S1=0 and S0=0 (no scaling)
then S = (A46 XOR A45) OR (B46 XOR B45)
S1=0 and S0=1 (scale down)
then S = (A47 XOR A46) OR (B47 XOR B46)
S1=1 and S0=0 (scale up)
then S = (A45 XOR A44) OR (B45 XOR B44)
S1=1 and S0=1 (reserved)
then the S flag is undefined.
where Ai and Bi means bit i in accumulator A or B.
Interrupt Masks (Bits 8 and 9)
The interrupt mask bits, I1 and I0, reflect the current IPL of the processor and indicate
the IPL needed for an interrupt source to interrupt the processor. The current IPL of the
processor can be changed under software control. The interrupt mask bits are set during
hardware reset but not during software reset.
Scaling Mode (Bits 10 and 11)
The scaling mode bits, S1 and S0, specify the scaling to be performed in the data ALU
shifter/limiter, and also specify the rounding position in the data ALU multiply-accumula-
5 - 12
PROGRAM CONTROL UNIT