Peripheral Interface: Difference between revisions
→The PI Bus
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|ROM (though this address range is huge, and ROM only typically occupied a small portion of it)
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There is no way to have more than two domains, nor to decide which domain is used for some specific address. The above table is hardcoded in the PI itself, and cannot the changed. In general, software that needs to change domain parameters before accessing a device is advised to do that in a transactional way, so that the default values are restored after the access for other peripherals.
=== Open bus behavior ===
Writes made to addresses with no "receiver" devices cause no harm; the writes are just ignored. As explained above, the PI has absolutely no notion if devices are attached or not (and whether they care about some addresses) so all writes will always be performed as if somebody cared about them. In particular, notice also that PI will also execute writes to the ROM address space (as it has no notion that the ROM is read-only, nor that a ROM is mapped to those addresses!): the cartridge will then ignore those writes.
Reads made to addresses with no "receiver" devices cause an open-bus behavior: the 32-bit word returned by PI is the 16-bit lowest part of the address put on the bus, repeated in both halves. For instance, a direct I/O 32-bit read from PI address <code>0x6666_DCBA</code> will return the value <code>0xDCBA_DCBA</code>. When reading unmapped areas via DMA, the rule is the same but the address returned is the address of the page being accessed (the only one physically put on the bus), and it is repeated for all words read until page change.
= Registers =
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