## Bit-Slice Design Approach

Digital systems have a fixed number of signals that can be used to represent binary numbers. Smaller, simpler systems might use 8-bit buses that can only represent 256 different binary numbers, while larger systems might use 16, 32, or even 64 bit buses. Whatever the number of bits, all systems have a finite number of wires, storage elements, and processing elements to represent and manipulate digital data. The number of available bits determines how many different numbers can be represented in a given system.

Digital circuits that perform arithmetic functions often must deal with negative numbers, so a method of representing negative numbers must be defined. An

Of all possible encodings of negative numbers, two have been used most often: signed magnitude, and 2’s compliment. Signed magnitude representations simply designate the MSB as the sign bit, and the remaining bits as magnitude. In an 8-bit signed-magnitude system, ‘16’ would be represented as ‘00010000’, and ‘-16’ as ‘10010000’. This system is easy for humans to interpret, but it has a major disadvantage for digital circuits: if the 0 to

In 2’s compliment encoding, the MSB still functions as a sign bit it is always ‘1’ for a negative number, and ‘0’ for a positive number. The 2’s compliment code has a single ‘0’ value, defined by a bit pattern containing all 0’s (including the leading ‘0’). This leaves

The disadvantage to 2’s compliment encoding is that negative numbers are not easily interpreted by humans (e.g., is it clear that ‘11110100’ represents a -12?). A simple algorithm exists for converting a positive number to a 2’s compliment-encoded negative number of the same magnitude, and for converting a 2’s compliment- encoded negative number to a positive number of the same magnitude. The algorithm, illustrated in Fig. 2 below, requires inverting all bits of the number to be converted, and then adding ‘1’ to the resulting bit pattern. The algorithm can be visualized in the 2’s compliment number wheel above by noting that inverting all bits reflects a number around an axis drawn through ‘0’ and the largest negative number, and adding one compensates for the 2’s compliment code containing one more negative code than positive code.

## Important Ideas

- Digital circuits that perform arithmetic functions often must deal with negative numbers, so a method of representing negative numbers must be defined. An N-bit system can represent 2N total numbers, so a useful encoding would use half the available codes (i.e.,
$2^N/2$ ) to represent positive numbers, and half negative numbers. - Signed magnitude representations simply designate the MSB as the sign bit, and the remaining bits as magnitude. In an 8-bit signed-magnitude system, ‘16’ would be represented as ‘00010000’, and ‘-16’ as ‘10010000’.
- In 2’s compliment encoding, the MSB still functions as a sign bit it is always ‘1’ for a negative number, and ‘0’ for a positive number. The 2’s compliment code has a single ‘0’ value, defined by a bit pattern containing all 0’s (including the leading ‘0’). This leaves
$2^N-1$ codes to represent all non-zero numbers, both positive and negative. - A simple algorithm exists for converting a positive number to a 2’s compliment-encoded negative number of the same magnitude, and for converting a 2’s compliment-encoded negative number to a positive number of the same magnitude.