UPD17P136ACT datasheet(58/292 Pages) NEC

58 / 292 page

CHAPTER 6 STACK

6.6.3 Operation on Execution of Interrupt Receipt and RETI Instruction

Table 6-4 shows stack operation during interrupt receipt and RETI instruction.

Table 6-4. Operation during Interrupt Receipt and RETI Instruction

Instruction

Operation

Receipt of interrupt

(1)

Stack pointer (SP) is decremented.

(2)

Value in the program counter (PC) is saved in the address stack register pointed to by the stack

pointer (SP).

(3)

Values in the PSWORD flags (BCD, CMP, CY, Z, IXE) are saved in the interrupt stack.

(4)

Vector address is transferred to the program counter (PC)

RETI

(1)

Values in the interrupt stack register are restored to the PSWORD (BCD, CMP, CY, Z, IXE).

(2)

Value in the address stack register pointed to by the stack pointer (SP) is restored to the program

counter (PC).

(3)

Stack pointer (SP) is incremented.

6.7 STACK NESTING LEVELS AND THE PUSH AND POP INSTRUCTIONS

During execution of operations such as subroutine calls and returns, the stack pointer (SP) simply functions as

a 3-bit counter which is incremented and decremented by one. When the value in the stack pointer is 0H and a CALL

or MOVT instruction is executed or an interrupt is received, the stack pointer is decremented to 7H. The

µPD17134A

subseries treat this condition as a fault and generates an internal reset signal.

In order to avoid this condition, when the address stack register is being used frequently, the PUSH and POP

instructions are used to save the address stack register.

Table 6-5 shows stack operation during the PUSH and POP instructions.

Table 6-5. Stack Operation during the PUSH and POP Instructions

Instruction

Operation

PUSH

(1)

Stack pointer (SP) is decremented.

(2)

Value in the address register (AR) is transferred to the address stack register pointed to by the

stack pointer (SP).

POP

(1)

Value in the address stack register pointed to by the stack pointer (SP) is transferred to the

address register (AR).

(2)

Stack pointer (SP) is incremented.

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