zx_saddrpup code question

derekfountain · Post by **derekfountain** » Mon Apr 27, 2020 5:17 pm

Just following up here on a thread over at Spectrum Computing. The Z88DK library code for finding the Spectrum screen address above a given one is in asm_zx_saddrpup. I noticed these instructions at the bottom of that function:

Code: Select all

and $18
cp $18

What are they for?

Post by **dom** » Mon Apr 27, 2020 6:03 pm

It will be to handle setting carry when it goes off screen.

H will have values from $40 to $57 in the valid exit case.

In the value range the bits corresponding to $18 can?t both be set.

derekfountain · Post by **derekfountain** » Mon Apr 27, 2020 8:34 pm

But the very next instruction is to clear carry...?

Post by **dom** » Mon Apr 27, 2020 9:14 pm

ccf = Complement Carry Flag.

Assuming we've stepped above the screen, then h=$3f. So $3f & $18 = $18. $18 cp $18 resets carry so it needs to be flipped for carry to be set to indicate above the screen.

derekfountain · Post by **derekfountain** » Tue Apr 28, 2020 7:40 am

lol... So much to learn... Thanks for the explanation.