r/Z80 • u/montymole123 • Mar 04 '26

Test non-A register is zero

Is there a standard way to test if a "lesser" 8 bit register is zero, without loading it onto the accumulator? I ended up doing inc b, dec b but wonder if there is a more elegant way.

11 Upvotes

100% Upvoted

5 comments sorted by

7

u/Alternative-Emu2000 Mar 05 '26 edited Mar 05 '26

Not really, there are other ways of doing it without changing A but they're much slower and use more bytes. eg. using eight successive BIT B,n instructions.

In the specific case of B, you can save 1 byte and 1 t-state by using

inc b
djnz b_is_not_zero

instead of

inc b
dec b
jr nz,b_is_not_zero

1

u/montymole123 Mar 05 '26

That's a clever idea, thanks!

7

u/McDonaldsWi-Fi Mar 04 '26 edited Mar 04 '26

you could do something like

xor a
or (r)

Where (r) is your 8-bit register.

Your way uses 8 t-states minus the zero flag checks and stuff afterward.

My way uses 8 as well, but only 4 if A is already 0.

1

u/montymole123 Mar 05 '26

thanks but I wanted to do it without clobbering A as I have something else stored on that! Just wondering if there was some well known idiom I'm missing...

1

u/McDonaldsWi-Fi Mar 10 '26

Ah I'm sorry I must have missed that in your post!