The SYSdev programming language is a combination of ladder, high-level (subset of "C") and assembly (MCS-96). All the files which comprise a SYSdev program are programmed in the same language format. Each file can be written in any combination of the language types. The typical M4500 PLC program consists of the following files:

1. Initialization file (optional): executed once at power up.
2. Main Program file (required): scanned continuously.
3. Timed Interrupt file (optional): executed once every 0.250 to 65.000 milliseconds as set by the user.
4. User Function file (optional): up to 100 user defined subroutines which can be called from any of the above files.
5. Input Interrupts (ufunc00 and ufunc01): If the input interrupts are enabled, user function 00 is called when IN0 is activated, user function 01 when IN1 is activated.
   

Each file is implemented as a series of consecutive blocks. Each block is defined as one of the three programming languages: ladder, high-level, or assembly. Blocks of the different languages can be intermixed as necessary within the file.

For more information download the complete M4500 Program Development Manual.