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SubjectOverflow Flag Reply to this message
Posted byfinaldave
Posted on09/12/07 03:18 AM

Trying to write (yet another!) cpu emulator, this time in C so you don't have the luxury of opcodes to get the flags for you! So I'm struggling with the Overflow flag...

The docs seems to say:
"overflow is set if signed overflow occured"
so here's some examples I came up with:

0x20   0x20 = 0x040 - no carry, no overflow
0xc0 0xc0 = 0x180 - carry, no overflow
0x40 0x40 = 0x080 - no carry, overflow (because it should be 128 not -128)
0x80 0x80 = 0x100 - carry, overflow (because it should be -256 not 0)
0x40 0x80 = 0x0c0 - no carry, no overflow

Some emulators seem to do something with boolean operations on the operands and result, so I tried building a Karnaugh map of bit 7 in operands and result:
bit 7 in all:

x y z x ^ ~y x ^ z
0 0 0, no carry, no overflow 1 0
0 0 1, no carry, overflow 1 1
0 1 0, carry, no overflow 0 0
0 1 1, no carry, no overflow 0 1
1 0 0, carry, no overflow 0 1
1 0 1, no carry, no overflow 0 0
1 1 0, carry, overflow 1 1
1 1 1, carry, no overflow 1 0
overflow = (x ^ ~y) & (x ^ z) ?

Weirdly when I plug this into a Karnaugh map solver I just get (~x)(~y)z xy(~z) which seems worse than the above, so not sure if I've done my maths wrong, or if an xor is just considered more expensive??

I'm Googling and I can't find much information about whether this is right... has anyone done this before? What method did you use for the carry, half-carry and overflow flags? (Not too bothered about half carry since I working on 68000, but just curious for Z80). Noticed further down the board something was mentioned about xoring all three for half carry?

Any help much appreciated :-)

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Entire Thread
Subject  Posted byPosted On
.Overflow Flag  finaldave09/12/07 03:18 AM
.*Re: Overflow Flag  Jonemaan09/17/07 09:00 AM
..*Re: Overflow Flag  finaldave09/19/07 06:19 PM