UPD17015 datasheet(17/120 Pages) NEC

17 / 120 page

µPD17015

3. ADDRESS STACK REGISTER (ASR)

3.1

OUTLINE AND CONFIGURATION OF ADDRESS STACK REGISTER

The address stack register is used to store the return address when a subroutine call instruction is executed.

Fig. 3-1 shows the configuration of the address stack registers.

As shown in Fig. 3-1, there is one address stack register, consisting of 11 bits.

Fig. 3-1 Configuration of Address Stack Registers

Address stack register

(ASR)

Bit

The return address of the program is stored

when a subroutine call instruction is executed.

3.2

ADDRESS STACK REGISTER OPERATION

When a subroutine call instruction is executed, the return address is stored in the address stack register.

When a return instruction is executed, the contents (return address) of the address stack register are read back

into the program counter (see Table 3-1).

At power-on reset, the contents of the address stack register are undefined. At CE reset, or when the clock stop

instruction is executed, the contents of the address stack register are retained.

Table 3-1 Program Counter Operation Upon Execution of a Subroutine Call Instruction

Instruction

Operation

CALL addr

Stores the program counter value to the address

stack register.

Transfers the value specified with an operand (addr)

to the program counter.

RET

Reads the value of the address stack register back into

RETSK

the program counter.

3.3

NOTES ON USE OF ADDRESS STACK REGISTER

Note that a subroutine call of more than one level cannot be used because the address stack register has a one-

UPD17015 Datasheet(PDF) 17 Page - NEC