CD Size Properties

[cksonic]

I have a query regarding the size of files contained in a CD and the properties shown by Windows Explorer. I have an installer CD, which tells me that it is 654,374,912 bytes in size when I query my CD-ROM drive icon (right click on CD-ROM icon -> Properties) from "My Computer" Explorer. However, when I explored into the CD ROM drive and query the size of all the files contained in the CD (select all files -> right-click Properties), it tells me the the following information:
Size: 812 MB (851,620,828 bytes)
Size on disk: 817 MB (857,339,904 bytes)

My doubts :confused: :
1) Why is there a discrepancy between the 2 Properties shown by Windows Explorer? Which one is the real size of the data in the CD?
2) Why is the "Size" of files on a CD different from "Size on disk"? I understand that for a hard disk, the difference between the 2 numbers is due to clustering of the files (the larger number in "Size on disk" is due to unused empty clusters). Does the same clustering structure work for CDs as well?

Thanks,

Damon

[satis]

the size on disk, size discrepancy has to do with how space is allocated in a file system. I'm not too up on CDFS, but in fat16/fat32/NTFS, for instance, all space on the drive is preallocated. Since you only have so much space in your FAT, the disk is broken into pieces (of, say, 512bytes a piece). If you have a 600 byte file, it has to take up two of these pieces (clusters? something like that), so it's size is 600 bytes, but its size on disk is 1024bytes.

Make sense?

As far as the other discrepancy, I don't know. That's a pretty big difference.

[cksonic]

hi satis,
thanks for your explanation! i do understand the logic behind file allocation in the case of a hard drive (FAT/NTFS), but i am not so sure for CDs. would anyone know the reason for the 2 discrepancies listed? (btw, those are real numbers that i obtained from a Microsoft installer CD)

[IanS]

hi satis,
thanks for your explanation! i do understand the logic behind file allocation in the case of a hard drive (FAT/NTFS), but i am not so sure for CDs. would anyone know the reason for the 2 discrepancies listed? (btw, those are real numbers that i obtained from a Microsoft installer CD)M$ have for years used their own special CDFS format for distribution CDs. Maybe this is part of the reason behind the differences? They manage to squeeze more data than theoretically possible onto a 640Mb CD a long time ago, so it doesn't surprise me that they now get 800Mb onto a CD. If I remember right, they also use a form of file compression on the file-system.