Clem Cole asks:

>> Did you know that before PCC the 'second' C compiler was a PDP-10
>> target Alan Snyder did for his MIT Thesis?
>> [https://github.com/PDP-10/Snyder-C-compiler]

I was unaware of that compiler until sometime in the 21st Century,
long after our PDP-10 was retired on 31-Oct-1990. 

The site

        https://github.com/PDP-10/Snyder-C-compiler/tree/master/tops20

supplies a list of some of Snyder's files, but they don't match
anything in our TOPS-20 archives of almost 180,000 files.

I then looked into our 1980s-vintage pcc source tree and compared
it with a snapshot of the current pcc source code taken three
weeks ago.  The latter has support for these architectures

        aarch64  hppa  m16c  mips64  pdp11    sparc64
        amd64    i386  m68k  nova    pdp7     superh
        arm      i86   mips  pdp10   powerpc  vax

and the pdp10 directory contains these files:

        CVS  README  code.c  local.c  local2.c  macdefs.h  order.c  table.c

All 5 of those *.c files are present in our TOPS-20 archives.  I then
grepped those archives for familiar strings:

        % find . -name '*.[ch]' | sort | \
               xargs egrep -n -i 'scj|feldman|johnson|snyder|bell|at[&]t|mit|m.i.t.'
        ./code.c:8: * Based on Steve Johnson's pdp-11 version
        ./code2.c:19: * Based on Steve Johnson's pdp-11 version
        ./cpp.c:1678:           stsym("TOPS20");        /* for compatibility with Snyder */
        ./local.c:4: * Based on Steve Johnson's pdp-11 version
        ./local2.c:4: * Based on Steve Johnson's pdp-11 version
        ./local2.c:209:         case 'A':               /* emit a label */
        ./match.c:2: * match.c - based on Steve Johnson's pdp11 version
        ./optim.c:318:                                           * Turn 'em into regular PCONV's
        ./order.c:5: * Based on Steve Johnson's pdp-11 version
        ./pftn.c:967:                    * fill out previous word, to permit pointer
        ./pftn.c:1458:  register        commflag = 0;  /* flag for labelled common declarations */
        ./pftn2.c:1011:                  * fill out previous word, to permit pointer
        ./pftn2.c:1502: register        commflag = 0;  /* flag for labelled common declarations */
        ./reader.c:632:         p2->op = NOASG p2->op;     /* this was omitted in 11 & /6 !! */
        ./table.c:128:          "       movei   A1,1\nZN",      /* ZN = emit branch */
        ./xdefs.c:13: * symbol table maintainence

Thus, I'm confident that Jay's work was based on Steve Johnson's
compiler, rather than Alan Snyder's.

Norman Wilson asks:

>> ...
>> How did that C implementation handle ASCII text on the DEC-10?
>> Were it a from-scratch UNIX port it might make sense to store
>> four eight- or nine-bit bytes to a word, but if (as I sense it
>> was) it was C running on TOPS-10 or TOPS-20, it would have had
>> to work comfortably with DEC's convention of five 7-bit characters
>> (plus a spare bit used by some programs as a flag).
>> ...

Our pcc compiler treated char* as a pointer to 7-bit ASCII strings,
stored in the top 35 bits of a word, with the low-order bit normally
zero; a 1-bit there meant that the word contained a 5-digit line
number that some compilers and editors would report.  Of course, that
low-order non-character bit meant that memset(), memcpy(), and
memmove() had somewhat dicey semantics, but I no longer recall their
specs.

kcc later gave us access to the PDP-10's 1- to 36-bit byte
instructions.

For text processing, 5 x 7b + 1b bits matched the conventions for all
other programming languages on the PDP-10.  When it came time to
implement NFS, and exchange files and data with 32-bit-word machines,
we needed the ability to handle files of 4 x 8b + 4b and 9 x 8b (in
two 36-bit words), and kcc provided that.

The one's-complement 36-bit Univac 1108 machines chose instead to
store text in a 4 x 9b format, because that architecture had
quarter-word load/store instructions, but not the general variable
byte instructions of the PDP-10.  Our campus had an 1108 at the
University of Utah Computer Center, but I chose to avoid it, because
it was run in batch mode with punched cards, and never got networking.
By contrast, our TOPS-20, BSD, RSX-11, SunOS, and VMS systems all had
interactive serial-line terminals, and there was no punched card
support at all.

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