sx20ac Parallax, Inc., sx20ac Datasheet - Page 33

• Manufacturer Part Number: sx20ac
• Description: Configurable Communications Controllers With Ee/flash Program Memory, In-system Programming Capability And On-chip Debug
• Manufacturer: Parallax, Inc.

Parallax SX20AC/SX28AC
15.10.1.
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.
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.
This means that the call destination must start in the lower
half of any page. For example, 00h-0FFh, 200h2FFh,
400h-4FFh, etc.
© Parallax Inc.
• The following happens when a CALL instruction is
• The current value of the program counter is
• The lower eight bits of the label address are copied
• The ninth bit of the Program Counter is cleared to
• The page select bits (in STATUS register) are
executed:
incremented and pushed onto the top of the stack.
into the lower eight bits of the program counter.
zero.
copied
Counter.
Jump Operation
Page Jump Operation
Call Operation
into the upper two bits of the Program
Page 33 of 51
15.10.4.
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.
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.
Page Call Operation
Push Operation
Rev 1.6 11/20/2006
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