The 800XL was my first Atari, before the ST. It was quite hackable, and within a few months I didn't need a disassembler any more. I could read the 6502 instructions directly out of a hex dump and could write them in hex, without an assembler (6502 has so few instructions, you can memorize them all). Fun times. Math functions in the ROM were all operating on BCD — there was no FPU, of course.