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The ND-500 Linkage-Loader (NLL) - what it is, what it makes, and why we need it

Purpose of this page: one place that ties together everything this repo knows about NLL. Every claim below is sourced; nothing here is new research. Written 2026-08-18.

1. What it is

NLL ("ND Linkage-Loader", product ND-210319) is the tool that turns compiler output into runnable ND-500 programs. The vendor manual defines it directly:

"NLL is a subsystem able to convert relocatable object files in ND Relocatable Format (NRF), created by language subsystems, to independent executable programs, or processes." - ND-60.136.04A ND-500 Loader Monitor

On the ND-500 there is no "executable file" the way the ND-100 has :PROG. A program is a domain - an addressing space of up to 32 segments - and a domain is not a file but an entry in a per-user index file, DESCRIPTION-FILE:DESC. NLL is the program that creates and maintains all of that:

  • it reads :NRF files from the compilers (FORTRAN-500, PLANC-500, NC, COBOL-500, ...),
  • it builds each segment as three SINTRAN files: :PSEG (instructions), :DSEG (data) and :LINK (label names, values and debug info),
  • it records the domain and its segments in DESCRIPTION-FILE:DESC,
  • and it resolves references between segments and domains (LINK-SEGMENT, LIBRARY-SEGMENT-LINK, common blocks, entry lists).

NLL itself is an ND-500 program: the domain LINKAGE-LOAD-H02 (in our H02 copy), started from SINTRAN with @ND-500-LINKAGE-LOADER or from the monitor with N500: LINKAGE-LOADER.

2. Why we need it

Every path onto the ND-500 side goes through the domain machinery NLL owns.

  1. Building anything. Compile with any ND-500 compiler and you get an :NRF file - not a program. Only NLL can load that into a domain you can run. The normal loop is three commands (see the operations guide, section 3.0):

    NLL: SET-DOMAIN "MY-DOMAIN"     double quotes = create NEW domain
NLL: LOAD-SEGMENT TESTPROG      load compiler output (:NRF file)
NLL: EXIT                       link libraries, update files, return

  2. Installing vendor products. The ND-500 product floppies in our corpus (COBOL-500, FORTRAN-500, LED, the RG family, NLL itself) ship as :PSEG/:DSEG/:LINK files plus a DESCRIPTION-FILE:DESC - the exact format NLL owns. Getting them onto a system means getting them into the user's description file. On our SINTRAN L image the ND-500 compiler chain could not be installed until NLL was in place - the NLL install was the gating step of the whole ND-5000 bring-up (NLL-INSTALL-ROOT-CAUSE-PLAN-2026-07-30, NLL-INSTALL-SWAPFILE-UNBLOCKED-5SWAP-PROTECT-VIOLATION-2026-07-31).

  3. Debugging. The :LINK file NLL writes is what the symbolic debugger reads to show names instead of addresses. No NLL, no symbols.

A system without NLL is recognizable by its symptoms - documented from a live session in Installing the ND-500 Linkage-Loader and Backup System: N500: LINKAGE-LOADER answers DESCRIPTION FILE ERROR: DESCRIPTION-FILE / NO SUCH FILE NAME (the honest error), while ND-500-LINKAGE-LOADER answers the misleading TOO LONG PARAMETER (21 characters treated as a domain name, limit 16).

3. What it is NOT

  • Not part of base SINTRAN. It is a separate product installed from its own floppy (210319H02-XX-01D in our archive, floppy image ND-disk-00042.img), and it hard-requires the Backup System (210337) to be installed first.
  • Not the same as LINKER-B01. LINKER-B01 (see ND500-APPS/LINKER-B01/userguide.md) is the later linkage editor that works on self-contained :DOM files - the newer domain format that replaced the :PSEG/:DSEG/:LINK trio. CONVERT-DOM converts the old format to :DOM (and does NOT read the DESC size fields - it asks the file system, see desc-format.json nonWitnesses).
  • Not the debug monitor. MON-DEBUG:PROG (nd-500-mon-j04) contains the LOOK-AT/debugger and PLACE machinery but none of the loader commands, and it never opens a :LINK file - verified byte-level 2026-08-17 (CARVE-ANSWER-FOUR-OPEN-QUESTIONS-2026-08-17, Q3).

