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Contended I/O: Difference between revisions
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Remove references to {{overline|WAIT}} as that's simply not how the Speccy works
m (Split out +3 sentence) |
(Remove references to {{overline|WAIT}} as that's simply not how the Speccy works) |
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When the [[Z80]] wishes to access an IO port, it places the port address on the address bus exactly as it does when accessing memory. On the 48K and 128K Spectrums, this causes delays to IO as the ULA pauses the processor by stopping its clock, even when the ULA's I/O port is not the one being accessed. | When the [[Z80]] wishes to access an IO port, it places the port address on the address bus exactly as it does when accessing memory. On the 48K and 128K Spectrums, this causes delays to IO as the ULA pauses the processor by stopping its clock, even when the ULA's I/O port is not the one being accessed. | ||
On the +3 Spectrum, no contention occurs as the +3 ULA applies contention only when the Z80's {{overline|MREQ}} line is active, which it is not during an IO operation. | On the +3 Spectrum, no contention occurs as the +3 ULA applies contention only when the Z80's {{overline|MREQ}} line is active, which it is not during an IO operation. [TODO: clarify whether port 0xfe can be accessed without delay.] | ||
# If the port address has its low bit reset, the ULA pauses the processor | On the 48K Spectrum and 128K Spectrum, two effects can occur here: | ||
# If the port being accessed is between 0x4000 and 0x7fff, this "looks like" an access to contended memory to the ULA and it again pauses the processor | |||
# If the port address has its low bit reset, the ULA pauses the processor by stopping its clock. | |||
# If the port being accessed is between 0x4000 and 0x7fff, this "looks like" an access to contended memory to the ULA and it again pauses the processor by stopping its clock. Contention is also applied if the port address is between 0xc000 and 0xffff on a 128K Spectrum with a contended RAM bank paged into that address range.<ref>http://www.worldofspectrum.org/forums/showpost.php?p=215386&postcount=88</ref><ref>http://www.worldofspectrum.org/forums/showthread.php?t=18214</ref> | |||
The combination of these two effects leads to the following pattern: | The combination of these two effects leads to the following pattern: | ||