Commander X16

In the C128 monitor you just need to type something like ".1300 lda #$00" and start coding. I tried the same on the X16 and no work, I didn't understand the commands as there was none for new program or something like that. Any help is greatly appreciated!

It's all documented here:

https://github.com/X16Community/x16-doc ... Monitor.md

To my recollection, X16 Monitor is virtually identical to the C128 monitor. There are probably some minor command differences, but they're both based on Jim Butterfield's Supermon.

To assemble, type
A <address> <instruction>

For example:
A 0400 LDA #$01

Remember that addresses must be exactly 4 hex digits, and 8-bit values must be exactly 2 hex digits. If you make a mistake, the line is simply not processed. There is no error message telling you what you did wrong.