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CHAPTER 3 CPU ARCHITECTURE
User’s Manual U19780EJ2V0UD
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(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
18.3 (3) Priority specification flag registers (PR0L, PR0H, PR1L, PR1H)
) can not be acknowledged.
Actual 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.
Figure 3-7. Format of Stack Pointer
15
SP SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0
0
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 3-8 and 3-9.
Caution Since reset signal generation makes the SP contents undefined, be sure to initialize the SP
before using the stack.