C505_9708 SIEMENS [Siemens Semiconductor Group], C505_9708 Datasheet - Page 46

• Manufacturer Part Number: C505_9708
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

4.6
4.6.1 The Importance of Additional Datapointers
The standard 8051 architecture provides just one 16-bit pointer for indirect addressing of external
devices (memories, peripherals, latches, etc.). Except for a 16-bit "move immediate" to this
datapointer and an increment instruction, any other pointer handling is to be handled bytewise. For
complex applications with peripherals located in the external data memory space (e.g. CAN
controller) or extended data storage capacity this turned out to be a "bottle neck" for the 8051’s
communication to the external world. Especially programming in high-level languages (PLM51,
C51, PASCAL51) requires extended RAM capacity and at the same time a fast access to this
additional RAM because of the reduced code efficiency of these languages.
4.6.2 How the eight Datapointers of the C505 are realized
Simply adding more datapointers is not suitable because of the need to keep up 100% compatibility
to the 8051 instruction set. This instruction set, however, allows the handling of only one single 16-
bit datapointer (DPTR, consisting of the two 8-bit SFRs DPH and DPL).
To meet both of the above requirements (speed up external accesses, 100% compatibility to 8051
architecture) the C505 contains a set of eight 16-bit registers from which the actual datapointer can
be selected.
This means that the user’s program may keep up to eight 16-bit addresses resident in these
registers, but only one register at a time is selected to be the datapointer. Thus the datapointer in
turn is accessed (or selected) via indirect addressing. This indirect addressing is done through a
special function register called DPSEL (data pointer select register). All instructions of the C505
which handle the datapointer therefore affect only one of the eight pointers which is addressed by
DPSEL at that very moment.
Figure 4-3 illustrates the addressing mechanism: a 3-bit field in register DPSEL points to the
currently used DPTRx. Any standard 8051 instruction (e.g. MOVX @DPTR, A - transfer a byte from
accumulator to an external location addressed by DPTR) now uses this activated DPTRx.
Special Function Register DPSEL (Address 92 H )
Bit
DPSEL.2-0
Semiconductor Group
Eight Datapointers for Faster External Bus Access
92 H
Bit No. MSB
7
–
Function
Data pointer select bits
DPSEL.2-0 defines the number of the actual active data pointer.DPTR0-7.
6
–
5
–
4
–
4-6
3
–
.2
2
External Bus Interface
.1
1
Reset Value : XXXXX000 B
LSB
.0
0
C505 / C505C
DPSEL
1997-08-01