Karoshi MSX Community

Hi!
A snippet for the beginners like me ;)

To add an 8 bit value (a) to a 16 bit register (hl) I used to do this:
That takes 22 cycles and destroys an additinoal register (de).

There is a way to do the addition without using any additional register, slightly faster (20 cycles) but 1 byte longer:
Note that bc register or de register can be used instead of hl.

Very optimized  :). Well done!

Oh, not mine; just found on the internet. Can't remember where; maybe here (http://stackoverflow.com/questions/2247906/z80-how-to-add-16-and-8-bit-registers).
Strangely, it is not in the classical "Z80 maths" pages ??? That's why I put it here.

But, What about the extra M1 cycle? In a MSX, both routines take 25 cycles, so the advantage is reduced to don't destroy aditional registres at price of one byte longer.

You're right. I'm a newbie; I forgot about the M1 cycle :(
Anyway, it's still useful if other registers cannot be detroyed (the push/pop pair would make the code longer and slower) or if the operation to accomplish is bc+=a or de+=a.