CD-ROM drives on os8088, and CDRack, a CD player

The compact disc arrived in 1982 as a music format. An audio CD holds tracks of sound, and a CD player turns them into sound itself. A CD-ROM is the same disc carrying files instead, about 650MB of them. That was more than four hundred 1.44MB floppy disks, which is why software, encyclopedias and games moved to CD in the 1990s.

The files on a data CD are laid out by ISO 9660, the standard file system for CDs since 1988. It is not the FAT file system DOS uses on floppies and hard disks. Its sectors are 2,048 bytes rather than 512, and its file names can be longer than the eight characters, a dot and three characters that DOS allows. Those are called 8.3 names, and they are what os8088 uses too.

From the mid-1990s most CD-ROM drives plugged into the same IDE connector as a hard disk and talked ATAPI. ATAPI is the set of commands a CD-ROM drive understands over that cable: the computer sends a 12-byte command, such as "read these sectors" or "play from this minute and second", and reads the answer back. The same drive reads both kinds of disc. For music it has a headphone socket on the front and a thin cable at the back to the sound card, and it plays the track out of those by itself.

os8088 now does both. A driver, CDROM.DRV, finds the drive and reads data discs. A program, CDRack, plays audio CDs through it.

Everything that works on a floppy works on the CD, except changing it. Folders open and .. goes back up. Programs start from it. Files copy off it byte for byte, including ones larger than 64KB. Saving to it, deleting from it or renaming on it is refused as write-protected, the answer a floppy with its tab open gives.

Long names are shortened the same way every time, so a name you saw in a listing still finds the same file later. A name that already fits 8.3 is kept, in capitals. A longer one becomes its first six usable characters, a ~, one letter or digit worked out from the whole name, and the first three letters of its extension.

Take the disc out and put another in, and the driver notices. A window still open on the old disc refuses its next step rather than show the new disc's files under the old disc's names. Opening the drive again shows the new disc. With no disc in the drive, the icon stays on the desktop and opening it does nothing, the way an empty floppy drive behaves.

An audio CD's music is 44,100 stereo samples a second, or 176,400 bytes every second. The computer does not have to move any of it. The drive turns the track into sound itself, as a CD player does, and sends it out of its headphone socket and its cable to the sound card. The computer sends one command to start a track, and while it plays, CDRack asks the drive where it is about twice a second to update the display. That is all the work the computer does for the music.

The root of an audio disc lists each track as a file: TRACK01.CDA, TRACK02.CDA and so on. Each is 44 bytes, laid out the way Windows lays them out, holding the track's number, where it starts and how long it is. Double-clicking one opens CDRack on that track.

CDRack's look comes from three mockups drawn by the project's author: a colour rack player, the same player in black and white, and a small mini player. It has a seven-segment LCD with a grid of track numbers, picture buttons, a volume slider and a list of the disc's tracks.

QEMU, the emulator these pictures come from, has a CD-ROM drive that holds one data track and cannot play audio. So these pictures use a test build of the driver that adds three simulated audio tracks, of 40, 30 and 20 seconds, after the data track. That build is never shipped. CDRack reads the tracks, then says the drive plays no audio and greys every button that would play. Actual playing was checked on 86Box, an emulator of period PCs whose CD-ROM drive does play audio, on an emulated 386. During development the driver played a test disc there on an emulated 386, track 2 into track 3 and on to the end, and the project's author watched CDRack play it through.

It needs a 286 or later. The drive moves its data 16 bits at a time and an IBM PC or XT's slots carry 8, so half of every transfer would be lost. That is also why the driver and CDRack are not on the 360KB system disk, which is the XT's. Drives on an XT-IDE adapter, and the older drives that plugged into their own cards or a Sound Blaster, such as Mitsumi's, Sony's and Panasonic's, are not supported.

It uses one drive, the first it finds. It shows 8.3 names only, and does not read Rock Ridge or Joliet, the two extensions that stored long file names on CDs. A file split into several pieces on the disc is read as far as its first piece.

It does not read the music off the disc as data, so a track cannot be played through a Sound Blaster instead of the drive's own output, or copied off as a sound file. Booting os8088 from a CD is a separate thing: the live CD on the download page works by pretending to be a hard disk.

1. The driver and CDRack are on the 1.44MB, 1.2MB and 720KB system disks and on the live USB image and CD. CDRack sits in the system disk's SYSTEM folder. Neither is on the 360KB system disk.

2. Boot os8088, open the Control Panel and go to its Drivers page. Tick CD-ROM. The driver loads at once, and the choice is saved for the next boot when you close the Control Panel. A CD-ROM icon appears on the desktop.

3. In QEMU, add a CD to the machine with -cdrom yourdisc.iso beside the floppy disks. QEMU can show a data disc but cannot play audio. In 86Box, give an AT-class machine an IDE CD-ROM drive and load a disc image into it. Its audio plays.

4. Double-click the CD-ROM icon to open the disc. For an audio CD, double-click a TRACK file to open CDRack on that track, and press Play. Closing CDRack does not stop the disc, because the drive plays on by itself. Press Stop first if you want quiet.