NEC PD754144, PD754244 3.2 Bank Configuration of General-Purpose Registers

CHAPTER 3 FEATURES OF ARCHITECTURE AND MEMORY MAP
45
User’s Manual U10676EJ3V0UM
RBE Register Bank
Fixed to 0
Remark × = don’t care
RBE is automatically saved or restored during subroutine processing and therefore can be set while subroutine
processing is under execution. When interrupt servicing is executed, RBE is automatically saved or restored, and
RBE can be set during interrupt servicing depending on the setting of the interrupt vector table as soon as the interrupt
servicing is started. Consequently, if different register banks are used for normal processing and interrupt servicing
as shown in Table 3-3, it is not necessary to save or restore general-purpose registers when an interrupt is serviced,
and only RBS needs to be saved or restored if two interrupts are nested. This means that the interrupt servicing speed
can be increased.
Table 3-3. Example of Using Different Register Banks for Normal Routine and Interrupt Routine
Normal processing Uses register bank 2 or 3 with RBE = 1
Single interrupt servicing Uses register bank 0 with RBE = 0
Nesting servicing of two Uses register bank 1 with RBE = 1
interrupts (at this time, RBS must be saved or restored)
Nesting servicing of Registers must be saved or restored by PUSH or POP instructions
three or more interrupts
.
3.2 Bank Configuration of General-Purpose Registers
The µ PD754244 is provided with four register banks with each bank consisting of eight general-purpose registers:
X, A, B, C, D, E, H, and L. The general-purpose register area consisting of these registers is mapped to the addresses
00H to 1FH of memory bank 0 (refer to Figure 3-5 Configuration of General-Purpose Registers (4-Bit Process-
ing)). To specify a general-purpose register bank, a register bank enable flag (RBE) and a register bank select register
(RBS) are provided. RBS selects a register bank, and RBE determines whether the register bank selected by RBS
is valid or not. The register bank (RB) that is enabled when an instruction is executed is as follows:
RB = RBE RBS
Table 3-2. Register Bank Selected by RBE and RBS
RBS
3210
0 00××Fixed to bank 0
1 0000Bank 0 selected
0 1 Bank 1 selected
1 0 Bank 2 selected
1 1 Bank 3 selected