🏠 Atari Jaguar Developer Reference ▸ CD-ROM Subsystem ▸ CD-ROM Subsystem Overview

CD-ROM Subsystem Overview

What the Jaguar CD is, how it differs from other CD systems, and the CD-ROM terminology used throughout these docs.

Source: Jaguar CD-ROM developer manual (scanned), © Atari Corp. 1995, pp. 1–3 (“The Jaguar CD-ROM”, “A Bit About CD-ROMs”, “Some Definitions”).

What the Jaguar CD is

The Atari Jaguar CD is a high-capacity optical storage device:

Property	Value
Capacity	746.9 MB
Speed	Double speed (≈353 kb/sec)
Sustained transfer rate	352,800 bytes/sec (full double-speed)
Uncorrectable error rate	< 1 in 10¹¹ (all errors flagged for re-read)
Sector / block size	2352 bytes (588 longs)
Frames per second	75 (at 1× single speed)
Format	“Raw” CD-DA, Red Book compliant, Motorola byte order
Sessions	Multi-session (Orange Book); appears as up to 40 stacked CDs
Tracks	≤ 99 total across the whole disc (TOC has ≤ 99 entries)

The high sustained rate (twice the MPC minimum of 150 kb/sec, and without the 40%-CPU-bandwidth penalty an MPC drive incurs) is achievable because of the Jaguar’s very large bus bandwidth.

How it differs from other CD systems

No file system. Data is accessed directly, not through a directory structure. You address data by its time stamp, not by filename.
Time-stamp addressing. Any position is reached via a time code mm:ss:ff (minutes : seconds : frames). Time stamps assume single-speed play and start at the beginning of the disc. The minimum addressable unit is one frame (588 longs / 2352 bytes); there are 75 frames/sec at single speed.
Reading is inexact. A read requested at a given time code is not guaranteed to land memory exactly at the start of that frame. The standard technique: start reading 6 frames early and search the first 31 frames (72,912 bytes) for your partition marker. Some writer software adds extra skew needing more pre-seeking; manufactured (pressed) discs stay well within tolerance. See Programming Guide and Disc & Data Format.
Long alignment only. CD data maintains long alignment, not phrase alignment. Graphics data that must be phrase-aligned has to be moved/realigned in your code. (Phrase = 64 bits; see Blitter.)
All access via the CD BIOS. To insulate titles from CD-vendor and data-transfer changes, every access to the CD and its controls must go through the CD BIOS. This is a hard requirement, not a recommendation.

A bit about CD-ROMs

CDs are constant linear velocity (CLV), single-data-track optical media with one data surface — a spiral roughly a mile long. Absolute position is encoded as a time code within the disc, resolvable to a single 2352-byte sector. Jaguar CDs are recorded in CD-DA “raw data” format with Motorola byte ordering: 2352 bytes per sector, all usable as data.

A standard CD divides into four region types: lead-in, tracks, pauses, and lead-out:

Lead-in (~10,000 sectors, inner diameter) repeats the Table of Contents endlessly in the Q subcode.
First pause follows lead-in, 150 or 225 sectors long.
Tracks are the data regions; multiple tracks are separated by 2–3 second pause regions.
Lead-out marks the end; primary data is all zeros with an alternating P subcode bit.

Multi-session CDs appear as up to 40 standard CDs arranged as sequential annular rings. Regardless of session count, the total tracks must be ≤ 99 for the whole disc. (In theory each session could hold 99 tracks for 3960 total, but that is not officially supported by Philips/Sony; the limit is usually worked around with a software “logical block / logical file” layer.)

See Disc & Data Format for how Jaguar lays sessions and tracks out, and the Atari header/tailer and partition-marker conventions.

Glossary of CD-ROM terms

Term	Definition
Absolute Time	Q-subcode time code, `00:00:00`–`73:59:75`, starting at the first pause region.
Area / Region	A physical ring-shaped portion of the disc surface.
Channel Frame	The 588-bit fundamental packet from the laser head: 24 bytes primary + 1 byte secondary (1 bit each of P–W subcodes) plus overhead.
Finalize	Writing the lead-in (with the main TOC) so a recordable CD is readable by standard players. Unfinalized CDs are generally unplayable except on players designed for it (Jaguar, Photo CD).
Index	A pointer within the currently playing track, used to access parts of a track independently of time code.
Lead-in	Inner-diameter region holding the TOC.
Lead-out	Region marking end of disc/session; primary data zeroed, P subcode alternates at 2 Hz.
Mode	Track type currently being read (audio, ROM, CD+G, Karaoke, CDI, …).
Open/Closed Session	A session is “closed” by writing its lead-in/lead-out; while open, data can be appended. Jaguar cannot access an open session.
Pause	Region of digital zeros with the P subcode all ones; some software calls it “Track Lead-in.”
Program	The main data region(s) of a CD.
Relative Time	Q-subcode time code from `00:00:00` to a track’s end-time while reading that track.
Sector / Block	Smallest addressable primary-data unit: 2352 bytes, readable without post-processing.
Session	An area with at least one full set of regions (lead-in, pause, track, lead-out). Up to 99 sessions per disc (~40 fit physically).
Subcode Data Channel	Secondary serial data at 1/192 the primary rate; 8 subcodes P–W. The Q subcode carries position info as `minutes:seconds:frames`.
Subcode Frame	Subcode info from one sector; 75/sec at 1×, 150/sec at 2×.
Table of Contents (TOC)	Disc directory read from the Q subcode; ≤ 99 entries, one per program, plus disc/manufacturer info.
Track Number	The number of a program (e.g. an audio selection) on the CD.

CD-ROM Subsystem Overview

What the Jaguar CD is

How it differs from other CD systems

A bit about CD-ROMs

Glossary of CD-ROM terms

See also