- Joined: 03 Aug 2017
- Posts: 61
- Location: Munich
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flags with I/O block instructions
 Posted: Sun Jul 21, 2019 11:58 am
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Hi there,
I'm trying to figure out how the Z80's status flags change with I/O block instructions (OUTI etc.)
Unfortunately I don't have a working Z80/SMS debugging-environment.
All my books and Zilog's documentation state that the C flag is unaffected, P, S and H are "unknown" for most instructions, N is unaffected (some instructions), set to 0 (IN r, (C)) or 1 (many output instructions).
For the "unknown" flags I looked into Sean Young's "The Undocumented Z80 Documented", namely the chapter "4.3 I/O Block Instructions". There I find all the answers for these flags, but nevertheless this is where my confusion begins:
1. Sean writes, the N flag is a copy of bit 7 of the value read from or written to an I/O port. This contradicts the Z80 documentation (see above).
2. Sean goes on to state H and C both are set if ((HL) + L > 255) (for output instructions, input instructions have 2 different conditions). This may be correct for the H flag (which is "unknown" according to Zilog), but the C should always be unaffected.
(as with H, determination of P and S may be as stated, they at least don't conflict with "unknown")
Can anyone help me with this?
Block transfer instructions (CPIR etc.) should work the same (no funny algorithm for N and C).
Bye
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- Joined: 03 Aug 2017
- Posts: 61
- Location: Munich
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Posted: Wed Jul 24, 2019 11:01 pm
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Hi again,
chapter 7 of Sean's documents seems to address most (all?) of my confusion. Seems, that I have a very flawed official documentation (found a lot of mistakes besids those mentioned by Sean) and my three Z80-books also get many flags wrong.
Did the Z80 flag behaviour change over the years?
Bye
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