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CHAPTER 3 CPU ARCHITECTURE
User’s Manual U15798EJ2V0UD
(a) Interrupt enable flag (IE)
This flag controls the interrupt request acknowledgement operations of the CPU.
When 0, the IE is set to the disable interrupt (DI) state, and only non-maskable interrupt request becomes
acknowledgeable. Other interrupt requests are all disabled.
When 1, the IE is set to the enable interrupt (EI) state and interrupt request acknowledgement enable is
controlled with an in-service priority flag (ISP), an interrupt mask flag for various interrupt sources and a
priority specification flag.
The IE is reset to (0) upon DI instruction execution or interrupt acknowledgement 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.
In these flags, the 2-bit information which indicates the register bank selected by the 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
18.3 (3) Priority specification flag registers (PR0L, PR0H, PR1L)
) are disabled for acknowledgement.
Actual request acknowledgement 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 manipulation 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 3-9. Stack Pointer Format
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0
SP SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0
The SP is decremented ahead of write (save) to the stack memory and is incremented after read (reset) from
the stack memory.
Each stack operation saves/resets data as shown in Figures 3-10 and 3-11.
Caution Since RESET input makes SP contents undefined, be sure to initialize the SP before using the
stack.
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