Memory map: Difference between revisions
→All other ranges 0x0500'0000 - 0xFFFF'FFFF (External SysAD bus via PI)
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Moreover, there are two important additional details:
* All writes are performed '''asynchronously''' by the PI. Making a write in this area will in fact just cause the PI to latch the value internally, and release the VR4300 immediately. The write will then happen in background. The status of the ongoing write will be reflected by the PI "I/O busy" status bit, which will be set to 1 until the write is finalized. While a write is ongoing, further writes are ignored, and reads (from any address) return the 32-bit value that is being written. For further information on this, please check the [[Peripheral Interface|PI page]].
* All external accesses are made by RCP through a 16-bit bus. Given that the RCP only knows of 32-bit accesses (as access size is ignored), this means that each read or write performed by the VR4300 will cause exactly two reads or two writes on the 16-bit bus: first the MSB at the address specified by the CPU (ignoring bit 0, so that the address is aligned to 16 bit), then the LSB at address+2. This might seem a small implementation detail, but it does actually cause an important and visible bug. For instance, if the VR4300 requests a 16-bit read at address <code>0x1000'0002</code>, the RCP (that ignores access sizes) will do two 16-bit reads on the cartridge bus at <code>0x1000'0002</code> and <code>0x1000'0004</code>, and will put on the SysAD bus the 32-bit word at <code>0x1000'0002 - 0x1000'0005</code>. This is a violation of the SysAD protocol explained above: in fact, the RCP should have put on the bus the 32-bit word at <code>0x1000'0000 - 0x1000'0003</code> instead. Because of this, effectively a 16-bit read at <code>0x1000'0002</code> returns the 16-bit word at <code>0x1000'0004</code> instead.
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