At least one of the IBM S/360 series (I _think_ it may have been the Model 40) had its field-upgradable instruction-set-defining microcode stored in "transformer-based" ROM known as "TROS" (transformer read-only storage). The "windings" were conductive patterns embedded in a plastic strip with square holes for the cores; Engineering Changes could be done on-site using a small leatherpunch-like tool to "cut through" some of these traces and leave others intact.
IIRC the Model 30 used CCROS (capacitive) which used a grid of conductive pads on a plastic sheet the same size and layout as the standard 80-column punch card. _These_ could be programmed using... (you guessed it!) the IBM 026/029 keypunch. Since an 80-column card had 12 rows of holes (960 bits), and since (IIRC) there were only
1/4 as many CCROS pads, that's 240 bits/card; this on a plastic card that occupied roughly the same volume as 2 CF cards (16Gb by now?).By the time the S/370s came along (and IIRC some of the later S/360s) IBM had moved on to 8"-floppy loadable microcode (160,000 bytes!!).
We've come a long way, in this area, at least.
Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
-- "Progress, far from consisting in change, depends on retentiveness-- when experience is not retained, as among savages, infancy is perpetual. Those who cannot remember the past are condemned to repeat it....This is the condition of children and barbarians, in whom instinct has learned nothing from experience. -- George Santayana, Life Of Reason (1905)
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