When DEC chose an implementation language, they knew about C but it had
not yet escaped from Bell Labs. PL/I was considered, but there were
questions of whether or not it would be suitable for a minicomputer. On
the other hand, by choosing BLISS, DEC could start with the BLISS-11
cross compiler running on the PDP-10, which is described in
https://en.wikipedia.org/wiki/The_Design_of_an_Optimizing_Compiler
BLISS-11 and DEC's Common BLISS had changes necessitated by different
word lengths and architectures, including different routine linkages
such as INTERRUPT, access to machine-specific operations such as INSQTI,
and multiple-precision floating point operations using builtin functions
which used the addresses of data instead of the values.
In order to port VMS to new architectures, DEC/HP/VSI retargeted and
ported the BLISS compilers to new architectures.
There have in general been two approaches to achieving language portability (machine independence).
One of them is to provide only abstract data types and operations on them and to completely hide the machine implementation. PL/I and especially Ada use this approach.
BLISS does the exact opposite. It takes the least common denominator. All machine architectures have machine words and ways to pick them apart. BLISS has only one data type--the word. It provides a few simple arithmetic and logical operations and also syntax for operating on contiguous sets of bits within a word. More complicated things such as floating point are done by what look like routine calls but are actually implemented in the compiler.
BLISS is also a true, full-blown expression language. Statement constructs such as if/then/else have a value and can be used in expressions. In C terminology, everything in BLISS is a lvalue. A semicolon terminates an expression and throws its value away.
BLISS is also unusual in that it has an explicit fetch operator, the dot (.). The assignment expression (=) has the semantics "evaluate the expression to the right of the equal sign and then store that value in the location specified by the expression to the left of the equal sign".
Supposing that a and b are identifiers for memory locations, the expression:
a = b;
means "place b (the address of a memory location) at the location given by a (also a memory location)". This is the equivalent of:
a = &b;
in C. To get C's version of "a = b;" in BLISS you need an explicit fetch operator:
a = .b;
Forgetting to use the fetch operator is probably the most frequent error made by new BLISS programmers familiar with more conventional languages.
DEC used four dialects of BLISS as their primary software development language: BLISS-16, BLISS-32, BLISS-36, and BLISS-64 the numbers indicating the BLISS word size in bits. BLISS-16 targeted the PDP-11 and BLISS-36 the PDP-10. DEC did implementations of BLISS-32 for VAX, MIPS, and x86. BLISS-64 was targeted to both Alpha and Itanium. VSI may have a version of BLISS-64 that generates x86-64 code.
-Paul W.