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Some personal computer ports (I/O addresses) are read-only. It may be desirable to modify the contents of such a port under program control.
English (United States)
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Substituted Write to Read-Only Port
Some personal computer ports (I/O addresses) are read-only. It may be
desirable to modify the contents of such a port under program control.
The method described here does not modify a port's contents but makes the
application program that reads the port act as if the port's content is what is
Before an application program reads a read-only port, a resident program
gains control at an appropriate event (e.g., a hardware interrupt). This resident
program sets a particular CPU flag indicating instruction trapping mode and sets
the associated trap interrupt vector to point to the interrupt handler routine in that
The trap flag operates to put the processor in single step mode whereby
control will be given to the interrupt handler. The associated trap routine will be
executed before the next instruction of the application program is fetched by the
processor. The trap routine provides the desired change in the interpretation of
the data in the port.
For example, the data read from the port may be coded in EBCDIC, but the
application program may be designed to operate on ASCII coded data, into which
the data read by the port must therefore be translated. To accomplish this, the
trap routine includes a translation operation which converts the data before it is
operated upon by the next instruction of the application program.