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The Apple //e originally did not support Double HiRes graphics (DHR). The engineers realized they could double their hires and lores X resolution shortly after the introduction of the machine, so the feature is only available on Revision B and later motherboards. However, to use DGR/DHR you needed the Extended 80-Column Card which added 64KB of RAM to the machine in the AUX slot and you had to close a jumper on the card to enable the feature.

While I've seen some screenshots of what happens when DHR is attempted on Rev A machines (basically a horizontally mangled normal HiRes mode), I became curious as to why it required a jumper since it appears to just fail visually otherwise.

Is there something in the firmware which is reflected by the jumper that would otherwise "protect" DHR on a non-Rev A machine? I'm just really curious what the jumper really does since the failure mode on a Rev A machine (with correct jumper setting) simply seems to be the horizontally smushed display.

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    My guess is that with the jumper in place it would cause the card to pull pin 55 low in certain or different circumstances. In Rev B motherboard this forced TEXT processing but in Rev A motherboards it disabled the motherboard's ROMs. – user722 Mar 22 '19 at 0:22
  • @RossRidge so I guess the question would evolve into what would happen in that case - would ROM processing of text output cease or something else? – bjb Apr 21 '20 at 20:14

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