Re: A86: Negative numbers, sign bits, and the sign flag.
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Re: A86: Negative numbers, sign bits, and the sign flag.
On Thu, 24 Jun 1999 23:27:35 EDT CurtG6@aol.com writes:
>
> I have read that a negative byte is represented by counting
>down from
>255. So -1 would be 254, -2 would be 253, etc. But how do you know
>if
>%11111101 represents 253 or -2? I have also read that bit 7 of a byte
you don't. to the processor, they're exactly the same (as long as you're
working with 8 bits) the only difference is how you choose to interpret
it. there's basically two kinds of numbers, signed and unsigned. if
you're using signed numbers, 8 bits can hold -127 to 128 and bit 7 is the
sign bit. if you're using unsigned, it's 0 to 255 and the bit 7 is the
128ths place.
check it out:
255+1=256
%11111111+%00000001=%00000000 (bit 8 is lost to carry)
-1+1=0
%11111111+%00000001=%00000000 (carry doesn't mean much here)
>as -1 which goes back to my first question.) And also, when and how
>is the
>sign flag affected and how can you check it? Thanks for your help.
i'm not absolutely certain, but i believe the sign bit always reflects
bit 7 (of a/the last operation that affected it) and it's more or less
meaningless if you're using unsigned numbers.
-josh
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