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Saturday, November 19, 2005


One of the major design decisions I had to make for Q1 was how to handle two things that seem unrelated, but really are: arithmetic overflow and conditions. Arithmetic overflow occurs when the result of arithmetic (either addition or subtraction) is a number that is too large to be represented in a single word (a register; as Q1 is a 32-bit CPU, its words are 32-bit).

Take the case of the unsigned addition of 0xFFFFFFFF and 0xFFFFFFFF (the largest possible numbers). The correct result of this addition is 0x1FFFFFFFE. However, this result requires 33 bit, and is thus truncated to 0xFFFFFFFE when placed in a register; one bit of significant data is loss.

Now, at the risk of confusing you, I should make it clear that the lost 33rd bit is not always significant. Take, for example, the subtraction of 5 from 10. In two's complement math this is performed by negating the 5 (to get 0xFFFFFFFB) and then adding it to 10. The result of this is 0x100000005, which is 33 bits. In this case, one bit is lost, but it contains no actual information. A single example such as this isn't sufficient to prove it is so, so I'll tell you straight out: for unsigned subtraction, overflow occurs if and only if there is NO loss of the 33rd bit - exactly the opposite of the case for addition.

However, it gets even more complicated. Consider the signed addition of 0x40000000 and 0x50000000. Both of these numbers are positive, so the result must also be positive. However, the result of addition is 0x90000000; the fact that the highest bit has been set indicates that the number is negative. Overflow has occurred, even though the 33rd bit hasn't been touched. Now consider the addition of -1 and -1 (0xFFFFFFFF). In this case the result is 0x1FFFFFFFE, or 0xFFFFFFFE (-2) when truncated. Here, the 33rd bit is lost, but no overflow has occurred.

What this means is that there are different methods of detecting overflow for signed and unsigned arithmetic. Unsigned arithmetic is fairly simple: if ((33rd_bit != 0) != is_subtraction), overflow has occurred. For signed arithmetic, it's more complicated. First of all, let me tell you that this equation, although it appears in some computer architecture books (like mine), is NOT correct: if (33rd_bit != 32nd_bit) overflow. The correct equation is: if ((32nd_bit_of_operand_1 == 32rd_bit_of_operand_2) && (32nd_bit_of_result != 32nd_bit_of_operand_1)) overflow. In other words, if both operands have the same sign (remember that with subtraction one operand will be negated; this must be taken into account), but the sign of the result is different, then overflow has occurred. Traditionally, unsigned overflow is referred to as a carry, and signed overflow is referred to as overflow (neither of which are particularly good names).

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