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