What’s the difference between unbuffered and fully buffered memory

DDR,DDR2 and DDR3 memories follow the DDRxxx/PCyyyy classification.

xxx indicates the maximum clock speed the memory chips support.eg DDR400 memories work at 400 MHz at the most,DDR2-800 can work up to 800 MHz and DDR3-1333 can work up to 1,333MHz.Though this is not thr real clock speed of the memory.The real clock of DDR,DDR2, and DDR3 memories if half of the labeled clock speed.Therefore DDR400 memories work at 200 MHz,DDR2-800 memories at 400 MHZ.

yyyy number indicates the maximum transfer rate that the memory reaches in MB/s.

The bus clock rate is the actual speed of ur FSB(The FSB connects the processor (CPU) in your computer to the system memory)

The physical address space is huge nowadays due to 64 bit addressing. Many devices, for example AHCI-compliant disk controllers, require quite big chunks of address space to be mapped to the device registers.

Also, the IO address space is not accessible with usual assembler instructions. It is accessible only with special instructions that can read from and write to the IO ports. This is not really convenient and inefficient in many cases.

For example the above mentioned ACPI compliant controller has some functionality that requires the processor to execute locked read-modify-write operation to change a single bit in a device register provided no other cores will be able access and modify the same register in between. With IO address space this is not possible at all.

One more point we should take to account is that the PCI bus is not unique to the x86 architecture. There are other architectures exist that has no special IO bus at all. So the device vendors prefer not to use IO addresses to stay compatible with most systems. Thus no one is using the IO address space. Then why not make de-jure the thing that is de-facto for a long time already?