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ADDRESS SPACES S3C84E5/C84E9/P84E9
2-10
USING THE REGISTER POINTERS
Register pointers RP0 and RP1, mapped to addresses D6H and D7H in set 1, are used to select two movable 8-byte
working register slices in the register file. After a reset, RP# point to the working register common area: RP0 points
to addresses C0H–C7H, and RP1 points to addresses C8H–CFH.
To change a register pointer value, you load a new value to RP0 and or RP1 using an SRP or LD instruction.
(see Figures 2-6 and 2-7).
With working register addressing, you can only access those two 8-bit slices of the register file that are currently
pointed to by RP0 and RP1. You cannot, however, use the register pointers to select a working register space in set
2, C0H–FFH, because these locations can be accessed only using the Indirect Register or Indexed addressing
modes.
The selected 16-byte working register block usually consists of two contiguous 8-byte slices. As a general
programming guideline, it is recommended that RP0 point to the "lower" slice and RP1 point to the "upper" slice
(see Figure 2-6). ). In some cases, it may be necessary to define working register areas in different (non-contiguous)
areas of the register file. In Figure 2-7, RP0 points to the "upper" slice and RP1 to the "lower" slice.
Because a register pointer can point to either of the two 8-byte slices in the working register block, you can flexibly
define the working register area to support program requirements.
F PROGRAMMING TIP — Setting the Register Pointers
SRP #70H ; RP0 ← 70H, RP1 ← 78H
SRP1 #48H ; RP0 ← no change, RP1 ← 48H,
SRP0 #0A0H ; RP0 ← 0A0H, RP1 ← no change
CLR RP0 ; RP0 ← 00H, RP1 ← no change
LD RP1,#0F8H ; RP0 ← no change, RP1 ← 0F8H
FH (R15)
0H (R0)
8-Byte Slice
16-Byte Contiguous
Working Register
block
Register File
Contains 32
8-Byte Slices
RP0
RP1
8H
7H
0 0 0 0 1 X X X
0 0 0 0 0 X X X
8-Byte Slice
Figure 2-6. Contiguous 16-Byte Working Register Block