NEC 78K0 User Manual page 55

(a) Interrupt enable flag (IE)
This flag controls the interrupt request acknowledge operations of the CPU.
When 0, the IE flag is set to the interrupt disabled (DI) state, and all maskable interrupts are disabled.
When 1, the IE flag is set to the interrupt enabled (EI) state and interrupt request acknowledgment is
controlled with an in-service priority flag (ISP), an interrupt mask flag for various interrupt sources, and a
priority specification flag.
The IE flag is reset (0) upon DI instruction execution or interrupt acknowledgment and is set (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.
In these flags, the 2-bit information that indicates the register bank selected by SEL RBn instruction
execution is stored.
(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 this flag is 0, low-
level vectored interrupt requests specified by a priority specification flag register (PR0L, PR0H, PR1L) (refer
to 15.3 (3) Priority specification flag registers (PR0L, PR0H, PR1L)) cannot be acknowledged. Actual
interrupt request acknowledgment is controlled by the interrupt enable flag (IE).
(f) Carry flag (CY)
This flag stores 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 operation instruction
execution.
(3) Stack pointer (SP)
This is a 16-bit register to hold the start address of the memory stack area. Only the internal high-speed RAM
area can be set as the stack area.
CHAPTER 3 CPU ARCHITECTURE
User's Manual U16227EJ2V0UD
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