zx_saddrpup code question

ZX80, ZX 81, ZX Spectrum, TS2068 and other clones
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derekfountain
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zx_saddrpup code question

Post by derekfountain »

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:

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and $18
cp $18
What are they for?
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dom
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Post by dom »

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
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Post by derekfountain »

But the very next instruction is to clear carry...?
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dom
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Post by dom »

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
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Post by derekfountain »

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