XC886/888CLM
Memory Organization
User’s Manual
3-14
V1.3, 2010-02
Memory Organization, V 1.2
In order to access a register located in a page other than the current one, the current
page must be exited. This is done by reprogramming the bit field PAGE in the page
register. Only then can the desired access be performed.
If an interrupt routine is initiated between the page register access and the module
register access, and the interrupt needs to access a register located in another page, the
current page setting can be saved, the new one programmed, and the old page setting
restored. This is possible with the storage fields STx (x = 0 - 3) for the save and restore
action of the current page setting. By indicating which storage bit field should be used in
parallel with the new page value, a single write operation can:
•
Save the contents of PAGE in STx before overwriting with the new value
(this is done at the beginning of the interrupt routine to save the current page setting
and program the new page number); or
•
Overwrite the contents of PAGE with the contents of STx, ignoring the value written
to the bit positions of PAGE
(this is done at the end of the interrupt routine to restore the previous page setting
before the interrupt occurred)
Figure 3-6
Storage Elements for Paging
With this mechanism, a certain number of interrupt routines (or other routines) can
perform page changes without reading and storing the previously used page information.
The use of only write operations makes the system simpler and faster. Consequently,
this mechanism significantly improves the performance of short interrupt routines.
The XC886/888 supports local address extension for:
•
Parallel Ports
•
Analog-to-Digital Converter (ADC)
•
Capture/Compare Unit 6 (CCU6)
•
System Control Registers
PAGE
ST0
ST1
ST2
ST3
value update
from CPU
STNR
*
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