S3F80P5_UM_ REV1.00 ADDRESS SPACE
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, they 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, we recommend 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 the either of the two 8-byte slices in the working register block, you can
define the working register area very flexibly to support program requirements.
PROGRAMMING TIP — Setting the Register Pointers
SRP #70H ; RP0 ← 70H, RP1 ← 78H
SRP1 #48H ; RP0 ← no change, RP1 ← 48H,
SRP0 #0A0H ; RP0 ← A0H, RP1 ← no change
CLR RP0 ; RP0 ← 00H, RP1 ← no change
LD RP1,#0F8H ; RP0 ← no change, RP1 ← 0F8H
0FH (R15)
00H (R0)
8-Byte Slice
16-byte
contiguous
working
register block
Register File
Contains 32
8-Byte Slices
8-Byte Slice
RP0
RP1 08H
07H
0 0 0 0 1 X X X
0 0 0 0 0 X X X
Figure 2-7. Contiguous 16-Byte Working Register Block
2-11