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p-System File Volumes

The p-System is an operating system in its own right, and naturally has its own file system. It consists of a set of flat (rather than directory-tree) file volumes, one volume per storage device or hard disc partition. Each volume has a name by which the system accesses it, no matter on which on-line device it may be mounted. Volume names can be up to 7 characters long, and each volume can hold up to 77 files -- an implementation limit of the directory structure.

(This type of volume, block-structured volumes, is for file storage. A second type, byte-structured volumes, is for I/O channels. I/O operations on both types are contained in the p-machine emulator, in native code, so the interpreted nature of the p-machine does not slow down primitive I/O operations. Unlike DOS, which assigns letters to disc or network drives, but names other I/O channels independently, the p-System assigns a volume number to every volume, whether a byte-oriented I/O channel or a block-oriented storage volume.)

The number of volumes that can be mounted at once is typically limited mainly by the configuration information from the boot volume's (see under 1.1) file SYSTEM.CONFIG, where the user can specify how many device numbers are to be available to them. Though the limit varies with p-System implementations, it's usually higher than the number of volumes at a time that the user could keep track of. When the p-System boots, it automatically mounts every volume it finds on-line, assigning device numbers to them in sequence. The user may refer to any volume by its name or its number. (Programs, in general, should only use the name, to avoid depending on the volume's being mounted at a specific number).

The p-System allocates all files sequentially and contiguously on the medium, rather than distributing the data of each among a pool of available storage components, linked by the directory structure, as in more recent operating systems. This may owe to the speed of the floppy disc media on which it was originally typically run: disc access was optimised if a seek to the head of the file was always followed by the entire body of the file, with no need to return to the directory blocks to read anything further from them.

File names can be up to 15 characters long. Though the full 15 are available for the name itself, they may also include extensions. Such extensions as .TEXT and .CODE are recognised by the file system, and users are free to create others.

The file system also notes a file type for each file, which is assigned, according to its extension, when the file is created. While this includes differentiating volumes from files themselves, it also distinguishes text files from executable code files, and some lesser-used types such as graphics files.

The user works with files and volumes in the Filer program SYSTEM.FILER which is run from the main command promptline.

(Although a newer, enhanced filing system was advertised as available from Pecan Systems toward the end of ETC's active use of the p-System, it was never acquired.)

• p-System Boot Volume and Prefixed Volume