The USSC90, the index-registers.

                                         Programmers manual.

The index-registers.

The three index-registers were meant to enhance the programming of loops. Clear appointments had to be respected to the indexregisters, else different program-parts would interfere, using the same index register. A card-reading routine needed two indexregisters, one as a band-pointer for buffer-tranfers and one as memory-pointer. For the main-program there remained one indexregister. Our USSC was equipped with three index registers: IR1, IR2 and IR3. These indexregisters were optional. For indexing operand-addresses the programmer had to write :
- 04 bit set in the instruction-code = to make use of IR1, - a negative sign before the instruction = to make use of IR2, - both, negative sign and 04 = to make use of IR3. On execution the content of an index-register was added to the operand address. When in such addition a carry occurred from a 100 addition, it was ignored: 0187+0015=0002. This caused that indexed operands were always addressed within the same band. All instructions with an operand-address or a band-number in M could be indexed. Instruction 02 loaded an index register with (M). To load an indexregister with 500, one had to program: +0Y 0500 1123 to load 500 into IR1, - 02 0500 1123 to load 500 into IR2, - 0Y 0500 1123 to load 500 into IR3. ( Y stands for the bit-pattern (2 AND 4) and 1123 was the address of the next instruction.) Instruction 07 incremented the content of an index register with (M). To add 12 to an index-register one had to program: +0y 0012 0590 to add 12 to (IR1), ( y stands for (7 AND 4) ). - 07 0012 0590 to add 12 to (IR2), - 0y 0012 0590 to add 12 to (IR3). Instruction 07 also placed the sum in M of rA for end-of-loop comparisons. To decrement the value in an indexregister an addition with the numbers complement of 10000 or 200 was made. +0y 9997 0590 subtract three from the indexregister IR1 +0y 0197 0590 did the same.

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