How would I go about adding, subtracting, multiplying and dividing decimal numbers? When I do add, sub, etc. and output it I get a character (which is kind of to be expected) How could I output it so it's still a number?

you work only with binary numbers stored in registers. Usual way is to convert input from ascii decimal string to number in register, work with it, and convert it back to decimal on output

http://board.flatassembler.net/topic.php?t=6330

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