AT8xC51SND2C
Dual Data Pointer
Description
The AT8xC51SND2C implement a second data pointer for speeding up code execution
and reducing code size in case of intensive usage of external memory accesses.
DPTR0 and DPTR1 are seen by the CPU as DPTR and are accessed using the SFR
addresses 83h and 84h that are the DPH and DPL addresses. The DPS bit in AUXR1
register (see Table 22) is used to select whether DPTR is the data pointer 0 or the data
pointer 1 (see Figure 18).
Figure 18. Dual Data Pointer Implementation
0
1
DPL0
DPL1
DPL
DPTR0
DPTR1
AUXR1.0
DPTR
DPS
0
1
DPH0
DPH1
DPH
Application
Software can take advantage of the additional data pointers to both increase speed and
reduce code size, for example, block operations (copy, compare, search …) are well
served by using one data pointer as a “source” pointer and the other one as a “destina-
tion” pointer.
Below is an example of block move implementation using the 2 pointers and coded in
assembler. The latest C compiler also takes advantage of this feature by providing
enhanced algorithm libraries.
The INC instruction is a short (2 Bytes) and fast (6 CPU clocks) way to manipulate the
DPS bit in the AUXR1 register. However, note that the INC instruction does not directly
force the DPS bit to a particular state, but simply toggles it. In simple routines, such as
the block move example, only the fact that DPS is toggled in the proper sequence mat-
ters, not its actual value. In other words, the block move routine works the same whether
DPS is '0' or '1' on entry.
; ASCII block move using dual data pointers
; Modifies DPTR0, DPTR1, A and PSW
; Ends when encountering NULL character
; Note: DPS exits opposite of entry state unless an extra INC AUXR1 is added
AUXR1
move:
EQU
0A2h
mov
inc
mov
DPTR,#SOURCE ; address of SOURCE
AUXR1
DPTR,#DEST
AUXR1
; switch data pointers
; address of DEST
mv_loop: inc
; switch data pointers
; get a Byte from SOURCE
; increment SOURCE address
; switch data pointers
; write the Byte to DEST
; increment DEST address
; check for NULL terminator
movx A,@DPTR
inc
inc
movx @DPTR,A
inc
jnz
DPTR
AUXR1
DPTR
mv_loop
end_move:
27
4341D–MP3–04/05
ATMEL [ ATMEL ]
