IRIS partitioned disk drives in a manner very similar to today’s drive partitioning.
Each partition was referred to as a LOGICAL UNIT (LU).
Logical Unit zero (LU0) contained the operating system files, and any other desired user files (similar to C:\ drive on windows)
Other Logical Units (LUn) could contain any type of file.
LU’s have the following characteristics:
- An LU consists of one or more cylinders on a given drive type.
- LU’s must begin and end on cylinder boundaries.
- RDA’s on LU zero are limited to: 0 – 77777 (32768 blocks).
- RDA’s on other LU’s are limited to: 0 – 177776 (65535 blocks).
IRIS supported a wide variety of file types, each with its own features and handling.
- Processors (e.g. RUN, BASIC, COPY, LIBR)
- Device Drivers (terminal, printer, mag tape, etc)
- System function drivers to extend system abilities
(decimal arithmetic, advanced terminal functions, etc)
- Basic programs (interpreted)
- Text files
- Formatted Data Files (defined later)
- Contiguous unformatted data files
- Contiguous Indexed files
- Stand-alone assembly utilities (e.g. backup)
- Other types as defined by developers
Internal File Structure
Every file contains a Header block and one or more ‘data’ blocks. The header block contains a lot of info about the file, including the location of its data blocks. All of the above file types are structured in one of three ways:
- Contiguous: The entire file is made up of contiguous disk blocks.
- Random: File blocks can be scattered all over, yet treated as a unit.
- Hybrid: Structured like Random, but with contiguous blocks.
(only DMAP and DISCSUBS are in this group)
A contiguous file can be any length, and its starting and ending points are defined by the RDA of the Header block and the number of blocks in the file.
A random file can be in one of two formats.
- Simple: Header block identifies up to 200 (128) data blocks (block list is in the last half of the header block).
- Extended: Header block identifies up to 200 (128) extender blocks, each of which identifies up to 400 (256) data blocks for a total of 100000 (65536) data blocks.
Random files are created in simple format, and modified to extender format automatically by the system when the file exceeds 200 (128) data blocks.