Cell 00 is the constant 1

Say you're counting cards until you hit a negative one.

You'd keep a cell holding 1 and add it to the count each time. An instruction names a cell, never a value, so on CARDIAC every constant costs you a cell.

cell  contents  notes
 10      030    INP 30  cells 10-18: count cards until a negative one
 11      130    CLA 30
 12      317    TAC 17
 13      131    CLA 31
 14      232    ADD 32  count + 1, from a cell holding 1
 15      631    STO 31
 16      810    JMP 10
 17      531    OUT 31
 18      900    HRS 00
 30      000
 31      000    the count
 32      001

Step through the natural way in Cardigan

Except one. Cell 00 is wired to hold 001, so there's a 1 sitting there that you never have to supply. ADD 00 and SUB 00 add and take away one for free.

cell  contents  notes
 10      030    INP 30  cells 10-18: count cards until a negative one
 11      130    CLA 30
 12      317    TAC 17
 13      131    CLA 31
 14      200    ADD 00  count + 1, from cell 00
 15      631    STO 31
 16      810    JMP 10
 17      531    OUT 31
 18      900    HRS 00
 30      000
 31      000    the count

Step through the CARDIAC way in Cardigan

Both count three cards. Bringing your own 1 takes 12 cells, and using cell 00 takes 11. They both run 26 instructions, so the saving here is the cell, which on a hundred-cell machine is worth having.

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