Pretty much what KTB said.
Asar is like an xkas upgrade. It adds a couple of new functions such as read, which allows you to read the value at a SNES address and then use it for stuff, usually conditional assembling by checking if a byte has a certain value.
Or freespace which automatically searches for freespace in the ROM and puts the code there including RATS tags.
Other changes are that they handle math opperations differently with asar being more accurate.
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