
64
CHAPTER 3
CPU ARCHITECTURE
User’s Manual U12790EJ2V0UD
(a) Interrupt enable flag (IE)
This flag controls the interrupt request acknowledgment operations of the CPU.
When IE = 0, all the interrupts are disabled (DI) except the non-maskable interrupt.
When IE = 1, the interrupts are enabled (EI). At this time, the acknowledgment of interrupts is controlled
by the in-service priority flag (ISP), the interrupt mask flag corresponding to each interrupt, and the
interrupt priority specification flag.
The IE flag is reset to (0) upon DI instruction execution or interrupt acknowledgment and is set to (1) upon
EI instruction execution.
(b) Zero flag (Z)
When the operation result is zero, this flag is set (1). It is reset (0) in all other cases.
(c) Register bank select flags (RBS0 and RBS1)
These are 2-bit flags to select one of the four register banks.
The 2-bit information that indicates the register bank selected by SEL RBn instruction execution is stored
in these flags.
(d) Auxiliary carry flag (AC)
If the operation result has a carry from bit 3 or a borrow at bit 3, this flag is set (1). It is reset (0) in all
other cases.
(e) In-service priority flag (ISP)
This flag manages the priority of acknowledgeable maskable vectored interrupts.
When ISP = 0, acknowledging the vectored interrupt requests to which a low priority is assigned by the
priority specification flag registers (PR0L, PR0H, PR1L) (refer to 18.3 (3) Priority specification flag
registers (PR0L, PR0H, PR1L)) is disabled. Whether an interrupt request is actually acknowledged
depends on the status of the interrupt enable flag (IE).
(f)
Carry flag (CY)
This flag stores an overflow and underflow upon add/subtract instruction execution. It stores the shift-
out value upon rotate instruction execution and functions as a bit accumulator during bit manipulation
instruction execution.