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Chapter 5. External Control [Category: External Control]
The External Control category of facilities and instructions
permits a program to communicate with a specialpurpose
device. Two instructions are provided, both of
which must be implemented if the facility is provided.
External Control In Word Indexed (eciwx), which
does the following:
- Computes an effective address (EA) like most
X-form instructions
- Validates the EA as would be done for a load
from that address
- Translates the EA to a real address
- Transmits the real address to the device
- Accepts a word of data from the device and
places it into a General Purpose Register
External Control Out Word Indexed (ecowx),
which does the following:
- Computes an effective address (EA) like most
X-form instructions
- Validates the EA as would be done for a store
to that address
- Translates the EA to a real address
- Transmits the real address and a word of data
from a General Purpose Register to the
device
Permission to execute these instructions and identification
of the target device are controlled by two fields,
called the E bit and the RID field respectively. If attempt
is made to execute either of these instructions when
E=0 the system data storage error handler is invoked.
The location of these fields is described in Book III.
The storage access caused by eciwx and ecowx is
performed as though the specified storage location is
Caching Inhibited and Guarded, and is neither Write
Through Required nor Memory Coherence Required.
Interpretation of the real address transmitted by eciwx
and ecowx and of the 32-bit value transmitted by
ecowx is up to the target device, and is not specified by
the Power ISA. See the System Architecture documentation
for a given Power ISA system for details on how
the External Control facility can be used with devices
on that system.
Example
An example of a device designed to be used with the
External Control facility might be a graphics adapter.
The ecowx instruction might be used to send the
device the translated real address of a buffer containing
graphics data, and the word transmitted from the General
Purpose Register might be control information that
tells the adapter what operation to perform on the data
in the buffer. The eciwx instruction might be used to
load status information from the adapter.
A device designed to be used with the External Control
facility may also recognize events that indicate that the
address translation being used by the processor has
changed. In this case the operating system need not
“pin” the area of storage identified by an eciwx or
ecowx instruction (i.e., need not protect it from being
paged out).
Chapter 5. External Control [Category: External Control]
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