pdp11-xdevalpha

Documentation

apsim: syscall parity

A category-by-category comparison of pdp11-apsim's system-call coverage against the reference vax11-apsim, and the calls both deliberately refuse.

A category-by-category comparison of what pdp11-apsim implements against the reference vax11-apsim. The two simulators target different machine lines, so "parity" means: everything the VAX apsim does that is applicable to the PDP-11 UNIX era set (V5 → 2.11BSD) is done here, and the calls both refuse are refused for the same reasons. The VAX apsim's extra machinery is, with one exception, for ABIs that never existed on the PDP-11 (NetBSD, OpenBSD, 4.4, SysV threads).

Measured coverage (this tree): every syscall in every PDP-11 era's own table is answered — gaps.py reports zero unanswered for v5, v6, v7, bsd28, bsd29; bsd210 and bsd211 have only deliberate refusals. Empirically, sweep.py runs 47/47 of 2.11 /bin and 41/41 of v6 /bin with no unimplemented syscall.

Category by category

category vax11-apsim pdp11-apsim notes
File I/O (open/read/write/close/lseek/dup/dup2/fcntl/access/ioctl) full full plus directory read(2) served from an era-format snapshot
File metadata (stat/fstat/lstat, chmod/chown/fchmod/fchdir, link/unlink/symlink/readlink, rename, mkdir/rmdir, truncate/ftruncate, umask, sync/fsync, utimes) full full per-era struct stat shapes (V6 36-byte, V7 30-byte, 2.10/2.11 52-byte)
Process (fork/vfork/exec/execve/exit/wait/wait4, getpid/getppid, kill/killpg, nice, alarm/pause) full (real host fork) full (real host fork) guest pid ↔ host pid map
Signals (signal/sigvec/sigaction/sigreturn, sigblock/sigsetmask/sigsuspend) full 4.3 sigtramp frame full 4.3 sigtramp frame 32-bit masks; the frame sendsig leaves; drives real csh job control
Job control (setpgrp/getpgrp, TIOCSPGRP/GPGRP, stop signals, wait WUNTRACED) full full real host process groups + tcsetpgrp
IDs (getuid/geteuid/getgid/getegid, setuid/setgid, setre[ug]id, getgroups, getlogin) full full setre[ug]id accepted (identity model), as on the VAX
Time (gettimeofday, time, ftime, times) full full $APSIM_TIME deterministic clock
Sockets (socket/socketpair/bind/connect/listen/accept, send/recv/sendto/recvfrom, sendmsg/recvmsg, get/setsockopt, getsockname/getpeername, shutdown) full, real host sockets full, real host sockets verified AF_UNIX socketpair + AF_INET TCP loopback
Filesystem stats (statfs/fstatfs/getfsstat) full (host statvfs) full (host statvfs) 232-byte 2.11 struct; df works
select/poll select + poll select 2.11 has no poll (a 4.3+ call)
ptrace (debug channel) cooperative channel (drives gdb) cooperative channel (drives real adb) opt-in $APSIM_PTRACE; PT_READ/WRITE_I/D/U, CONTINUE, STEP, KILL over an abstract socket; wait reports synthetic WIFSTOPPED; the u-page's saved registers are synthesized so adb's $r reads them
sbrk/brk full full flat 64 KB space
__sysctl rich node table + /proc-scan ps common nodes (CTL_KERN/CTL_HW strings, hostname) 2.11's sysctl is far smaller than 4.4's; the common facts are answered
Overlays (auto-overlay text) n/a (VAX has no overlays) 0430 + 0431 (2.10/2.11 15-overlay, separate-I&D window) how csh fits 64 KB
Floating point bit-exact VAX F/D/G/H softfloat bit-exact FP11 D/F softfloat + FIS no host FP on either side
Interpreter scripts (#!) (gap on the VAX) implemented one interpreter + one arg, plus the command-line /bin/sh courtesy

Calls both simulators deliberately refuse

pdp11-apsim's entire refusal set (bsd210/bsd211) falls in the same categories the VAX apsim refuses:

  • Privileged system administration — adjtime, settimeofday, quota/qquota, mount/umount, reboot, chroot, acct: a user-mode simulator has no authority to honor them. (The VAX returns EPERM; pdp11-apsim currently returns ENOSYS for the same set.)
  • Kernel-internal pokes — fetchi (fetch from user I-space), ucall (call a kernel subroutine): no user-mode equivalent, exactly like the VAX's nfssvc/getkerninfo/audgen refusals.

(sigwait — which the VAX apsim refuses — is instead implemented here: it blocks for a signal in the given set and returns it.)

The VAX features that don't apply to the PDP-11 line

The VAX apsim's remaining headline extras are, by category, inapplicable to the PDP-11 UNIX era set:

  • LWP threads on real pthreads behind a GIL — no PDP-11 UNIX had kernel threads. Not applicable.
  • kqueue/kevent — a 4.4BSD/NetBSD facility; absent from 2.11. N/a.
  • mmap (anonymous + file-backed) and the *at family — 4.3+/POSIX; 2.11 has neither. N/a.
  • Multi-personality NetBSD/OpenBSD/SVR2 remap tables with shaped struct marshallers — those ABIs don't run on a PDP-11.

The one applicable feature that was the outstanding parity item — the cooperative ptrace debug channel — is now implemented (opt-in via $APSIM_PTRACE). A traced guest, instead of host-stopping, parks inside apsim serving an abstract AF_UNIX socket; the tracer's ptrace ops and its wait() (which reports a synthetic WIFSTOPPED) go over that channel, so one apsim process can read/write and single-step another across their separate address spaces. This runs 2.11's classic ptrace(2) (PT_READ/WRITE_I/D/U, CONTINUE, STEP, KILL) — the adb model. Verified two ways: tests/ptrace.s (a parent reads a word out of a traced child's D-space, single-steps it, continues it to exit), and the genuine 2.11 adb under apsim (tests/run.sh 211-adb) — set a breakpoint, run /bin/echo under ptrace, hit the breakpoint, read the registers with the PC exactly at it, single-step, continue to exit, and produce a $c stack backtrace (walking the r5 frame chain back to crt0 -- r5 read via PT_READ_U, the frame/return-pc pairs via PT_READ_D). For adb's registers to read right, apsim synthesizes the saved-register block at the top of the emulated u-page (ctob(USIZE)=3968; R0 at byte 3962, the rest at their reg.h word offsets) so PT_READ_U lands where adb's uar0[regloc[i]] expects.

Summary

For the universe range this project targets (First Edition through 2.11BSD), pdp11-apsim is at syscall parity with vax11-apsim: the same file, process, signal, job-control, socket, filesystem-stat, and time coverage; the same set of deliberate refusals; the cooperative ptrace debug channel; plus PDP-11-specific work the VAX never needed (auto-overlays, the FP11/FIS softfloat, the era struct stat and directory shapes). Every syscall in every PDP-11 era's own table is answered.