On Thu, Oct 3, 2019 at 12:04 PM Guy Sotomayor Jr via cctalk < cctalk@classiccmp.org> wrote:
> So does the PDP-11. The 8 registers are mapped to the top 8 words of > memory so you can do some quite interesting things. It is also possible to > run a (small) program in only the registers (e.g. no memory at all). > That's not an actual PDP-11 architectural feature, and it only works on the KD-11B CPU (PDP-11/05 and 11/10). No other models can execute code from the general purpose registers, or access the GPRs via a memory address. When an instruction word is fetched from a GPR, the PC is only incremented by one, because the 16-bit registers are at consecutive addresses, rather than multiples of two as one would expect. The GPRs are not byte-addressable. On all other Unibus PDP-11 processors made from TTL (i.e., not based on the F11 or J11), the general-purpose registers can be accessed at the same addresses from the console, but NOT from software. Code can be executed from the MMU PAR registers on processors with 22-bit addressing (11/23, 11/24, 11/44, 11/70, and J-11 based systems). These are word registers at word addresses, so they don't have the increment-by-one hack of the 11/05 executing from GPRs.
