40
CHAPTER 3 CPU
MB89620 series
3.2 Dedicated Registers
3.2.2 Register Bank Pointer (RP)
The register bank pointer (RP) located in the upper 8 bits of the program status (PS)
indicates the address of the general-purpose register bank currently in use. The RP is
converted to form the actual address in general-purpose register addressing.
n
Structure of Register Bank Pointer (RP)
Figure 3.2.2a shows the structure of the register bank pointer.
Figure 3.2.2a Structure of Register Bank Pointer
The register bank pointer indicates the address of the register bank currently in use. Figure
3.2.2b shows the relationship between the pointer contents and the actual address is based on
the conversion rule.
Figure 3.2.2b Rule for Conversion of Actual Addresses
of General-Purpose Register Area
The register bank pointer points to the memory block (register bank) in the RAM area that is
used for general-purpose registers. A total of 32 register banks are available. A register bank is
specified by setting a value between 0 and 31 in the upper 5 bits of the register bank pointer.
Each register bank contains eight 8-bit general-purpose registers. Registers are specified by the
lower 3 bits of the operation codes.
Using the register bank pointer, the addresses 0100
H
to 01FF
H
can be used as the general-
purpose register area. However, the available area is limited on some products if internal RAM
only is used. The initial value after a reset is indeterminate.
Check: The register bank pointer is part of the program status (PS) and cannot be accessed
independently.
Bit 15
Bit 14
Bit 13 Bit 12 Bit 11Bit 10 Bit 9 Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
R4
R3
R2
R1
R0
—
—
—
H
I
IL1
IL0
N
Z
V
C
RP initial value
XXXXXXXX
B
RP
CCR
PS
X: Indeterminate
A15 A14 A13 A12 A10 A11
A9
A8
A7
A6
A5
A4
A3
A2
A1
A0
“0”
“0”
“0”
“0”
“0”
“0”
“0”
“1”
R4
R3
R2
R1
R0
b2
b1
b0
Upper bits of RP
Lower operation codes
Generated addresses
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