4. Its files - all decoded in this repo

File Contents Format doc
DESCRIPTION-FILE:DESC Per-user index: domain entries (56 bytes) and segment entries (192 bytes), fully byte-mapped from the monitor's own print code DESCRIPTION-FILE-FORMAT.md, desc-format.json
<segment>:PSEG Instruction segment, raw image; size rule PLB + PSIZE + 1 = file size same docs, section 5
<segment>:DSEG Data segment; file maps from logical address 0, DLB + DSIZE + 1 = file size same docs
<segment>:LINK NLL's loader table dumped at CLOSE-SEGMENT: 32-byte label records in ascending value order; the universal 32k+1 size is the SMAX off-by-one LINK-FILE-FORMAT.md, link-format.json
<file>:NRF Its input - relocatable output of the compilers NRF-FILE-FORMAT.md, nrf-format.json

5. Command surface (H02)

91 commands, enumerated from the binary's own dispatch table (descriptor table at DSEG 0x1368, handler table at 0x4A0C). The families:

  • Domain/segment lifecycle: SET-DOMAIN, OPEN-SEGMENT, APPEND-SEGMENT, CLOSE-SEGMENT, END-DOMAIN, DEFINE-SEGMENT-SIZE, DELETE/CLEAR/RENAME/COPY/RELEASE-DOMAIN, DELETE/CLEAR/RENAME-SEGMENT, LIST-DOMAIN, LIST-SEGMENT.
  • Loading: LOAD-SEGMENT, TOTAL/LIBRARY/OMITTED/SELECTED-SEGMENT-LOAD, RELOAD-SEGMENT, SET-AUTO-LOAD-FILE, RUN.
  • Linking: LINK-SEGMENT, LIBRARY-SEGMENT-LINK, FORCE-SEGMENT-LINK, SET-AUTO-LINK-SEGMENT, COMMON-SEGMENT-OPEN/APPEND/CLOSE/NUMBER, MATCH-RTCOMMON, MATCH-COMMON-RT-SEGMENT, LINK-RT-PROGRAM, DEFINE-COMMON, DEFINE-ENTRY, PROGRAM/DATA-REFERENCE.
  • Symbol control: KILL-ENTRIES, GLOBAL-ENTRIES, VALUE-ENTRIES, SYSTEM-ENTRIES-ON, LIST-ENTRIES-DEFINED/UNDEFINED, LIST-MAP (these decide what survives onto the :LINK file).
  • Memory placement: LOW/HIGH-ADDRESS, SET-SEGMENT-LIMITS, FIX-SEGMENT- SCATTERED/CONTIGUOUS/ABSOLUTE, SET-SEGMENT-NUMBER.
  • Traps: LOCAL/SYSTEM-TRAP-ENABLE/DISABLE, ENTRY-ROUTINES.
  • NRF library maintenance: NEW/DELETE/APPEND/FETCH-NRF-MODULES, LIST-NRF-ENTRIES, LIST-NRF-CODE, WRITE-NRF-EOF-AFTER-MODULE, INSERT-NRF-MESSAGE, PREPARE-NRF-LIBRARY-FILE.
  • Modes/misc: COMPUTER-MODE (100/500/pioc - NLL also loads ND-100 and PIOC targets), LIST-MODE, CHECK-SYNTAX-MODE, PAGE-MODE, LIST-OCTAL, LIST-SYMBOLIC, OUTPUT-FILE, HELP, EXIT, RESET, WRITE-DOMAIN/SEGMENT-STATUS, SET-MONITOR-NAME, SET-IO-BUFFERS, SUPPRESS-DEBUG-INFORMATION, ABORT-BATCH-ON-ERROR.

6. Install and operation - the repo's own guides

7. Reverse-engineering assets in this repo

We hold NLL H02 itself and have started carving it:

Asset Path
NLL program segment (ND-500 code, 123,989 bytes) SINTRAN/ND500/nll-re/LINKAGE-LOAD-H02.PSEG
Utilities note (7-bit text) SINTRAN/ND500/nll-re/LINKAGE-LOAD-H02.UTIL
NLL data segment (2,184,977 bytes: command tables, strings, error texts) NOT committed - re-extract from the floppy image, see nll-re/README.md
Full disassembly, 31,747 lines (nd500-dis, base 0xB0000000 = segment 22) NOT committed - regenerates exactly from the PSEG, command in nll-re/README.md
Carved: the :LINK serializer, CLOSE-SEGMENT chain, MON-call wrappers CARVE-ANSWER-FOUR-OPEN-QUESTIONS-2026-08-17.md, Q3

Tooling note: ND-500 code is carvable from a Windows session even without a Ghidra processor - nd500-dis from the pcc-nd500 tree (a WSL binary, callable from Windows as wsl <path-to-pcc-nd500>/bin/nd500-dis; the tree is indexed in the nd500x repository's docs/EXTERNAL-ARTIFACTS.md) handles raw PSEGs and annotates monitor calls inline.

8. Open questions

  • The L-era :LINK variant (SL202-FO-L27) has undecoded string/module regions - needs an L-series NLL binary (LINK-FILE-FORMAT.md section 6).
  • Write-side proof of the DESC size-1 encoding must come from NLL's DESC writer (the read side and the file bytes are proven; desc-format.json openQuestions).