CARDIAC › Tips and tricks › Instructions are numbers

SFT 22 reads the address out of an instruction

Say you want to walk through a table in cells 40 to 43 and print each entry.

CARDIAC has no indirect addressing, so walking a table means rewriting the address in one of your own instructions. The usual way also keeps a counter, builds the instruction from it each time round, and checks the counter against the end.

cell  contents  notes
 10      130    CLA 30  cells 10-13: build OUT 40 + i from the counter
 11      232    ADD 32
 12      613    STO 13
 13      500    OUT 00  OUT 40 + i, written by the cell before
 14      130    CLA 30  cells 14-19: count, and stop at 4
 15      200    ADD 00
 16      630    STO 30
 17      731    SUB 31
 18      310    TAC 10
 19      900    HRS 00
 30      000    the counter, kept in step with the instruction
 31      004
 32      540    OUT 40
 40      007
 41      003
 42      009
 43      001

Step through the natural way in Cardigan

But the instruction already knows where it's pointing, so let it be the pointer. Add 1 to the instruction itself, and when you want to know where it's got to, load it and do SFT 22, which drops the opcode and leaves the address. OUT 41 is 541; shift 0541 two places left and two right and you've got 0041. There's no counter to keep in step.

cell  contents  notes
 10      540    OUT 40  cells 10-17: print cells 40-43; cell 10 is the pointer
 11      110    CLA 10
 12      200    ADD 00
 13      610    STO 10
 14      422    SFT 22  drop the opcode: just the address
 15      730    SUB 30
 16      310    TAC 10
 17      900    HRS 00
 30      044    one past the end
 40      007
 41      003
 42      009
 43      001

Step through the CARDIAC way in Cardigan

Both print 7, 3, 9 and 1. Keeping a counter takes 17 cells and runs 37 instructions; using the instruction as the pointer takes 13 cells and runs 29.

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