Thread: Zx81
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Old 28th Mar 2020, 8:57 pm   #213
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: Zx81

Offcially this can never happen, ie, the user should never be able to do anything from the keyboard which ends up damaging the hardware.

In practice, maybe it is possible with badly designed address decoding, such as would allow the user to try to write to ROM, which is read-only. If the ROM activates (in read / output mode) at the same time as the system is trying to write to it you end up with a bus battle between devices. One of them could potentially cause damage to the other.

With regard to the TRS80, I assume this is based on the Z80 which usually begins executing from address 0000 after reset, so the ROM is usually the first thing in the memory map and everything else comes afterwards. It is unlikely therefore that the ROM was mapped at a higher address than the screen RAM but I don't know the TRS80 at all, so who knows?
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