Binary Dinosaurs Computer Museum
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ZX81 'Piggyback' Fix
During my visit to Nine Tiles I discovered what this mod actually did. When John Grant was coding the ROM for the ZX81 he made a tiny mistake in the division routines that resulted in an error appearing. It was fixed late on a friday night and this was the finished product - extra ROMs on top of the original ROM that had enough power to take over when said division routine was called. This means this particular machine is a pretty early one.

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