Park the number you subtract; keep the other in the accumulator

Say you've worked out two values, a = x + y and b = 10z, and want to know whether a is below b.

The tidy way is to work out both, store both, then load b back and subtract a.

cell  contents  notes
 10      040    INP 40  cells 10-15: a = x + y, stored
 11      041    INP 41
 12      042    INP 42
 13      140    CLA 40
 14      241    ADD 41
 15      643    STO 43
 16      142    CLA 42  cells 16-18: b = 10z, stored
 17      410    SFT 10
 18      644    STO 44
 19      144    CLA 44  cells 19-26: load b again and test b - a - 1
 20      743    SUB 43
 21      700    SUB 00
 22      325    TAC 25
 23      500    OUT 00  a < b: print 1
 24      900    HRS 00
 25      545    OUT 45
 26      900    HRS 00
 40      000
 41      000
 42      000
 43      000
 44      000
 45      000

Step through the natural way in Cardigan

Here's the thing: CARDIAC can only subtract a cell, so only the number you subtract has to be stored. Store a, work out b, and while b is still sitting in the accumulator, subtract a straight away. For "a < b" you test b - a - 1 against zero, and the 1 comes free from cell 00.

cell  contents  notes
 10      040    INP 40  cells 10-15: a = x + y, stored, because it is the one subtracted
 11      041    INP 41
 12      042    INP 42
 13      140    CLA 40
 14      241    ADD 41
 15      643    STO 43
 16      142    CLA 42  cells 16-24: b = 10z stays in the accumulator; test b - a - 1
 17      410    SFT 10
 18      743    SUB 43
 19      700    SUB 00
 20      323    TAC 23
 21      500    OUT 00  a < b: print 1
 22      900    HRS 00
 23      544    OUT 44
 24      900    HRS 00
 40      000
 41      000
 42      000
 43      000
 44      000

Step through the CARDIAC way in Cardigan

Both compare 19 with 20 and print 1. Storing both takes 23 cells and runs 15 instructions; parking only a takes 20 cells and runs 13.

From the tips in cardiac-code on GitHub. All the tips.