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ZXI standard: Difference between revisions
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The Z80 provides 16-bit I/O decoding, so in theory the Spectrum has 65536 ports available for hardware devices. In practice the Spectrum does not fully decode the I/O and there are no spare I/O ports available that do not clash with at least one existing peripheral. For example, the ULA alone uses 32768 ports (every even port). It is worth noting that the Timex clones do fully decode the I/O ports. | The Z80 provides 16-bit I/O decoding, so in theory the Spectrum has 65536 ports available for hardware devices. In practice the Spectrum does not fully decode the I/O and there are no spare I/O ports available that do not clash with at least one existing peripheral. For example, the ULA alone uses 32768 ports (every even port). It is worth noting that the Timex clones do fully decode the I/O ports. | ||
During the development of the ULAplus replacement ULA, a comprehensive search was carried out to determine a 16-bit port range that clashed with the least devices. An 8-bit port was located that clashed with a single device, the ZX Printer, and a hardware fix created to prevent the ZX Printer using that address. | During the development of the [[ULAplus]] replacement ULA, a comprehensive search was carried out to determine a 16-bit port range that clashed with the least devices. An 8-bit port was located that clashed with a single device, the ZX Printer, and a hardware fix created to prevent the ZX Printer using that address. | ||
The following modification to the ZX Printer circuit prevents it being triggered by access to port xx3B. | The following modification to the ZX Printer circuit prevents it being triggered by access to port xx3B. | ||
