Some cleanup an the decode code

Added a bunch of comments and clarified some variables in the decoding
logic.

rom/decoder.go

@@ -10,17 +10,18 @@ import (
 // Returns packet, next state, and error (if any)
 type decodeFunction func(page *Page, data []byte, startIdx int) (Packet, int, error)
 
-// map of states.  each state is a map of types
+// Map of states.  Each state is a map of types
 var definedPackets = map[int]map[byte]decodeFunction{
-	0: map[byte]decodeFunction{
+
+	0: map[byte]decodeFunction{ // state 3 in firmware.org
 		0x01: decodeHeader,
 	},
 
-	1: map[byte]decodeFunction{
+	1: map[byte]decodeFunction{ // state 1 in firmware.org
 		0x00: decodeMarkDataEnd,
 	},
 
-	2: map[byte]decodeFunction{
+	2: map[byte]decodeFunction{ // state 2 in firmware.org
 		0x02: decodeSetWorkRamLoad,
 		0x03: decodeMarkDataStart,
 		0x04: decodeMarkDataStart,
@@ -45,18 +46,19 @@ func (page *Page) decode(data []byte) error {
 			return nil
 		}
 
+		stateArg := data[idx+1]
 		var packet Packet
-		if page.state == 1 && data[idx+1] != 0x00 {
+		if page.state == 1 && stateArg != 0x00 {  // stateArg is length here?
 			// bulk data
 			packet, page.state, err = decodeBulkData(page, data, idx)
 			if err != nil {
 				return err
 			}
 		} else {
-			df, ok := definedPackets[page.state][data[idx+1]]
+			df, ok := definedPackets[page.state][stateArg]
 			if !ok {
 				return fmt.Errorf("State %d packet with type %02X isn't implemented",
-					page.state, data[idx+1])
+					page.state, stateArg)
 			}
 			packet, page.state, err = df(page, data, idx)
 			if err != nil {
@@ -149,7 +151,7 @@ func decodeMarkDataStart(page *Page, data []byte, idx int) (Packet, int, error)
 func decodeMarkDataEnd(page *Page, data []byte, idx int) (Packet, int, error) {
 	packet := &packetMarkDataEnd{
 		Type:     data[idx+2],
-		Reset:    (data[idx+2]&0xF0 == 0xF0),
+		Reset:    (data[idx+2]&0xF0 == 0xF0),  // what is this?
 		checksum: data[idx+3],
 		address:  page.DataOffset + idx,
 	}
@@ -195,6 +197,9 @@ func decodeSetWorkRamLoad(page *Page, data []byte, idx int) (Packet, int, error)
 }
 
 func decodeBulkData(page *Page, data []byte, idx int) (Packet, int, error) {
+	// idx+1: length of data
+	// idx+2: data start
+
 	if data[idx+1] == 0 {
 		return nil, 0, fmt.Errorf("Bulk data packet has a length of zero at offset %08X",
 			page.DataOffset+idx)
@@ -206,8 +211,9 @@ func decodeBulkData(page *Page, data []byte, idx int) (Packet, int, error) {
 
 	datalen := int(data[idx+1])
 	packet.Data = data[idx+2 : idx+2+datalen]
-	packet.checksum = data[idx+len(packet.Data)+2]
+	packet.checksum = data[idx+2+len(packet.Data)]
 
+	// checksum includes the packet ID and data length values
 	checksum := calcChecksum(data[idx : idx+int(data[idx+1])+2])
 	if checksum != packet.checksum {
 		data := []string{}

rom/export.go

@@ -13,6 +13,10 @@ func (sb *StudyBox) Export(directory string) error {
 		Audio:   directory + "/audio" + sb.Audio.ext(),
 	}
 
