--
-- based on Kilburn's Highest Factor Routine
-- https://www.computerhistory.org/revolution/birth-of-the-computer/4/87/360
--
01: LDN 24      -- \
02: SUB 22      --  | initialization:
03: STO 27      --  | $27 - initial subtraction step (a - 1)
04: LDN 27      --  | $26 - supplementary value -$27
05: STO 26      -- /
06: LDN 24      -- \
07: SUB 27      --  | subtraction loop
08: CMP         --  | subtract $27 from `a` until negative
09: JRP 21      -- /
10: SUB 26      -- \
11: STO 25      --  | compare negative rest with -$27
12: LDN 25      --  | stop if equal, then $27 is the answer
13: CMP         -- /
14: STOP        -- stops when C=0, $27 (or $26) is the result
15: LDN 26      -- \
16: SUB 22      --  \
17: STO 27      --   | decrement $27
18: LDN 27      --   | jump to division by subtraction
19: STO 26      --  /
20: JMP 23      -- /
21: NUM -3      -- subtraction loop iteration jump
22: NUM 1       -- probe value decrement
23: NUM 5       -- subtraction loop absolute line number (preceeding one)
24: NUM -262144 -- -a ; a=2^18 -- value to factor in negated form
-- $25 register to negate result after subtraction loop to compare with iteration step
-- $26 supplementary value -$27
-- $27 iteration step