When the Z80 wishes to access an I/O 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 I/O 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 MREQ line is active, which it is not during an I/O operation. [TODO: clarify whether port 0xfe can be accessed without delay.]
On the 48K Spectrum and 128K Spectrum, two effects can occur here:
- 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.
The combination of these two effects leads to the following pattern:
|High byte in 0x40 (0xc0) to 0x7f (0xff)?||Low bit||Contention pattern|
|Yes||Set||C:1, C:1, C:1, C:1|
The "Contention pattern" shows the sequence of (non-)contention which occurs: an entry of "N:x" means that the Z80 simply continues for x T states, while "C:x" means that the Z80 is paused for the same number of T states as with a normal contended memory access, before continuing for x T states.
Access to ports between 0x4000 and 0x7ffe with the low bit clear (i.e. when the ULA's own port is not being accessed) is subject to contention on every T state, as neither the contention cancellation mechanism for memory requests nor the contention cancellation mechanism for ULA I/O port access is triggered. Every T state of the I/O access is therefore treated as if it were the first T state of a memory access.
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This article uses material from the "Contended IO" article on the ZX Spectrum technical information wiki at Fandom (formerly Wikia) and is released under the Creative Commons Attribution-Share Alike License.