DISC physical format

The physical format complies with the CD-ROM standard (YELLOW BOOK) and CD-ROM XA standard.
This section describes the points that SEGA specifically limits within this standard. It also describes the items necessary for developing an application.

■Region division within the disk and arrangement of each area

The disc is divided and arranged in the following order starting from the inner circumference.

1. Lead-in area
2. program area
3. Lead out area

TOC information is written in the lead-in area according to the "CD-ROM XA standard."

Figure 2.1 Layout of each area on the disk

Only one track is required for the CD-ROM area (Mode 1). The CD-DA area requires at least one track.

■Track layout of the program area (Program Area)

The disc consists of one CD-ROM track followed by a CD-ROM XA track, followed by a CD-DA track.
Therefore, the program area is divided and arranged in the following order.

• CD-ROM area (Mode1 track)
• CD-ROM XA area (Mode2 Form1 & Form2 track)
• CD-DA area

This is to prevent a CD-ROM track from suddenly playing while a CD-DA track is being played when played on an audio CD player.
In addition, the Mode 1 track will be placed at the beginning so that there will be no trouble even if this disc is set in a CD-ROM drive that only supports Mode 1 tracks. Therefore, each track must be placed in the order listed above.
Do not place CD-ROM tracks and CD-DA tracks alternately or place CD-ROM areas and CD-DA areas oppositely.

●Track number

The track number in the CD-ROM area is "01".
Track numbers in the CD-ROM XA area can be set consecutively from "02" to "98".
Track numbers in the CD-DA area must be set consecutively starting from "02" if there is no CD-ROM XA area, and continuing from the next consecutive track number to "99" if there is a CD-ROM XA area. I can.
Do not place them discontinuously.
Each track must be at least 4 seconds long .

One track must be at least 4 seconds long, excluding the pause area, pre-gap, and post-gap areas.

●Pause between tracks

A 2-second pause (silence period) must be placed before and after the CD-ROM track, before and after the CD-ROM XA area, before each CD-DA track, and after the last CD-DA track according to the "YELLOW BOOK". It doesn't have to be. This is to prevent the last note of the previous song from playing when seeking in an audio CD player.

●CD-DA track

Data for at least one song must be placed in the CD-DA area. If you do not need CD-DA data,

Audio sample example:

"This CD is a GAME-CD. Please play it on a dedicated game console."

Please include a warning message. This is intended to tell you what CD it is when played in an audio CD player. If there is music data for even one song, this message is not necessary. Sometimes users play just to listen to the songs.

●Program area recording range

	ABS TIME	LSN	F.A.D.
first frame	00:00:00	−	0
first sector	00:02:00	0	150
final sector	63:01:74	283499	283649
final frame	63:03:74	−	283799
Lead-out area start time	63:04:00

The "first frame, first sector" will always be the time shown above. "Last sector, last frame, lead-out area start time" will always be the above time or a smaller value.

Figure 2.2 Track image when data track is maximized
(When audio track is minimized)

The area that can actually be used as a data sector is from the first sector to one sector before the start of the post gap. (approx. 566MByte)
Every additional minute of audio track usage consumes 9MB of data sectors.

■Key to specify location on disk

Introduces frame addresses (FAD) in addition to absolute time (Atime) and logical sector numbers (LSN) to specify locations on the disk.

●Logical sector number (LSN)

Numbers are assigned consecutively to each sector (frame), starting at 00:02:00 in absolute time.

●Frame address (FAD)

Sectors (frames) are numbered consecutively, starting at 00:00:00 in absolute time. There is a one-to-one correspondence with absolute time.

The relationship between logical sector numbers and frame addresses is as follows.

Logical sector number = frame address - 150

The CD block is accessed using the frame address as a key.
To clarify the meaning of frame addresses, etc., Figure 2.4 shows the relationship between track structure and access keys.

Figure 2.3 General GAME-CD structure

(Note) In the CD-ROM XA standard, when POINT=A0H, PSEC=20H, but in this Semi CD-ROM XA standard, PSEC=00H.

■Sector structure

Figure 2.4 shows the sector physical format of CD-ROM and CD-ROM XA.
GAME-CD uses three types of sectors: Mode1, Mode2 Form1 and Mode2 Form2.

Figure 2.4 CD-ROM and CD-ROM XA sector format

Mode2 format without form specification cannot be used.

■Header field

The header consists of a sector address (absolute time) and a mode byte. Table 2.1 shows the structure of the header fields.
There may be an error between the ATIME value of the subcode Q channel and the sector address in the header. Therefore, when identifying a sector, use the header value instead of the subcode Q channel.

Table 2.1 Header field layout

byte index	value
12 13 14 15	minutes seconds flame mode

■Subheader field

The subheader consists of 8 bytes, and 1 byte each for file number, channel number, submode, and coding information (4 bytes in total) are written twice. The reason for writing twice is for data reliability.

●File number

File numbers are used to identify sectors that belong to one file. Certain files can be recorded interleaved with other files. At this time, the file number of each sector of the logical file will be the same value.
File numbers can be used to select sectors that belong to the same file or to exclude unwanted files.
The following restrictions apply depending on the file number value.

Table 2.2 File number constraints＞

file number	explanation
0	Used for the following files or areas ・ Files recorded continuously on disk ・ directory ・ Others (path table, volume descriptor) Cannot interleave with other files
1~255	Can be interleaved or continuous (It is not clear whether it is interleaved with other files or not)

●Channel number

Channel numbers are used to identify sectors within a file that belong to one channel.

Figure 2.5 Sector interleaving by channel number

●Sub mode

Submodes are used by the system to allocate sectors, terminate files and records, and synchronize.
Table 2.3 shows the configuration of the submode byte.

Table 2.3 Bit configuration of submodes

bit number	Bit name	Default value
7 6 5 4 3 2 1 0	End Of File(EOF) Real Time Sector(RT) Form(F) Trigger(T) Data(D) Audio(A) Video(V) End Of Record(EOR)	0 0 0 0 1 0 0 0

• End of File (EOF):
  Only the last sector of the file is set to 1.
  Other sectors will be 0.

• Real Time Sector(RT)：
  When this bit is 1, processing must be performed without interrupting real-time processing of CD-ROM XA.

• Form(F)：
  0 indicates that it is recorded in Form1, and 1 indicates that it is recorded in Form2.

• Trigger(T)：
  This bit is used for synchronization in applications with diverse coding information.

• Data(D):
  This bit is set to 1 for data sectors related to the program.
  When this bit is 1, the Form bit must be 0.

• Audio(A):
  Set to 1 for the audio sector.
  When this bit is 1, the Form bit is also set to 1.

• Video(V):
  Set to 1 for video sectors.

• End of Record (EOR):
  Set to 1 when it is the last sector of a logical record.

For all sectors except empty sectors, only one of bits 1, 2, and 3 (Video, Audio, Data) must be 1.

●Coding information

The coding information byte defines the data type details of the sector. It is not specified in GAME-CD.

●Empty sector information

It is a sector where the channel number and coding information are 0, and all bits of the submodes Data, Audio, and Video are 0.
Empty sectors are required to be Form2 sectors with all 0 user data areas.