Optimize the work without changing the arithmetic

On a machine with a word size that is a multiple of 16bits, partial sums can use that wider width and then fold the16-bit portions together, retaining any further end-around carries. The §3 words 0001f203 andf4f5f6f7 sum to f4f7e8fa; foldf4f7+e8fa to 1ddf1, then to ddf2 (§2(C), §3).

• On a machine with a word size that is a multiple of 16bits, partial sums can use that wider width and then fold the16-bit portions together, retaining any further end-around carries. The §3 words 0001f203 andf4f5f6f7 sum to f4f7e8fa; foldf4f7+e8fa to 1ddf1, then to ddf2 (§2(C), §3).
• Deferred carries require enough accumulator width or proper carry handling. Loop unrolling reduces loop overhead; combining copying with summation can fetch each source byte once for both tasks. None of these techniques permits dropping bytes or their even/odd positions (§2, coding techniques1–3). Our helper copies and sums; it is not a speed benchmark.
• The C, Motorola68020, Cray and IBM370 examples are historical cores with omitted environmental/special-case code (§§4.1–4.4). Their old CPU timings are not modern performance promises. Verified editorial errata560/4918 correct C pointer code and 7711 corrects a typo; held technical3133 addresses the odd-byte C handling. This guide uses explicit bytes, not a verbatim copy of that old C.
• Incremental updates are a separate companion topic. Official metadata lists RFC 1141 as updating1071, despite1141’s printed “Obsoletes” header (held erratum2102). RFC 1624 in turn updates1141 and corrects its zero-boundary behavior (§§2–5). Neither relationship obsoletes the whole1071 publication. Follow the existing1624 guide for that method; do not apply1071’s uncomplemented-sum notation directly to the stored field.
• The reproduced IEN45 appendix is historical design discussion, including alternative product codes and outboard processing. It is not a new TCP wire-format requirement. This guide covers the current arithmetic and implementation properties of the1071 body, not executable historical assembly or a modern offload specification.