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Page description languages allowing infinite loops or calculations that require a lot of computing time. Even minimalist languages like [PCL](http://hacking-printers.net/wiki/index.php/PCL) can be used to upload permanent macros or fonts until the available memory is consumed.
By setting `showpage`– which is used in every document to actually print the page – to do nothing at all, PostScript jobs are processed they won't print anything.
Welcome to the pret shell. Type help or ? to list commands.
printer:/> disable
Disabling printing functionality
```
Both attacks code can also be written into Sys/Start, startup.ps or similar files to cause **permanent DoS** on devices with a writable disk.
## PJL
### PJL jobmedia
Proprietary PJL commands can be used to set the older HP devices like the LaserJet 4k series into service mode and completely disable all printing functionality as shown below:
In addition, the PJL standard defines the `OPMSG` command which ‘prompts the printer to display a specified message and go offline’ \cite{hp1997pjl}. This can be used to simulate a paper jam as shown in below:
**Learn more about these attacks in** [**http://hacking-printers.net/wiki/index.php/Document\_processing**](http://hacking-printers.net/wiki/index.php/Document\_processing)
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