Memory map: Difference between revisions
→Memory Map accesses
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The SysAD bus is described at the hardware level in the [[SysAD Interface|SysAD page]], but to understand the effects on memory map it is sufficient to understand how data is marshalled for reads and writes. Since SysAD is a 32-bit bus, 32-bit accesses are the "native" ones, and the others are made to work around a 32-bit data exchange.
* Reads: VR4300 puts the address on the bus and the size of the access (8, 16, 32, 64). The RCP typically returns a full (aligned) 32-bit word address (or two, in case of a 64-bit read), from which the VR4300 extracts the correct portion. For instance, when reading 8-bit from address <code>0x0000'0001</code>, the RCP will put on the bus the 32-bit values at <code>0x0000'0000 - 0x0000'0003</code>, and the VR4300 will then just isolate the requested 8 bits.
* Writes: VR4300 puts the address on the bus, the size of the access, and then the 32-bit value to be written. When the access is made using 8 or 16 bits, the value on the bus is prepared to match with the aligned 32-bit address. This is the same of what happens for reads, but this time it is the VR4300 to prepare the data. For instance, if register <code>S0=0x1234'5678</code>, <code>A0=0x0000'0001</code> and the opcode <code>SB S0, 0(A0)</code> is run, the VR4300 puts on the bus the value <code>S0 << 24</code>, that is <code>0x5678'0000</code>. It is then up to the RCP to see that, since the address is <code>0x0000'0001</code>, it needs to isolate the the second byte <code>0x78</code>. So even if it is a 8-bit write opcode, other bits of the register <code>S0</code> "leak" on the bus.
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