F.I.G. Forth

* FIG FORTH FOR SERIES-16 MACHINES PAGE 11

0434 * THE CARRY OUT OF THE 15-BIT ADDITION IS AVAILABLE IN THE TOP BIT 0435 * OF THE RESULT. HOWEVER, IT'S NOT DIRECTLY APPLICABLE TO 0436 * PERFORMING 32-BIT ADDITION AND SUBTRACTION. 0437 * 0438 * THE APPROACH TAKEN IS TO NOTE THAT: 0439 * 0440 * IF A, B ARE INPUTS TO THE TOP BIT OF ADDITION AND S IS THE TOP 0441 * BIT FROM THE SUM, THEN THE CARRY INTO THE TOP BIT CIN=(A^B^S) 0442 * SINCE OVERFLOW, V=CIN^COUT, IT FOLLOWS THAT COUT=V^CIN 0443 * SO CARRY CALCULATED AS (A^B^S^V) 0444 * 0445 * CIN A B | S V |COUT 0446 * 0 0 0 | 0 0 | 0 0447 * 0 0 1 | 1 0 | 0 0448 * 0 1 0 | 1 0 | 0 0449 * 0 1 1 | 0 1 | 1 0450 * 1 0 0 | 1 1 | 0 0451 * 1 0 1 | 0 0 | 1 0452 * 1 1 0 | 0 0 | 1 0453 * 1 1 1 | 1 0 | 1 0454 * 0455 * THE SAME PROCEDURE WILL ALSO YIELD BORROW FROM A 16-BIT 0456 * SUBTRACTION. 0457 * 0458 * WHERE THE HIGH SPEED ARITHMETIC (HSA) OPTION IS AVAILABLE 0459 * IT IS USED TO IMPROVE SPEED OF THE MULTIPLY AND DIVIDE 0460 * OPERATIONS. HOWEVER, THERE IS INEVITABLY SOME MUCKING 0461 * AROUND TO CONVERT BETWEEN FORTH'S 32-BIT FORMAT AND THE 0462 * NATURAL SERIES-16 31-BIT FORMAT, EVEN FOR NUMBERS THAT 0463 * FIT WITHIN THE 31-BIT RANGE. 0464 * THE HSA ONLY PROVIDES SIGNED OPERATIONS, BUT UNSIGNED 0465 * MULTIPLY CAN STILL BE ACCELERATED USING THE SIGNED MULTIPLY 0466 * OPERATION. NO SATISFACTORY WAY TO USE THE SIGNED DIVIDE 0467 * OPERATION COULD BE FOUND TO ACCELERATE THE UNSIGNED DIVIDE. 0468 * 0469 EJCT