d2e8555 100644
--- a/cardDisk2.go
+++ b/cardDisk2.go
@@ -126,13 +126,16 @@ func (c *cardDisk2) assign(a *Apple2, slot int) {
 	c.cardBase.assign(a, slot)
 }
 
+// Q6: shift/load
+// Q7: read/write
+
 func (c *cardDisk2) softSwitchQ6Q7(index uint8, in uint8) uint8 {
 	switch index {
 	case 0xC: // Q6L
 		c.q6 = false
 	case 0xD: // Q6H
 		c.q6 = true
-	case 0xE: // Q/L
+	case 0xE: // Q7L
 		c.q7 = false
 	case 0xF: // Q7H
 		c.q7 = true
@@ -151,13 +154,13 @@ func (c *cardDisk2) processQ6Q7(in uint8) {
 	if d.diskette == nil {
 		return
 	}
-	if !c.q6 {
+	if !c.q6 { // shift
 		if !c.q7 { // Q6L-Q7L: Read
 			c.dataLatch = d.diskette.read(d.tracksStep, c.a.cpu.GetCycles())
 		} else { // Q6L-Q7H: Write the dataLatch value to disk. Shift data out
 			d.diskette.write(d.tracksStep, c.dataLatch, c.a.cpu.GetCycles())
 		}
-	} else {
+	} else { // load
 		if !c.q7 { // Q6H-Q7L: Sense write protect / prewrite state
 			// Bit 7 of the control status register means write protected
 			c.dataLatch = 0 // Never write protected
diff --git a/diskette16sector.go b/diskette16sector.go
index 9c8e60d..9eb50c8 100644
--- a/diskette16sector.go
+++ b/diskette16sector.go
@@ -20,28 +20,58 @@ const (
 	nibImageSize     = numberOfTracks * nibBytesPerTrack
 	dskImageSize     = numberOfTracks * numberOfSectors * bytesPerSector
 	defaultVolumeTag = 254
+	cyclesPerBit     = 4
 )
 
 type diskette16sector struct {
-	track    [numberOfTracks][]byte
-	position int
+	track     [numberOfTracks][]byte
+	timeBased bool
+	// Not time based implementation
+	position int // For not time based implemenation
+	// Time based implementation, expermiental
+	cycleOn uint64 // Cycle when the disk was last turned on
 }
 
-func (d *diskette16sector) powerOn(_ uint64) {
-	// Not needed
+func (d *diskette16sector) powerOn(cycle uint64) {
+	d.cycleOn = cycle
 }
 func (d *diskette16sector) powerOff(_ uint64) {
 	// Not needed
 }
 
-func (d *diskette16sector) read(quarterTrack int, _ uint64) uint8 {
-	track := quarterTrack / stepsPerTrack
-	value := d.track[track][d.position]
+func (d *diskette16sector) getBitPositionInTrack(cycle uint64) int {
+	// Calculate how long the disk has been spinning. We move one bit every 4 cycles.
+	// In this implementation we don't take into account hot long the motor takes to be at full speed.
+	cycles := cycle - d.cycleOn
+	position := cycles / cyclesPerBit
+	return int(position % (8 * nibBytesPerTrack)) // Ignore full turns
+}
+
+func (d *diskette16sector) read(quarterTrack int, cycle uint64) uint8 {
+	track := d.track[quarterTrack/stepsPerTrack]
+	if d.timeBased {
+		bitPosition := d.getBitPositionInTrack(cycle)
+		bytePosition := bitPosition / 8
+		shift := uint(bitPosition % 8)
+		if shift == 1 {
+			// We continue having the previous data for a little longer
+			shift = 0
+		}
+		value := track[bytePosition]
+		value >>= shift
+		//fmt.Printf("%v, %v, %v, %x\n", bitPosition, shift, bytePosition, uint8(data))
+		return value
+	}
+	value := track[d.position]
 	d.position = (d.position + 1) % nibBytesPerTrack
+	//fmt.Printf("%v, %v, %v, %x\n", 0, 0, d.position, uint8(value))
 	return value
 }
 
 func (d *diskette16sector) write(quarterTrack int, value uint8, _ uint64) {
+	if d.timeBased {
+		panic("Write not implmented on time based disk implementation")
+	}
 	track := quarterTrack / stepsPerTrack
 	d.track[track][d.position] = value
 	d.position = (d.position + 1) % nibBytesPerTrack
@@ -50,6 +80,9 @@ func (d *diskette16sector) write(quarterTrack int, value uint8, _ uint64) {
 func loadDisquette(filename string) (*diskette16sector, error) {
 	var d diskette16sector
 
+	// Experimental
+	d.timeBased = true
+
 	data, err := loadResource(filename)
 	if err != nil {
 		return nil, err