Hand-wired core for ROM?

[Eric Smith]

On Fri, Apr 10, 2015 at 10:08 AM, Johnny Billquist <bqt at update.uu.se> wrote:
> Hand threaded or not, core is not rom...

Wrong.  There does exist core ROM. It works differently than "normal"
core memory, and doesn't require the hysteresis loop used by normal
core, nor does it need to flip the magnetization of the cores. Instead
it uses each core as a transformer.  It is called "core rope" memory
or "wire braid" memory, and is closely related to the IBM TROS
(Transformer Read Only Store) used as control store in some System/360
CPUs and peripheral control units. It was used in the Apollo Guidance
computer, the DEC PDP-9 control store, some PDP-16 configurations, and
the HP 9100 desk calculator. It was probably used in many other
machines of that era also.

Each "word" of core ROM has one drive line.  That line is threaded
through some of the cores and around others. The core acts as a
transformer, so a pulse on the drive line is coupled to the sense
lines of the cores that the drive line is threaded through, but is not
coupled to the sense lines of the cores that the drive line bypasses.
Typically about 64 drive lines are used for one set of cores; beyond
that another set of cores would be used. That makes it 64 times as
dense, in terms of bits per core, as normal read-write core memory.

The HP 9100 has two levels of control store. The top level is a 32Kbit
inductively coupled PCB, which is amazing in its own right. The lower
level store is a core rope memory of 64 words of 29 bits, so there are
64 drive lines each threaded through or around 29 cores, with 29 sense
amplifiers.

To change the contents of the core rope memory, you have to rewire it.