Hello, all. This is my first post here, so my apologies if I do anything wrong.

I have been reading Rodnay Zak's "Programming the 6502", and so far it's been helpful. However, one thing I don't think the book explains is when it tells me for an exercise to write a positive integer in "two's complement" (Binary). I understand that if the leftmost bit (bit 7?) is 1, then the number is negative, and that two's complement is the same thing as one's complement but you add 1 (BIN). So my question is, how do I write a positive integer/number in two's complement if inverting the bits turns bit 7 from a 0 to a 1? Again, as much as I like the book, I don't recall it being covered, though I could be wrong.

For example, if I wanted to write 52 in two's complement (00110100), would it be something like 01001011 (not inverting the 7th bit), or would I change the 7th bit as well? (11001011)

Thank you for your time.
-Nick

A binary number with the highest bit clear is considered to be a positive two's complement number.

A single byte represents unsigned numbers in the range 0..255.

There are several ways to represent signed numbers.

One is sign/magnitude in which the upper bit is the sign, 1 for negative. The rest represents the absolute value of the number. The problem with this format is that arithmetic involving negative numbers is awkward. Not to mention there are two representations for zero, positive and negative.

The most common representation supported by computer hardware is two's complement is which a byte represents numbers in the range -128..127. 0 is represented by 0, naturally. 1 by 1 and -1 by $FF. Adding or subtracting two's complement numbers is very easy.

There are other representations with specialized uses.

Maybe this will help to clarify: http://www.mathcs.emory.edu/~cheung/Cou ... ess-n.html

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Thank you for the responses!

So the 7th bit would just be 0, still. Thanks!

Welcome! Yes indeed: bits only get flipped when dealing with a negative number.

In fact two's complement wraps around zero in a nicely regular way: there's no discontinuity as you count up from negative numbers into positive numbers:
1111 1110
1111 1111
0000 0000
0000 0001
That is, the usual rules for adding one and propagating carries.