Two's Complement — How are negative numbers handled?

It is my understanding that numbers are negated using the two's compliment, which to my understanding is: !num + 1. So my question is does this mean that, for variable 'foo'=1, a negated 'foo' will be the exactly the same as variable 'bar'=255. f we were to check if -'foo' == 'bar' or if -'foo' == 255, would we get that they are equal? I know that some languages, such as Java, keep a sign bit -- so the comparisons would yield false. What of languages that do not? And I'm assuming that assembler/native machine does not have a sign bit.

In addition to all of this, I read about a zero flag or a carry-over flag that is set when a 'negative' number is added to another (of any sign) number. This flag being set whenever it is added because of the way two's complement works, 0x01 + 0xff = 0x00 (with the leading 1 truncated). What exactly is this flag used for?

And my last question, for other math operations (such as multiplication), would I have to re-negate the number (so it is now positive), perform the operation, and negate the result? E.g., !((!neg + 1) * pos) + 1.

Edit Finished the question, so feel free fire away.

Answers


Yes, in two’s complement, the number x is represented as ~x+1, where ~x is the bitwise complement of the binary numeral for x in some fixed number bits. E.g., for eight bits, the binary numeral for x is 000000001, so the bitwise complement is 11111110, and adding one produces 11111111.

There is no way to distinguish -1 in eight-bit two’s complement from 255 in eight-bit binary (with no sign). They both have the same representation in bits: 11111111. If you are using both of these numbers, you must either separately remember which one is eight-bit two’s complement and which one is plain eight-bit binary or you must use more than eight bits. In other words, at the raw bit level, 11111111 is just eight bits; it has no value until we decide how to interpret it.

Java and typical other languages do not maintain a sign bit separate from the value of a number; the sign is part of the encoding of the number. Also, typical languages do not allow you to compare different types. If you have a two’s complement x and an unsigned y, then either one must be converted to the type of the other before comparison or they must both be converted to a third type. Thus, if you compare x and y, and one is converted to the other, then the conversion will overflow or wrap, and you cannot expect to get the correct mathematical result. To compare these two numbers, we might convert each of them to a wider integer, such as 32-bits, then compare. Converting the eight-bit two’s complement 11111111 to a 32-bit integer produces -1, and converting the eight-bit plain binary 11111111 to a 32-bit integer produces 255, and then the comparison reports they are unequal.

The zero flag and the carry flag you read about are flags that are set when a comparison instruction is executed in a computer processor. Most high-level languages do not give you direct access to these flags. Many processors have an instruction with a form like this:

cmp a, b

That instruction subtracts b from a and discards the difference but remembers several flags that describe the subtraction: Was the result zero (zero flag)? Did a borrow occur (borrow flag)? Was the result negative (sign flag)? Did an overflow occur (overflow flag)?

The compare instruction requires that the two things being compared be the same type (two’s complement or unsigned), but it does not care which type. The results can be tested later by checking particular combinations of the flags depending on the type. That is, the information recorded in the flags can distinguish whether one two’s complement number was greater than another or whether one unsigned number was greater than another, depending on what tests are made. There are conditional branch instructions that test the desired flag properties.

There is generally no need to “un-negate” a number to perform arithmetic operations. Processors include arithmetic instructions that work on two’s complement numbers. Usually the add and subtract instructions are type-agnostic, the same way the compare instruction is, but the multiply and divide instructions are not (except for some forms of multiply that return partial results). The add and subtract instructions can be type-agnostic because the wrapping that occurs in the arithmetic works for both two’s complement and unsigned. However, that wrapping does not work for multiplication and division.


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