C509-L_97 Datasheet, PDF (59/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

External Bus Interface
C509-L
- - - - - .2 .1 .0
DPSEL(92 _H)
DPSEL
Selected
Data-
.2 .1 .0 pointer
0 0 0 DPTR 0
0 0 1 DPTR 1
0 1 0 DPTR 2
0 1 1 DPTR 3
1 0 0 DPTR 4
1 0 1 DPTR 5
1 1 0 DPTR 6
1 1 1 DPTR 7
DPTR7
DPH(83H) DPL(82H)
DPTR0
External Data Memory
MCD00779
Figure 4-3
Accessing of External Data Memory via Multiple Datapointers
4.3.3 Advantages of Multiple Datapointers
Using the above addressing mechanism for external data memory results in less code and faster
execution of external accesses. Whenever the contents of the datapointer must be altered between
two or more 16-bit addresses, one single instruction, which selects a new datapointer, does this job.
lf the program uses just one datapointer, then it has to save the old value (with two 8-bit instructions)
and load the new address, byte by byte. This not only takes more time, it also requires additional
space in the internal RAM.
4.3.4 Application Example and Performance Analysis
The following example shall demonstrate the involvement of multiple data pointers in a table
transfer from the code memory to external data memory.
Start address of ROM source table:
Start address of table in external RAM:
1FFFH
2FA0H
Semiconductor Group
4-7
1997-10-01