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Hans-Peter Diettrich wrote on Tue, 07 Feb 2012 09:12:53 +0100
> Base / Displacement consists of a 16 bit value, the first 4 bits enumerate a
> register, and the other 12 bits hold a displacement from 0 to 4095. The actual
> or Effective address for each storage operand is calculated as the unsigned
> addition of the value held in the base register to the displacement from the
> instruction itself.
This reminds me of 8086 "real" mode, and 16 bit mode with (far)
addresses splitted into segment:offset. Using an 16 bit compiler and
syntax (near/far pointers and seg:off notation) would be easier to
support a segmented-memory architecture. I also don't see right now how
data structures > 4KB could be addressed, without subroutines to map
virtual into physical (here seg:off) addresses.
There are certain similarities, but System 360 onwards are true 32-bit processors. It's compilers, and there must be hunderds of them by now are 32-bit compilers. (Probably 64 bit too, I have never worked on the 64 bit types, so I don't know)
Data structures -> see episode 5!
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When I had a first look at the 8086 CPU, decades ago, I didn't
understand how to handle such a memory architecture in an compiler. Only
when I came across the Microsoft compilers and their memory models, I
saw how "hints" in the source code would allow an compiler to create
efficient machine code. But all these syntax externsions have been
stripped from nowadays compilers and languages, so that it will be very
hard to support such a dinosaur architecture in an 32/64 bit compiler.
These systems have had had 32 bit compilers since the mid 1960's, it's not rocket science, just computer science. A version of C was implemented on 360 machines at Bell Labs long before it became a portable system. The architecture produces problems, but these are surmountable. I won't have answers to everything; I don't know enough (actually hardly anything) about the internals of the FreePascal code generator. I am confident enough in my abilities to know that these items are soluble; After all VS/Pascal solved them; GCC solved them (and there is a version of GCC available for 370 processors) etc
