Memory Mapped I/O Versus Isolated I/O

Memory Mapped I/O versus Isolated I/O Although this concept was explained earlier as well, it will be useful to review it again in this context. In isolated I/O, a separate address space of the CPU is reserved for I/O operations. This address space is totally different from the address space used for memory devices. In other words, a CPU has two distinct address spaces, one for memory and one for input/output. Unique CPU instructions are associated with the I/O space, which means that if those instructions are executing on the CPU, then the accessed address space will be the I/O space and hence the devices mapped on the I/O space. The x86 family with the in and the out instructions is a well known example of this situation. Using the in instruction, the Pentium processor can receive information from a peripheral device, and using the out instruction,the Pentium processor can send information to a peripheral device.

Thus, the I/O devices are mapped on the I/O space in case of the Pentium processor. In some processors, like the SRC, there is no separate I/O space. In this case, some address space out of the memory address space must be used to map I/O devices. The benefit will be that all the instructions which access memory can be used for I/O devices. There is no need for including separate I/O instructions in the ISA of the processor. However, the disadvantage will be that the I/O interface will become complex. If partial decoding is used to
reduce the complexity of the I/O interface, then a lot of memory addresses will be consumed. The given figure shows the memory address space as well as the I/O address space for the Pentium processor. The