SAB80C517-M16 SIEMENS [Siemens Semiconductor Group], SAB80C517-M16 Datasheet - Page 28

• Manufacturer Part Number: SAB80C517-M16
• Description: 8-Bit CMOS Single-Chip Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

External Bus Interface
5.2
Eight Datapointers for Faster External Bus Access
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 done byte by byte. For
complex applications with numerous external peripherals 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, "C", 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.
How the Eight Datapointers of the SAB 80C517 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 SAB 80C517 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
SAB 80C517 which handle the datapointer therefore affect only one of the eight pointers which is
addressed by DPSEL at that very moment.
Figure 5-1 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.
Semiconductor Group
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