The Arithmetic Unit. Erard 6502

2.2 THE ARITHMETIC UNIT

One of the functions to be expected from any computer is the ability to compute or perforin arithmetic operations. Even in a simple control problem, one often finds it useful to add 2 numbers in order to determine that a value has been reached, or subtract 2 numbers to calculate a new value which must be obtained. In addition, many problems involve some rudimentary form of decimal or binary arithmetic; certainly many applica­ tions of the microprocessor will involve both. The MCS650X has an 8-bit arithmetic unit which interfaces to the accumulator as shown in Figure 2.2

Partial Block Diagram including Arithmetic Logic Unit o f MCS650X

FIGURE 2.2

The arithmetic unit is composed of several major parts. The most important of these is the circuitry necessary to perform a two's comple­ ment add of 8-bit parallel values and generate an 8 parallel bit binary result plus a carry. A review of binary and binary coded decimal (BCD) arithmetic is presented in Appendix H. However, a quick review of the concept of "carry" is in order. The largest range than can be repre­ sented in an 8-bit number is 256 with values ranging between 0 and 255.

If we add any 2 numbers which result in a sum which is greater than 255, we represent the result with a ninth bit plus the 8 bits of the excess over 255. The ninth bit is called "carry."

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