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µPD750008 USER'S MANUAL
3.2 GENERAL REGISTER BANK CONFIGURATION
The µPD750008 contains four register banks, each consisting of eight general registers: X, A, B, C, D,
E, H, and L. These registers are mapped to addresses 00H to 1FH in memory bank 0 of the data memory
(see Figure 3-5). To specify a general register bank, a register bank enable flag (RBE) and a register bank
select register (RBS) are contained. The RBS is a register used to select a register bank, and the RBE is
a flag used to determine whether a register bank selected using the RBS is to be enabled. The register bank
(RB) enabled at instruction execution is determined as
RB = RBE·RBS
Table 3-2. Register Bank to Be Selected with the RBE and RBS
Remark x: Don't care
The contents of the RBE are automatically saved or restored at the beginning or end of subroutine
processing, so that the RBE can be freely modified during subroutine processing. In interrupt processing,
the RBE is automatically saved or restored, and when interrupt processing is started, the contents of the RBE
can be specified for the interrupt processing by setting the interrupt vector table. Therefore, as indicated in
Table 3-3, by selecting a register bank depending on whether the processing is normal or interrupt, the general
register need not be saved and restored for the level-one interrupt processing, and only the RBS needs to
be saved and restored for the level-two interrupt processing, thus speeding up interrupt processing.
Table 3-3. Recommended Use of Register Banks with Normal Routines and Interrupt Routines
Normal processing
Level-one interrupt processing
Level-two interrupt processing
Multiple (triple or more) interrupt processing
3 4
RBS
RBE
3
2
1
0
0
0
x
1
0
0
0
0
1
1
Always 0
Save and restore the registers with PUSH or POP.
Register bank
0
x
Bank 0 is always selected.
0
Bank 0 is selected.
1
Bank 1 is selected.
0
Bank 2 is selected.
1
Bank 3 is selected.
Use register banks 2 and 3 with RBE = 1.
Use register bank 0 with RBE = 0.
Use register bank 1 with RBE = 1.
(In this case, the RBS needs to be saved and restored.)
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