When UNPK Broke – Diagnosing an IBM Mainframe Hardware Failure

Texas, 1978. Robert was working at a cotton co-op running what may have been one of the world's earliest online auction systems. Farmers received punch cards representing their bales of cotton - occasionally complete with enough stray cotton fibres to jam the card readers - while buyers placed bids remotely using leased IBM 3270 terminals and acoustic coupler modems. After an upgrade, the mainframe refused to come back properly. An IPL produced a corrupted device address before dropping the machine into a hard wait. Single-cycling the processor indicated that an UNPK (Unpack) instruction was producing the wrong output from perfectly good input. The instruction itself had effectively broken. And yes, this episode contains a lot of old IBM hardware and mainframe terminology. We know. That's part of the fun. Want even more info ? Read our show notes and related blog post related to this episode : https://blog.ithorrorstories.eu/episode-24-when-unpk-broke/ 00:00 Introduction 00:43 Robert's Background 02:14 IBM DOS 03:27 Cotton Auction System 05:37 Online Auction 07:14 SNA Protocol 09:05 Hardware Issue 13:20 Debugging Unpack 17:13 IBM Fix 21:23 Conclusion