Editing Commodore DOS (section)

===Command channel===
As previously noted, the Commodore DOS itself is accessed via the "command channel", using syntax like that used to access files. Issuing commands to the DOS and retrieving status and error messages generated in response to commands is accomplished by opening a file to the device using 15 as the secondary address, for example:

:{{code|lang=cbmbas|OPEN 1,8,15}}

To retrieve and display the device status, one could code:

:{{code|lang=cbmbas|OPEN 1,8,15:INPUT#1,E,E$,T,S:PRINT E,E$,T,S:CLOSE 1}}

In the above example, <code>E</code> will hold the error number (if any; it will be zero if no error exists), <code>E$</code> will be a terse text description of the error, <code>T</code> will represent the disk track where the error occurred, and <code>S</code> will be the sector on track <code>T</code> to which the error refers. If no error exists, the equivalent of {{samp|00,OK,00,00}} will be returned in the four variables.  Note that <code>INPUT#</code> is a run mode only verb. Also, in programs that issue many disk commands it is customary to open a file to the device's command channel at the start of the program and not close it until the program has finished.

Commodore BASIC versions 4.0 and later provide a pseudo-variable referred to as <code>DS$</code> that may be used to retrieve drive status in lieu of the above code. This reserved variable is not available on earlier versions of BASIC, so the command channel must be manually read as demonstrated above. Note that immediately after power-on or reset, the DOS revision will be returned. For example, a 1541 will return {{samp|73,CBM DOS V2.6 1541,00,00}}. Error code 73 is common to all drive models and may be used to determine if the drive has been reset to its power-on state.