QuoteThese days I'd recommend using one of the following
TI's MSP430 is about as close to a PDP-11 as you can get for less than $1, and I think it still qualifies as a "current" architecture.It looks like assembly would be pretty nice on it (and potentially useful, since they sell some Very Tiny (512B) chips), and you can get those LaunchPad boards for under $15, some with FRAM (which is also "interesting technology")
http://www.oocities.org/westfw/trip-report-msp430.txt (quite old; predates cheap, predates launchpad, predates FRAM)
It's definitely decent. Twice the registers of the PDP11 (which was always a bit short), in exchange for half the addressing modes. It's a bit annoying that the destination can only use half the addressing modes that you *do* have -- and in particular that you can't do register indirect for the destination but have to waste a word on a zero offset.
Super-H (especially SH4) is similarly PDP11-ish, but getting to 16 registers by keeping a full set of addressing modes, but using them only for (one operand of) MOV. All the arithmetic is register-to-register, RISC style. I think I find this preferable.
Both MSP430 and SH4 would be vastly better for learning assembly language than any 8 bit CPU (except AVR), anything ever designed by Intel, Thumb2 or Aarch64, SPARC, PowerPC. Many of those are fine CPUs and no problem at all if you've got a compiler to do the work. But manuals thousands of pages long are not good for assembly langauge programming, especially by beginners.
I shouldn't omit m68000 (forget the 020/040 additions). Also PDP11-ish, but this time getting 16 registers by specialising them into use as pointers or for arithmetic.
With that bit of sometime annoying asymmetry I think I'd put 68k on the same level as MSP430, and SH4 just above both. All are preferable to PDP11 or Thumb1 which are just a little bit too tight on usable registers (but at least Thumb gets PC, SP, LR out of the working registers).
There's quite a lot of good choices really :-)