UPD17P136ACT datasheet(57/292 Pages) NEC

57 / 292 page

CHAPTER 6 STACK

6.6 STACK OPERATION

Stack operation during execution of each instruction is explained in 6.6.1 to 6.6.3.

6.6.1 On Execution of Instructions CALL, RET, RETSK

Table 6-2 shows operation of the stack pointer (SP), address stack register, and the program counter (PC) during

execution of CALL, RET, and RETSK.

Table 6-2. Operation of the Instructions CALL, RET, and RETSK

Instruction

Operation

CALL addr

(1)

Stack pointer (SP) is decremented.

CALL @AR

(2)

Program counter (PC) is saved in the address stack register pointed to by the stack pointer

(SP).

(3)

Value specified by the instruction operand (addr or @AR) is transferred to the program

counter.

RET

(1)

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

RETSK

program counter (PC).

(2)

Stack pointer (SP) is incremented.

When the RETSK instruction is executed, the first instruction after data restoration becomes a NOP instruction.

6.6.2 Table Reference (MOVT DBF, @AR Instruction)

Table 6-3 shows the operation during table reference.

Table 6-3. Stack Operation during Table Reference

Instruction

Instruction cycle

Operation

MOVT DBF, @AR

First

(1)

Stack pointer (SP) is decremented.

(2)

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

the stack pointer (SP).

(3)

Value in the address register (AR) is transferred to the program counter (PC).

Second

(4)

Contents of the program memory (ROM) pointed to by the program counter

(PC) is transferred to the data buffer (DBF).

(5)

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

restored to the program counter (PC).

(6)

Stack pointer (SP) is incremented.

Caution When the “MOVT DBF, @AR” instruction is executed, one level of the address stack is temporarily

used. Exercise care not to exceed the usable stack level.

Remark Two instruction cycles are required to execute the “MOVT DBF, @AR” instruction. This is an exception.

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