Virtual I/O

DETAILED DESCRIPTION A method and apparatus are disclosed for coupling external, bidirectional ports to an onboard port of a single chip microprocessor such that the external ports can be addressed in the same manner as onboard ports.
Using the disclosed method and apparatus, external ports, which are referred to herein as virtual ports, a single onboard port may be configured that it can be used to address up to twelve virtual ports.
The onboard port is used as a multiplexed bus to communicate with the virtual ports.
Read (RD) and write (WR) signals are used to perform read and write operations on the virtual ports.
Functionally, the virtual ports behave as if they are internal to the microprocessor.
However, the port "select" and "data" signals are latched by external hardware.
When the processor is directed to access virtual port addresses, it automatically initiates proper control signals to perform read/write operations on the selected virtual port.
Virtual input/output ("VIO") offers the following advantages: Since the number of virtual ports is not fixed in the chip, it is possible to configure as many virtual ports as desired.
The cost is not built into the chip.
Although instruction execution time is doubled for VIO, a single instruction is required to access it.
As contrasted with prior art methods, where multiple instructions are needed, the overall speed of execution is faster using VIO.
Since no extra instructions are needed, the code density is not affected.
In the prior art, more than one instruction is required, increasing the size of code.
The additional hardware needed to support VIO is simple and inexpensive compared to prior art techniques.
Quasi-bidirectional ports with read-modify-write functionality is available in virtual ports.
It is not available using prior art techniques.
Since the number of instructions remains the same, a previously written program does not change with VIO.