SX28ACSLASHSS datasheet(33/51 Pages) PARALLAX

33 / 51 page

Parallax SX20AC/SX28AC

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Page 33 of 51

Rev 1.6 11/20/2006

15.10.1.

Jump Operation

When a JMP instruction is executed, the lower nine bits

of the program counter is loaded with the address of the

specified label. The upper two bits of the program counter

are loaded with the page select bits, PA1:PA0, contained

in the STATUS register. Therefore, care must be

exercised to ensure the page select bits are pointing to the

correct page before the jump occurs.

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

executed:

• The current value of the program counter is

incremented 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, 200h2FFh,

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 values

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 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.

15.11. 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

memory. This instruction can be used for table lookup

operations. 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.

15.12. Subroutine Operation

15.12.1.

Push Operation

When a subroutine is called, the return address is pushed

onto the subroutine stack. Specifically, each address in the

stack is moved to the next lower level in order to make

room for the new address to be stored. Stack 1 receives

the contents of the program counter. Stack 8 is

overwritten with what was in Stack 7. The contents of

stack 8 are lost.

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