+	// A "Page" here does not correspond to the entered "Page" number on the
+	// title screen.  These are really segments.  The "Page" that is entered on
+	// the title screen is stored in the header of a segment.  Multiple
+	// segments can have the same "Page" number.
 	for pidx, page := range sb.Data.Pages {
 		jp := jsonPage{
 			AudioOffsetLeadIn: page.AudioOffsetLeadIn,
@@ -20,7 +24,7 @@ func (sb *StudyBox) Export(directory string) error {
 			Data:              []jsonData{},
 		}
 
-		file, err := os.Create(fmt.Sprintf("%s/page%02d_0000.txt", directory, pidx))
+		file, err := os.Create(fmt.Sprintf("%s/segment-%02d_packet-0000.txt", directory, pidx))
 		if err != nil {
 			return err
 		}
@@ -74,13 +78,13 @@ func (sb *StudyBox) Export(directory string) error {
 
 				switch jData.Type {
 				case "pattern":
-					jData.File = fmt.Sprintf("%s/page%02d_%04d_chrData.chr", directory, pidx, dataStartId)
+					jData.File = fmt.Sprintf("%s/segment-%02d_packet-%04d_chrData.chr", directory, pidx, dataStartId)
 
 				case "nametable":
-					jData.File = fmt.Sprintf("%s/page%02d_%04d_ntData.dat", directory, pidx, dataStartId)
+					jData.File = fmt.Sprintf("%s/segment-%02d_packet-%04d_ntData.dat", directory, pidx, dataStartId)
 
 				case "script":
-					jData.File = fmt.Sprintf("%s/page%02d_%04d_scriptData.dat", directory, pidx, dataStartId)
+					jData.File = fmt.Sprintf("%s/segment-%02d_packet-%04d_scriptData.dat", directory, pidx, dataStartId)
 
 					//script, err := DissassembleScript(scriptData)
 					//if err != nil {
@@ -118,7 +122,7 @@ func (sb *StudyBox) Export(directory string) error {
 				rawData = append(rawData, p.Data...)
 
 			default:
-				return fmt.Errorf("Encountered an unknown packet: %s page: %d", p.Asm(), pidx)
+				return fmt.Errorf("Encountered an unknown packet: %s segment: %d", p.Asm(), pidx)
 			}
 		}
 

rom/packets.go

@@ -26,7 +26,7 @@ func (ph *packetHeader) RawBytes() []byte {
 }
 
 func (ph *packetHeader) Asm() string {
-	return fmt.Sprintf("header %d ; Checksum: %02X", ph.PageNumber, ph.Checksum)
+	return fmt.Sprintf("header %d [Page %d] ; Checksum: %02X", ph.PageNumber, ph.PageNumber+1, ph.Checksum)
 }
 
 func (ph *packetHeader) Address() int {
@@ -81,7 +81,7 @@ func newPacketWorkRamLoad(bank, addressHigh uint8) *packetWorkRamLoad {
 }
 
 func (p *packetWorkRamLoad) Asm() string {
-	return fmt.Sprintf("work_ram_load $%02X $%02X ; Checksum %02X",
+	return fmt.Sprintf("work_ram_load bank:$%02X addr:$%02X00 ; Checksum %02X",
 		p.bankId, p.loadAddressHigh, p.checksum)
 }
 
@@ -192,7 +192,7 @@ func (p *packetMarkDataStart) Address() int {
 
 type packetMarkDataEnd struct {
 	//Arg   uint8
-	Reset bool
+	Reset bool // what does this mean?  what is special about the value 0xF0?
 	Type  uint8
 
 	address  int
@@ -251,7 +251,8 @@ func (p *packetMarkDataEnd) Asm() string {
 	for _, b := range p.RawBytes() {
 		s = append(s, fmt.Sprintf("%02X", b))
 	}
-	return fmt.Sprintf("mark_datatype_end %s ; %s Checksum: %02X", tstr, strings.Join(s, " "), p.checksum)
+	return fmt.Sprintf("mark_datatype_end %s ; %s Checksum: %02X",
+		tstr, strings.Join(s, " "), p.checksum)
 }
 
 func (p *packetMarkDataEnd) Address() int {