4 Apr
1999
4 Apr
'99
6:18 p.m.
For example, if you
pulled the ROMs on the 11/45 and reprogrammed them, you'd not be able to
make the machine into a P850 (for all they are both 16 bit machines, and
both have 16 machine registers). The PDP11 instruction set uses some
3-bit fields to select registers, and those bitfields are connected (via
logic) to the register select lines in the CPU circuity. The microcode
can't change that.
Actually, the microarchitecture of the 11/45 is Turing-complete, so you
could in fact make it run P850 code. It just wouldn't be particularly good
at it.
Just two weeks ago while researching something else, I found an old product
announcement for a third-party writable control store option for the 11/45.
However, in order to make this option more usable, they also added several
new data paths to the 11/45.