THE MODE BIT — a dart

THE MODE BIT one bit chooses the machine’s width — the pattern under the whole lineage

Strip the histories away and one mechanism recurs across every chip in this batch: a mode bit (or a small mode field) that selects the machine’s width and capabilities. The 65816’s E and M/X; the 286’s PE; x86-64’s LMA — each is one flag that folds a wider machine behind the old one, reachable and reversible without a new instruction set. It is what makes a permutation a permutation rather than a rewrite: the past stays underneath, selected by a bit. This dart names the pattern the whole batch is made of.

THE TECHNIQUE a mode field selects width — 8/16/32-bit by one value

The demo uses a mode value to select the width mask — 8, 16, or 32 bits — the mechanism the whole lineage shares: live demo

HISTORY & CREDIT the mode bit · E/PE/LMA

“Each of these chips solved widening differently.” — they share one mechanism: a mode bit that selects the width. cited

the mechanism · a mode bit/field selects width — E, M/X, PE, LMA.
the property · the wider machine folds behind the old one, reachable and reversible by one flag.
the meaning · this is what makes a widening a permutation, not a rewrite — the past stays underneath.

One bit choosing how wide the machine is — the single mechanism the whole lineage is built from. mode

RECOMMEND FOR I-13 the width select, on the compiler

On the canonical compiler, a mode value selects the width mask — 0xFF, 0xFFFF, or 0xFFFFFFFF:

$ i13 run ln_modebit.i13 # mode selects width RUN OK · 54 step(s) · peak stack 3 · call depth 1 m0 = 255 -- mode 0: 8-bit m1 = 65535 -- mode 1: 16-bit m2 = 4294967295 -- mode 2: 32-bit one_bit_selects = 1

Recommend as the batch’s truest keeper shot — then NULL. The mode bit is the abstraction of the whole lineage (and generalizes the 65816’s XCE, dart 455). But selecting a width by a flag is configuration over a representation (B44): the mode chooses which fixed-width function runs, not an extra channel two same-function mechanisms carry differently. And a toggle bit is B43/B44 territory, not a new axis. NULL — the pattern is real and unifying, but it is representation-selection, not a sixth pillar.