DISC physical format

The physical format conforms to the CD-ROM standard (YELLOW BOOK) and the CD-ROM XA standard.
Describes the specific restrictions of SEGA within this standard. It also describes what you need to do to develop your application.

■ Area division in the disk and arrangement of each area

The discs are divided and arranged in the following order from the inner circumference.

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

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

Figure 2.1 Arrangement 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.

■ Program Area track layout

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

• CD-ROM area (Mode 1 track)
• CD-ROM XA area (Mode2 Form1 & Form2 tracks)
• CD-DA area

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

● Track number

The track number in the CD-ROM area is "01".
The track number in the CD-ROM XA area can be set continuously from "02" to "98".
The track number of the CD-DA area should be set continuously from "02" if there is no CD-ROM XA area, and from the next consecutive track number to "99" if there is a CD-ROM XA area. I can.
Do not place them discontinuously.
Make sure that one track is at least 4 seconds.

One track always requires 4 seconds or more, excluding the pause area, pre-gap area, and post-gap area.

● Pose between tracks

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, a 2-second pause (silence period) must be placed according to the "YELLOW BOOK". Must be. This is to prevent the last note of the previous song from being heard when seeking with an audio CD player.

● CD-DA track

At least one song's worth of data must be placed in the CD-DA area. If you do not need the CD-DA data

Audio sample example:

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

Please enter a caution message such as. This is intended to tell you what the CD is when it is played on an audio CD player. If you have music data for even one song, you don't need this message. The user may play to listen only to the song.

● Recording range of the program area

	ABS TIME	LSN	FAD
First frame	00:00:00	−	0
First sector	00:02:00	0	150
Last sector	63:01:74	283499	283649
Final frame	63:03:74	−	283799
Lead-out area start time	63:04:00

"First frame, first sector" will always be the above time. "Last sector, last frame, readout area start time" will always be the above time or less.

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

The actual data sector can be used from the first sector to one sector before the start of the post gap. (Approximately 566MByte)
Every minute of audio track usage consumes 9MB of data sectors.

■ Key to specify the position on the disk

Introduces the frame address (FAD) in addition to the absolute time (Atime) and logical sector number (LSN) to specify the position on the disk.

● Logical sector number (LSN)

The number is continuously assigned to each sector (frame) with the absolute time of 00:00:02:00 as 0.

● Frame address (FAD)

It is a continuous numbering in sector (frame) units with the absolute time 00:00:00 as 0. There is a one-to-one correspondence with absolute time.

The logical sector number and the frame address have the following relationship.

Logical sector number = frame address -150

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

Figure 2.3 General GAME-CD structure

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

■ Sector structure

Figure 2.4 shows the sector physical formats for 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 the sector address (absolute time) and mode bytes. The structure of the header fields is shown in Table 2.1.
There may be an error between the ATIME value of the subcode Q channel and the sector address in the header. Therefore, when identifying the sector, use the header value instead of the subcode Q channel.

Table 2.1 Header field layout

Byte subscript	value
12 13 14 15	Minute second frame mode

■ Subheader field

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

● File number

The file number is used to identify the sectors that belong to one file. One file can be recorded interleaved with another file. At this time, the file number of each sector of the logical file will be the same value.
The file number can be used to select sectors that belong to the same file or to exclude files that are not covered.
Depending on the value of the file number, there are the following restrictions.

Table 2.2 File number restrictions>

File number	explanation
0	Used for the following files or areas-Files and directories recorded continuously on disk-Others (pathtable, volume descriptor) Cannot interleave with other files
1-255	It can be interleaved or continuous (it is not clear if it is interleaved with another file).

● Channel number

The channel number is used to identify the sector that belongs to one channel in the file.

Figure 2.5 Sector interleaving by channel number

● Sub mode

Submodes are used in the system for sector allocation, file and record termination, synchronization, and more.
Table 2.3 shows the configuration of submode bytes.

Table 2.3 Submode bit configurations

Bit number	Bit name	Default value
7 6 Five Four 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 are 0.

• Real Time Sector (RT):
  When this bit is 1, the CD-ROM XA must be processed in real time without interruption.

• Form (F):
  When it is 0, it indicates that it is recorded in Form1, and when it is 1, it 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 the data sector related to the program.
  When this bit is 1, the Form bit must be 0.

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

• Video (V):
  1 is set for the video sector.

• End Of Record (EOR):
  1 is set at the last sector of the logical record.

All sectors except empty sectors must have only one bit of bits 1, 2, 3 (Video, Audio, Data) being 1.

● Coding information

The coding information byte defines the details of the sector's data type. Not specified in GAME-CD.

● Empty sector information

A sector where the channel number and coding information are 0, and all bits of submode Data, Audio, and Video are 0.
The empty sector is required to be a Form2 sector with all 0 user data areas.