section 7 - Index of

tors allow word growth so that the most positive number that can be used is approximately256 and the most negative number is approximately -256. When the accumulatorextension registers are in use, the data contained in the accumulators cannot be storedexactly in memory or other registers. In these cases, the data must be limited to the mostpositive or most negative number consistent with the size of the destination and the signof the accumulator (the most significant bit (MSB) of the extension register).To maintain alignment of the binary point when a word operand is written to accumulatorA or B, the operand is written to the most significant accumulator register (A1 or B1), andits MSB is automatically sign extended through the accumulator extension register. Theleast significant accumulator register is automatically cleared. When a long-word operandis written to an accumulator, the least significant word of the operand is written to theleast significant accumulator register AO or BO and the most significant word is written toA1 or B1 (see Figure 3-8).DATAALUWORD OPERANDX1, XOY1, YOA1, AO81, 80IZ-242--47LONG - WORD OPERANDX1:XO = XY1:YO = YA1:AO = A1081:80 = 810I'_2 8 2 0I2-24ACCUMULATOR A OR 8SIG~ EXTENSIONA1,81,,OPER~ANDAO,80IZE~OFigure 3-7 Bit Weighting and Alignment of Operands