Lets say you have a brand new hard drive. It is completly blank with no partitions defined. Now, also, lets say that you are going to install DOS 6 onto this drive. The largest partition DOS 6 can see is 2 Gigs. But your hard drive has 8 Gigs of free space. You will have to make 4 partitions of 2 Gigs each on this drive to use it all up. Now we enter the concept of logical drives. There is only 1 phisycal drive, but you will have 4 logical drives on your hard disk; "C: D: E: F:" (Four individual partitions, get it? Good.) Now, there can only be 4 primary partitions on a single hard disk. To have any more rquires an extended partition. It can contain virtually limitless amounts of partitions. Also, since a partition is completly isolated from all other partitions on your hard drive, you can have different operating systems in each partition. You can have OS/2
in one, win95 in another, Linux in another, and so on. "But why can't DOS 6 see a single partition if it's bigger than 2 Gigs?" Because DOS 6 and DOS 5 use whats called a 16 bit FAT (File Allocation Table). DOS 3.3 had a 12 bit FAT, which limited partitions to 33Megs! (Back then a 40Meg hard disk was like having a 14Gig drive now!) The FAT is a place DOS stores the address of all your files and programs. When you run a program, DOS looks at the FAT to see which cluster on your hard disk that program lies. Since the FAT only has 16 bits to store the location of individual clusters, it can only store physical locations of files within 2 Gigs of hard drive space. Anything bigger would require a 17th bit. Ok, now most of you should have heard about windoze 95/98's 32 bit FAT. Since there are more bits more phisical space can be allocated. It also allows for smaller clusters to be used. (Now we will delve into what clusters are. Clusters are made of x number of sectors. Sectors are 512 bytes in size. Ok, say you have a 200 byte file. The smallest space DOS can place your file is 1 cluster (x sectors) which is 20K bytes! A 32 bit FAT allows clusters as small as 4K, so you went from 20K to 4K for that 200 byte file. That frees up alot of space if you have many small files laying around!) Now back to fdisk! When you run fdisk from DOS 7 (Windoze 95B or 98) you are first greeted with a prompt to enable large disk support (Install a 32 bit FAT). Remember, DOS 6 and below as well as the A version of windoze 95 can NOT see a drive partitioned with FAT32. Windows NT is also unable to see FAT32, although you can obtain third party drivers that fix this problem. (FYI, Linux 2.0.35 and up DOES support FAT32, as well as many other OS partitions!)
File Allocation Table
The File Allocation Table (Otherwise know as a FAT) is where DOS keeps the location of every file and subdirectory on your disk (Floppies, hard drives, CD's, ZIP disks, ect.). If all you wanted to know is what FAT stood for, you can stop reading here. Now I will go into a little more detail on the DOS file system. Lets say you just wrote a love letter. When DOS writes your love letter file to the disk, it puts the cluster('s) your letter is located on into the FAT. Now suppose you copy a program to your disk, the start of this program is at the very end of your love letter. After a couple hours you decide to expand on your feelings to your sweety. But the program you just copied to your disk is immediately after your letter, how can DOS make you letter longer without moving your program to another spot further down the disk? Simple, DOS just starts adding the rest of your love letter after the program you copied. In order to keep track of this "fragment", DOS puts a notice in the FAT telling it where the next ajoining "fragment" is. Notice I used the word "fragment"... Have you ever wondered where DEFRAG got its name? Well, to get off track a little, DEFRAG moves all your programs and data so they are not scattered about in pieces. That way your hard disk can get your entire file at once without constantly referring back to the FAT to find more pieces, and also it takes less time to get the entire file if the hard drive doesn't have to go all about the platters to gather data. Well, now you know..... the ressst of the story.