SX20AC75 datasheet(32/52 Pages) ETC

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SX18AC/SX20AC/SX28AC/SX18AC75/SX20AC75/SX28AC75

15.10.2 Page Jump Operation

When a JMP instruction is executed and the intended

destination is on a different page, the page select bits

must be initialized with appropriate values to point to the

desired page before the jump occurs. This can be done

easily with SETB and CLRB instructions or by writing a

value to the STATUS register. The device also has the

PAGE instruction, which automatically selects the page in

a single-cycle execution.

Note: “N” must be 0, 1, 2, or 3.

15.10.3 Call Operation

The following happens when a CALL instruction is exe-

cuted:

• The current value of the program counter is increment-

ed and pushed onto the top of the stack.

• The lower eight bits of the label address are copied into

the lower eight bits of the program counter.

• The ninth bit of the Program Counter is cleared to zero.

• The page select bits (in STATUS register) are copied

into the upper two bits of the Program Counter.

This means that the call destination must

start in the

lower half of any page. For example, 00h-0FFh, 200h-

2FFh, 400h-4FFh, etc.

15.10.4 Page Call Operation

When a subroutine that resides on a different page is

called, the page select bits must contain the proper val-

ues to point to the desired page before the call instruction

is executed. This can be done easily using SETB and

CLRB instructions or writing a value to the STATUS reg-

ister. The device also has the PAGE instruction, which

automatically selects the page in a single-cycle execu-

tion.

Note:“N” must be 0, 1, 2, or 3.

15.11

Return Instructions

The device has several instructions for returning from

subroutines and interrupt service routines. The return

from subroutine instructions are RET (return without

affecting W), RETP (same as RET but affects PA1:PA0),

RETI (return from interrupt), RETIW (return and add W to

RTCC), and RETW #literal (return and place literal in W).

The literal serves as an immediate data value from mem-

ory. This instruction can be used for table lookup opera-

tions. To do table lookup, the table must contain a string

of RETW #literal instructions. The first instruction just in

front of the table calculates the offset into the table. The

table can be used as a result of a CALL.

PAGE N

STATUS<6:5>

JMP LABEL

PC<10:9>

PC<8:0>

STATUS<6:5>

CALL LABEL

PC<10:9>

PC<8>

PC<7:0>

PAGE N

STATUS<6:5>

CALL LABEL

PC<10:9>

PC<8>

PC<7:0>

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