NEC UPD789026 Series User Manual page 55

(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
The SP is decremented ahead of write (save) to the stack memory and is incremented after read (restore)
from the stack memory.
Each stack operation saves/restores data as shown in Figures 3-14 and 3-15.
Caution Since RESET input makes SP contents undefined, be sure to initialize the SP before
SP _ 2
SP
SP _ 2
SP _ 1
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SP + 1
SP SP + 2
Figure 3-13. Stack Pointer Configuration
SP14 SP13 SP12 SP11 SP10 SP9
instruction execution.
Figure 3-14. Data to be Saved to Stack Memory
PUSH rp
Instruction
Lower Half
Register Pairs
Upper Half
Register Pairs
SP
Figure 3-15. Data to be Restored from Stack Memory
POP rp
Instruction
Lower Half
SP
Register Pairs
Upper Half
Register Pairs
CHAPTER 3 CPU ARCHITECTURE
SP8 SP7
CALL, CALLT
Instructions
SP _ 2
SP
SP _ 2
PC7 to PC0
SP _ 1
PC15 to PC8
SP
RET Instruction
SP PC7 to PC0
PC15 to PC8
SP + 1
SP SP + 2
User's Manual U11919EJ3V0UM00
SP6 SP5 SP4 SP3 SP2
SP _ 3
SP
SP _ 3
SP _ 2
SP _ 1
SP
SP
SP + 1
SP + 2
SP SP + 3
0
SP1 SP0
Interrupt
PC7 to PC0
PC15 to PC8
PSW
RETI Instruction
PC7 to PC0
PC15 to PC8
PSW
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