> For the complete documentation index, see [llms.txt](https://guide.cronuszen.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://guide.cronuszen.com/gpcscripting/gpc-script-guide/what-is-gpc.md).

# What is GPC?

The language behind every Cronus Zen script, and the three ideas that matter   most in a real-time report loop.

GPC is the scripting language used to read controller input, maintain state, transform values, schedule timed sequences, control device feedback, and write the outgoing controller report on Cronus Zen.

A GPC script is not a desktop program. Its `main` section runs repeatedly as part of the real-time report loop. That makes three ideas especially important:

{% stepper %}
{% step %}

#### Read the input layer you actually mean.

{% endstep %}

{% step %}

#### Keep one clear final writer for each output.

{% endstep %}

{% step %}

#### Perform expensive display or persistence work only when something changes.

{% endstep %}
{% endstepper %}

This guide begins with those ideas before introducing advanced storage, architecture, and VM details.


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