Acorn's BBC Micro series is well known for the range of add-on processors (or Second Processors) that could be connected through it's Tube interface. These included the 6502, Z80, 80186, and also the ARM Second Processor, effectively a development kit for the processor in Acorn's forthcoming Archimedes machines.
The Archimedes' successor, the RISC PC, was designed to support guest processors inside the main unit. Commerically available options included an x86-based PC card (to run PC-compatible software), and a second ARM processor for use with a multiprocessor operating system.
The Archimedes itself, however, saw very little in the way of add-on processors. The ARM2 processor was designed with a co-processor interface, and the podule expansion bus reserved a set of pins for use by a co-processor podule. The original podule specification notes that:
Coprocessor Podules share the same interface as other Podules, but they are not mapped into the I/O space as are other Podules. In addition to the normal Podule signals, coprocessors require access to the main system data bus, and extra control signals. These are provided by a special Coprocessor Podule connector, which has 96 pins. When a coprocessor is not required, this connector may be used as a normal Podule slot. Note that in this context the term `coprocessor' refers to a dedicated hardware processor, and not an additional general purpose microprocessor system.
I interpret this last sentence to mean they only supported floating-point coprocessors, similar to the x87 chips in PC-compatible systems. The only co-processor podule I'm aware of is Acorn's own (AKA20) We32206 co-processor. Later Archimedes machines (in particular the A series) had fewer podule slots, and the addition of a co-processor would require the use of a custom expansion board in the CPU's socket. Both of these are floating-point co-processors.
Given that Acorn designed both the ARM processor and the Archimedes platform, what reasons would Acorn have to remove the support for general-purpose co-processors in their Archimedes systems, given that its predecessors and successors did support them?