[Simh] Adding second copy of device to PDP-8

[Bob Supnik]

The way I've usually done it is embodied in the PDP11 RH and RQ 
controllers. Essentially, the code is made "re-entrant", with every 
reference to an absolute variable replaced with pointers or array 
references.

Basically, all the global DECtape variables get collected into a large 
struct, called a context. Then the DEVICE, UNIT, and context structures 
are replicated and given unique names. There's a routine to map from the 
IO address (IOT number) to an index, and some arrays to map from context 
index to context structure and DEVICE structure. (Reset typically puts 
the context index in a user-defined field of every unit.)

Now look at td77. It would start with a call to get the index from the 
device number and the context pointer from the index:

int32 cidx = td_map_dev (IR);
struct td_ctx *cp = td_ctxmap[cidx];

Now every global variable td_xxx becomes cp->td_xxx. The cidx variable 
gets added to every routine that gets called, so that the routine can 
find the proper DEVICE, UNITs, or CONTEXT.

For unit service, the proper context index is retried from the UNIT 
argument.

For reset, a reverse lookup is done to derive a context index by looking 
up the DEVICE structure in the DEVICE map.

You made need to add some additional navigation aids to make "getting 
around" more convenient.

A slightly different approach was taken in the RH and in the i7094 tape 
controllers. There, the DEVICE structures were arrayed, so that the 
context index could directly select a DEVICE structure; and conversely, 
a context index could be derived from a DEVICE pointer by simple pointer 
subtraction. That gets rid of one context index map.

Alternately, you could just clone the code and change every td_ and TD_ 
to tdb_ and TDB_, respectively. You'd get pretty close, with no 
possibility of damaging the existing TD.
/Bob Supnik