NEC 78K0 User Manual page 75

(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, PR1H)
(see 19.3 (3) Priority specification flag registers (PR0L, PR0H, PR1L, PR1H)) 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.
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SP SP15 SP14 SP13 SP12 SP11 SP10 SP9
The SP is decremented ahead of write (save) to the stack memory and is incremented after read (restored) from
the stack memory.
Each stack operation saves/restores data as shown in Figures 3-14 and 3-15.
Caution Since RESET input makes the SP contents undefined, be sure to initialize the SP before using
the stack.
CHAPTER 3 CPU ARCHITECTURE
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Figure 3-13. Format of Stack Pointer
SP8 SP7
User's Manual U15947EJ2V0UD
SP6 SP5 SP4 SP3 SP2
0
SP1 SP0
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