Re: A92: Question about flags...
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Re: A92: Question about flags...
At 19:16 1998-07-07 +0200, you wrote:
>
>
>I don't really understand the difference between the overflow flag (V) and
>the carry flag (C).
>Could someone explain me it ? (in fact, I think I don't understand the
>carry flag)
>
>And I have an other question :
>for example, if we have for d4.l=0x0000FFFE
>and we do : add.w #0x20,d4
>I know that the overflow flag will be set, but what will be the value of
>d4.l ???? 0x0000FFFF or 0x0001001E ?
d4.l will be 0x0000001E and the carry bit will be set.
The .w in add.w simply tells the CPU how much data it should operate on. If
your example was right, you should be able to do add.l #20,d4 with d4.l =
0xFFFFFFFE and get d4.? = 0x10000001E ;-) That extra bit is simply sent to
the
carry instead.
>From the Motorola 68000 Programmers Reference:
V—Overflow
------------------
Set if an arithmetic overflow occurs implying that the result cannot be
represented in the
operand size; otherwise clear.
I believe this goes for both signed and unsigned arithmetics.
C—Carry
--------------
Set if a carry out of the most significant bit of the operand occurs for an
addition, or if a
borrow occurs in a subtraction; otherwise clear.
Niklas Brunlid - http://www.efd.lth.se/~e96nbr
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