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CHAPTER 3
CPU ARCHITECTURE
User’s Manual U12697EJ4V1UD
(3) Interrupt request enable flag (IE)
This flag controls the CPU interrupt request acknowledgement.
If IE is 0, interrupts are disabled, and only non-maskable interrupts and unmasked macro services can be
acknowledged. Otherwise, everything is disabled.
If IE is 1, the interrupt enable state is entered. Enabling the acknowledgment of interrupt requests is controlled
by the interrupt mask flags that correspond to each interrupt request and the priority of each interrupt.
This flag is set to 1 by executing the EI instruction and is reset to 0 by executing the DI instruction or by interrupt
acknowledgement.
(4) Auxiliary carry flag (AC)
If the operation result has a carry from bit 3 or a borrow to bit 3, this flag is set to 1. Otherwise, the flag is reset
to 0.
This flag is used when the ADJBA and ADJBS instructions are executed.
(5) Register set selection flag (RSS)
This flag sets the general-purpose registers that function as X, A, C, and B and the general-purpose register pairs
(16 bits) that function as AX and BC.
This flag is used to maintain compatibility with the 78K/III Series. Always set this flag to 0 except when using
a 78K/III Series program.
(6) Zero flag (Z)
This flag indicates that the operation result is 0.
If the operation result is 0, this flag is set to 1. Otherwise, it is reset to 0. The state of the Z flag can be tested
by a conditional branch instruction.
(7) Sign flag (S)
This flag indicates that the MSB in the operation result is 1.
The flag is set to 1 when the MSB of the operation result is 1. If 0, the flag is reset to 0. The S flag state can
be tested by a conditional branch instruction.
(8) Register bank selection flags (RBS0 to RBS2)
This is a 3-bit flag that selects one of the eight register banks (register banks 0 to 7) (refer to Table 3-4).
Three-bit information that indicates the register bank selected by executing the SEL RBn instruction is stored.
Table 3-4. Register Bank Selection
RBS2
RBS1
RBS0
Set Register Bank
000
Register bank 0
001
Register bank 1
010
Register bank 2
011
Register bank 3
100
Register bank 4
101
Register bank 5
110
Register bank 6
111
Register bank 7
(9) User flag (UF)
This flag is set and reset by a user program and can be used for program control.