pdp11-as

The PDP-11 assembler — a C reimplementation of the 2BSD as (which is itself written in PDP-11 assembly), historically parameterized from First Edition UNIX (1972) to 2.11BSD.

pdp11-as [-o out.o] [-u] [-V] [-n] [-j] [-7]
         [--isa=v1|v4|bsd211|extended] [--sys=none|v1|v6]
         [--aout=v1|v2|v2+] [--std=v1,...,v7,bsd,newbsd,extended] file

• Three orthogonal era axes. --isa selects which instructions exist, --sys which syscall keywords, --aout the object format — from the First Edition 12-byte header with bit-stream relocation, through classic 16-byte 0407, to 2.11's string-table symbols (--aout=v2+, 32-char names). --std tokens are presets over the axes; see docs/std.md.
• Corpus-proven authenticity. A dozen fixes were found by sweeping entire vintage source trees and byte-comparing against the native assembler under apsim: .. relocation, .if-false interning, span off-by-one, 8-bit sys trap codes, and more.
• -V is ovas. Overlay assembly (undefined non-.globl text refs left for ld to resolve into thunks) is a mode of this one binary, as it was in 2.9's MENLO toolchain.
• Self-hosting, to the byte: assembling 2.9's own as0.s/as2.s reproduces the native /bin/as and /lib/as2 exactly (oracle/selfhost.sh).

Documentation

• Design — the token pipeline, the two-pass symbol/segment model, and the a.out + relocation emitter.
• User guide — invocation, the ISA/sys dialect flags, and per-universe examples.
• docs/std.md — the era-axis user guide.
• docs/porting.md — design and porting notes.

Build and test

make          # builds ../../bin/pdp11-as
make check    # covered by tests/as/encode.sh at the repo root, the
              # apsim CIS golden, and the as-corpus/tree-sweep oracles