
CENTRAL PROCESSING UNIT (CPU)
2.1 Central processing unit (CPU)
7700 FAMILY SOFTWARE MANUAL
2–6
2.1.7 Data bank register (DT)
Data bank register DT is an 8-bit register. With some addressing modes using the data bank register DT,
the contents of this register are used as the high-order 8 bits (bank) of a 24-bit address.
Use the
LDT
instruction to set the value in this register.
Set the only “00
16
” in single-chip mode because the access within bank 0
16
is only allowed.
This register is cleared to “00
16
” at reset.
2.1.8 Direct page register (DPR)
Direct page register DPR is a 16-bit register. The contents of this register indicate the direct page area
which is allocated in bank 0
16
or in the area across banks 0
16
and 1
16
. This area can be accessed with
2 bytes
C
by using the direct page addressing mode.
The contents of the DPR are the base address (the lowest address) of the direct page area which extends
to 256 bytes above this address. The DPR can contain a value from 0000
16
to FFFF
16
. However, set a
value from 0000
16
to FF00
16
in single-chip mode because the access within bank 0
16
is only allowed.
If it contains a value equal to or more than “FF01
16
”, the direct page area spans the area across banks
0
16
and 1
16
. If the low-order 8 bits of the DPR is “00
16
”, the number of cycles required to generate an
address is smaller by 1 cycle than the number if its contents are not “00
16
”. Accordingly, the low-order 8
bits of the DPR should usually be set to “00
16
”.
This register is cleared to “0000
16
” at reset. Figure 2.1.4 shows a setting example of the direct page with
the direct page register (DPR).
C
With 3 bytes for
DIV
and
MPY
instructions, and 1 byte is added for all instructions when using accu-
mulator B.
Fig. 2.1.4 Setting example of direct page with direct page register (DPR)
Note 1 :
The number of execution cycles is incremented by 1 when the low-order 8 bits of the
DPR are not “00
16
”.
Note 2 :
The direct page area spans the area across banks 0
16
and 1
16
when the DPR is “FF01
16
”
or more.
Bank 0
16
Bank 1
16
0
16
FF
16
123
16
222
16
FF10
16
1000F
16
0
16
FFFF
16
10000
16
DPR area when DPR=0123
16
(Note 1)
DPR area when DPR=FF10
16
(Note 2)
DPR area when DPR=0000
16