Stack Pointer Configuration - NEC PD789860 User Manual

(d) Carry flag (CY)
This flag stores overflow and underflow that have occurred 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.
15
SP SP15 SP14 SP13 SP12 SP11 SP10 SP9
The SP is decremented before writing (saving) to the stack memory and is incremented after reading
(restoring) from the stack memory.
Each stack operation saves/restores data as shown in Figures 4-8 and 4-9.
Caution Since RESET input makes SP contents undefined, be sure to initialize the SP before using
the stack.
PUSH rp
instruction
SP _ 2
SP
Lower half
SP _ 2
register pairs
Upper half
SP _ 1
register pairs
SP
POP rp
instruction
Lower half
SP
register pairs
Upper half
SP + 1
register pairs
SP SP + 2
CHAPTER 4 CPU ARCHITECTURE
Figure 4-7. Stack Pointer Configuration
SP8
Figure 4-8. Data to Be Saved to Stack Memory
SP _ 2
SP
SP _ 2
SP _ 1
SP
Figure 4-9. Data to Be Restored from Stack Memory
SP
SP + 1
SP SP + 2
User's Manual U14826EJ5V0UD
SP7 SP6 SP5 SP4
CALL, CALLT
instructions
SP _ 3
SP
SP _ 3
SP _ 2
PC7 to PC0
SP _ 1
PC15 to PC8
RET instruction
PC7 to PC0
PC15 to PC8
SP + 1
SP + 2
SP SP + 3
0
SP3 SP2 SP1 SP0
Interrupt
PC7 to PC0
PC15 to PC8
PSW
SP
RETI instruction
SP
PC7 to PC0
PC15 to PC8
PSW
45