EXDOS System Overview

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3. IS­DOS Organisation

The code of IS­DOS is on disk rather than being in ROM and will be loaded automatically at power on, or by an explicit EXDOS CLI command. When loaded it sets up a standard EXOS environment with certain defined channels open (for example video, keyboard and editor), and interprets commands typed by the user.

The way in which IS­DOS fits in with EXDOS is show in the diagram above, although only the most important connections are shown. The user's commands are interpreted by a section of IS­DOS called the IS­DOS CLI. This handles commands in various different ways, the important ones of which are:

• Passing them to the EXDOS CLI for interpretation.
• Interpreting them internally, using the filing system handler to carry out disk operations (and maybe using EXOS as well).
• Passing them to the EXOS scan system extensions function call for interpretation by other ROMs.
• Loading .COM files from disk and executing them as a transient program.
• Reading subsequent commands from .BAT files.

When IS­DOS loads a transient program, it provides it with a CP/M and MSX­DOS compatible environment in which to run. This is done by the resident portion of IS­DOS which remains in memory while the transient program is running (the IS­DOS CLI can be overwritten by the transient program). This resident portion translates CP/M function calls into calls to the filing system handler, and also handles the re­starting (and possibly reloading) of the IS­DOS CLI when the transient program finishes. This latter function includes controlling batch file processing.

Transient programs loaded by IS­DOS can make all important CP/M and MSX­DOS calls, and some CP/M BIOS calls. This ensures that programs written for CP/M or MSX­DOS will work under IS­DOS. However IS­DOS programs can also make EXOS function calls and can thus make full use of all the Enterprise's features. These calls can be mixed together in any way.