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Annotation of researchv10no/cmd/f77/alt/README, revision 1.1

1.1     ! root        1: this is a bit of a horror.
        !             2: 
        !             3: the stab interface (to sdb and pi) has been hacked mercilessly until it works.
        !             4: it's all pretty fragile, and probably somewhat buggy.  things to beware
        !             5: especially:
        !             6: 
        !             7: f77 is a pcc1 compiler.  c is a pcc2 compiler these days.  over the years,
        !             8: c has gained types, sort of; the pcc internal representation of types (which
        !             9: is put out in the type field of the stab) has changed too.  (and the printed
        !            10: V8 manuals get it wrong, by the way; the online manual is correct.)  f77,
        !            11: being an old compiler, uses the old representation; it has to convert it
        !            12: to the new one when it prints stabs.
        !            13: 
        !            14: pcc1 used to put out symbols at the top of each block, followed by the
        !            15: line number markers (N_SLINEs).  f77 used some horrible hackery to make
        !            16: that happen, as it didn't know enough about symbols until the end (think
        !            17: about implicit declarations).  pcc2 puts symbols after N_SLINEs.  sdb now
        !            18: wants things that way too.  f77 has followed along.
        !            19: 
        !            20: common and equivalence are nightmares.  one would like to simply spit out
        !            21: a stab for each piece of the common, pointing at the address of that piece;
        !            22: unfortunately, offsets to commons don't work in the symbol table (it
        !            23: requires the relocation info to make it work, and there's no relocation
        !            24: data for symbols, logically enough).  hence these monsters are put out as
        !            25: crypto-structures, in the following format:
        !            26:        .stabs  "cname",N_BCOMM,0,0,0
        !            27:        .stabs  "element",N_SSYM,0,type,offset
        !            28:        ...
        !            29:        .stabs  "cname",N_ECOMM,0,0,0
        !            30: when a debugger wants the address of `element', having noted that it is
        !            31: a N_SSYM within the BCOMM/ECOMM (or BCOMM/ECOML, see below) brackets, it
        !            32: should pick up `cname' from the ECOMM, prepend an `_' to make an external
        !            33: name of it, look that name up in the symbol table to find the address of
        !            34: the common region, and add the offset.
        !            35: 
        !            36: sometimes the endmarker isn't an ECOML, but is
        !            37:        .stabs  "",N_ECOML,0,0,addr
        !            38: e.g. when you've equivalenced two non-common variables together.  this
        !            39: means `element's address is addr+offset.
        !            40: 
        !            41: enough.  sdb works, sort of.  pi works, but not on commons nor equivalences.
        !            42: (tough.)
        !            43: 
        !            44: it is probably impossible to make a pdp11 compiler without lots of work.

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