Index registers may be used in conjunction with indirect addressing. In this case, the address after being modified by the selected index register is used to locate the indirect address.
The indirect address can be acted on by an index register and deferred again if desired. Each use of an index register or an indirect address extends the operating time of the original instruction by 5 microseconds.
This list includes the title of the instruction, the normal execution time of the instruction, i.e., the time with no indexing and no deferring, the mnemonic code of the instruction, and the operation code number. The notation used requires the following definitions. The contents of a register Q are indicated as C(Q). The address portion of the instruction is indicated by Y. The index register address of an instruction is indicated by x. The effective address of an operand is indicated by Z. Z may be equal to Y or it may be Y as modified by deferring or by indexing.
_Add_ (10 usec.) add x Y Operation Code 40
The new C(AC) are the sum of C(Z) and the original C(AC). The C(Z) are unchanged. The addition is performed with 1's complement arithmetic.
If the sum exceeds the capacity of the Accumulator Register, the overflow flip-flop will be set (see Skip Group instructions).
_Subtract_ (10 usec.) sub x Y Operation Code 42
The new C(AC) are the original C(AC) minus the C(Z). The C(Z) are unchanged. The subtraction is performed using 1's complement arithmetic.
If the difference exceeds the capacity of the Accumulator, the overflow flip-flop will be set (see Skip Group instructions).
_Multiply_ (approximately 25 usec.) mul x Y Operation Code 54
The C(AC) are multiplied by the C(Z). The most significant digits of the product are left in the Accumulator and the least significant digits in the In-Out Register. The previous C(AC) are lost.
_Divide_ (approximately 90 usec.) div x Y Operation Code 56
The Accumulator and the In-Out Register together form a 70 bit dividend. The high order part of the dividend is in the Accumulator. The low order part of the dividend is in the In-Out Register. The divisor is (Z).
Upon completion of the division, the quotient is in the In-Out Register. The remainder is in the Accumulator. The sign of the remainder is the same as the sign of the dividend. If the dividend is larger than C(Z), the overflow flip-flop will be set and the division will not take place.