Chapter 4
38
The shadow bank 0 / 1 bit operates with all available attribute modes. However, it
does not affect the border area since this is not stored in the Spectrum’s lower 16K
RAM bank.
Display mode compatibility
The new display modes provided by the SPECTRA interface are controlled through a
specific I/O port, but there is the possibility that this port address might also be used
by another peripheral. This could prevent the display mode register from being read, or
could cause the other device to become activated when a write to the display mode
register occurs. Should such a conflict be encountered, then either the peripheral must
be disconnected to allow the new display modes to be used, or the new display
modes must be disabled to allow the peripheral to operate correctly.
The new display modes can be disabled by setting configuration switch 6 to the off
position. This causes the SPECTRA interface to revert to the standard Spectrum
display mode and prevents further accesses to the display mode register, thus
ensuring full I/O compatibility with ‘conflicting’ hardware. Further details about the
compatibility of the SPECTRA interface with existing hardware and software can be
found in Appendix B.
Irrespective of the state of configuration switch 6, the SPECTRA interface will always
output a picture through the SCART socket.
Display mode availability
A program can test whether the new display modes are available by writing a value to
the display mode register and then attempting to read it back. If the values do not
match then the new display modes are not available. A value of $FF should be
avoided since this is the default value returned if the display modes are not enabled or
if a SPECTRA interface is not connected.
Should the value read back be different to that expected (but not $FF) then another
device must have responded to the I/O port read, making it impossible to identify
whether the new display modes are available. Note that such a device could also
respond with a value of $FF, thereby giving the impression that no device responded
to the I/O port read.
It is also possible that a conflicting device just happens to respond with the expected
value, leading to a false detection of the display modes functionality. The chances of
this occurring can be significantly reduced by performing the test several times using
a variety of different values.
Note that the display mode register uses an I/O port that is contended with the ULA,
i.e. it has an address between $4000 and $7FFF. This means that the reading of the
I/O port will be suspended whilst the ULA is fetching video data from the Spectrum’s
RAM and will only continue once the ULA has finished its read. As a result, if the
display mode register is read when a SPECTRA interface is not connected (or the
display modes have been disabled) then a value of $FF will always be returned. This
would not necessarily have been the case if a non-contending I/O port had been used
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