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The Parrot for the ZX80 / ZX81
R.I.S.T. Inc.
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The Parrot uses port $4F to write to and read the status back from the SP0256.
Only address lines A7, A5 and A4 are decoded, so it will respond to any port
address in the form of %0x00xxxx.

When reading from the SP0256, the 'busy' status is reported in bit 0. A value
of 0 indicates that the SP0256 is busy.