Disk Management, and File System In CP/M

                   The major task of CP/M was to provide a standard and portable file system. About 70 to 75% of OS system calls were related to the disk management and file system. (And rest of the OS systems calls were bypassed most of the times :D)

The Disk System:
                   Author’s Note: Before reading this post, users are kindly advised to read <<8 inch floppy disk>>. Or else, the information given in the following post might prove blinding.
                   In CP/M, there are two factors in the file system: disk drive and disk controller. The disk drive holds and rotates the floppy disk*1. The disk controller (device controller*2 for the disks) is usually placed on the motherboard. The disk drive has a magnetic head, which can read, and write data from track to track. To read or write data on a particular sector of a particular track, the drive/head must wait for that sector/track to come under the magnetic head (unless if it is already there).                                                                   
                   When a user gives read or command to the CP/M, CP/M commands the disk controller to do the task. After receiving the command, the device drive moves the head towards the target. Sometimes, the magnetic head might wrong the targeted track (it can go to the wrong track). The controller notices this and repositions the magnetic head correctly (each sector, and track has their number on it). Sometimes, the same thing might happen with a sector. All these activities are invisible to BIOS, and CP/M. These activities are carried out by a software, which is embedded on the device controller ROM. This type of software is often called as firmware.
After being formatted, such an 8” floppy disk contains:
77(tracks) * 26 (sectors per track) * 128 (bytes per sector) * 2 (sides) = 5,12,512 bytes.
Disk formatting is the process of writing the control information on the disk, by an operating system.
The File System of CP/M:
                   Explaining about file system in general or in relationship with any OS isn’t easy. But trust me; I’ll explain CP/M’s file system in very easy manner. CP/M’s file system is built on top of the BIOS. In CP/M’s file system the disk is divided in three parts: disk boot area, file directory area, and data storage area. The BIOS has a built in table, which has the size of each of these areas. A typical CP/M file system layout is shown in following diagram.

 

Disk Boot Area:

                   The first part of CP/M’s file system is this disk boot area. It contains the OS binary code used for booting. This area is not visible from the file system (so, not from anywhere else). Simply, this binary code is not part of ay file, and this code is invisible for normal users. Just like the Men In Black, MIB, this code is invisible, but doing its job promptly. The binary code of the OS contains loadable images of BIOS, BDOS, and CPP. These images are written in this disk boot area: sector by sector, track by track, starting at track 0, sector 1. Usually the BIOS is around 2000 bytes, the BDOS is around 3500 bytes, and the CCP is around 2000 bytes. So, all of them merrily fit together in the first three tracks of the floppy.
                   When the computer system is turned on, a small program, which is stored in a ROM chip on motherboard, is initialised. This program first initialises all the aspects of the computer hardware, and then copies OS executable images (binary code) from the disk to memory (RAM), and then starts the CP/M. This process is called as OS loading or more commonly as booting. Now-a-days there is a program which bears an uncanny resemblance with the program from the ROM mentioned above. This new program is called as bootstrap loader.

File Directory Area:
                   In CP/M, the size of the file directory area is fixed and recorded in a table in the BIOS. For an 8” floppy disk the file directory area holds up to 64 entries of 32 bytes each. A “file directory entry” is shown in the following diagram:

• User Number: This is actually a group number from 0 to 16, which allows multiple users or groups to share a disk and collect their files into a group. The tricky thing here is that there are actual sub-directories. Technically, all files are in one directory, the group numbers give an illusion of single level sub-directories. So that makes them “virtual sub-directories”.
• File Name and File Type: These two items can be considered as one item. A filename can consist 1 to 8 characters. (kill or killbill, but not killthebill).A file type can consist 0 to 3 characters (actually, it’s the OS, and application’s job to set the file type). The filename and file type is separated by a period (that’s what the computer nerds call the full stop). So, depending on the type of file, we get killbill.doc (a text document describing the various methods for killing the bill). This is something that we can still see in today’s operating systems. Sometimes, the file type is called as “extension”. Using some special characters some special characters in filename in not allowed in CP/M, including period, and blank space. Microsoft Windows XP doesn’t allow following items:  / \ : * ? ! < > | 
• Extent Counter: An “extent” is the portion of a file controlled by one directory entry. If a file takes more blocks than one directory entry can point, then that file is given additional directory entries. The extent counter is set to zero for the first part of the file, and then sequentially numbered for the each of the remaining parts of the file. Large files have multiple directory entries with the same file name, but different group of allocation pointers in each entry. As files can be deleted, and their directory entries can be re-used, the extents may not be in order in the directory. 
• The Number of the Records: No useful info could be found yet. (But soon, I will upload the information.)
• Reserved: Please read above entry.
• Allocation Map: This is a group of numbers, of (or, pointers to) the disk block that conation the data of the file. (This entry tells the OS that the data of killbill.doc file is stored on X block of the Y track.) There are eight pointers of 16-bits each. Each value (pointer) points to a sector which contains part of the file. If the file is small and if it can be stored in seven sectors, then seven pointers are used and the remaining one is set to zero. (the remaining pointers are set to zero, no matter how many are remaining.) If the file is too large, then additional pointers are allocated, and they are filled in another directory entry. In some CP/M systems, there were sixteen pointers of 8-bytes each.(Above, we were discussing about eight pointers of 16-bites each.)

Data Storage Area:
                   This is the area, which contains the actual data/files. When a user, or application tries to access a file, the file system searches the file directory entries to determine if a file with that name is stored on the disk or not. If the file name is found, then the directory entry would have the address of tracks and sectors (blocks), where the file is stored, so the data of the file can be accessed.
                   By now, if you are thinking that the file systems are too much easy, then let me correct you. Right now it seems easy, because the platform on which it is implemented, the CP/M, is not much complex in regard to file system. But still there is a complication in CP/M’s filesystem. As we saw in file directory area, there are 64 directory entries, and 8 file pointers per entry (each pointer of 16-bits.), which gives us 512 sectors to be used. But on the floppy, there are 26 sectors on each of the 77 tracks, which gives a total of 2002 tracks on the floppy! That simply means, we are not getting most of the space from our floppy. But Gary was a smart guy, do you think he could have overlooked this problem which was so easy to be found in theory? In practical finding this problem might have been easier. So, rather than pointing to an individual sector, Gary used a method in which, consecutive sectors are grouped together as “allocation blocks”. The size of these allocation blocks, is determined by the size of the disk. With the 8” floppies used with CP/M, it is eight sectors, that is 1024 bytes. So technically, in CP/M, each directory entry points to eight allocation blocks of 1024 bytes each.
    Whatever you studied above, should give you the answer of a question, this question bothers everybody, but nobody asks it loudly: “Where the heck are my A: and B: drives?” If you haven’t got your answer yet, then please visit: <<post will be uploaded soon.>>