4.4
STACK POINTER (SP)
The stack pointer
is
a 12-bit register that contains the address of the next free location
on
the stack.
During
an
MCU reset or the reset stack pointer
(RSP)
instruction, the stack pointer
is
set to location
$07F.
The stack pointer
is
then decremented
as
data
is
pushed onto the stack and incremented
as
data
is
pulled from the stack. The
seven
most significant bits of the stack pointer are permanently
set to
0000011.
Subroutines and interrupts may
be
nested down
to
location
$061
(31
bytes maxi-
mum) which allows the programmer to
use
up to
15
levels
of
subroutine calls (less if interrupts
are
allowed).
4.5 CONDITION CODE REGISTER (CC)
The condition code register
is
a 5-bit register
in
which four bits
are
used
to indicate the results
of
the
instruction just executed. These bits
can
be
individually tested by a program and specific action
taken
as
a result of their state.
Each
bit
is
explained
in
the following paragraphs.
4.5.1 Half Carry (H)
Set during ADD and ADC operations to indicate that a carry occurred between bits 3 and
4.
4.5.2 Interrupt (I)
When this bit
is
set, the timer and external interrupts (INT and INT2)
are
masked (disabled), If
an
interrupt occurs while this bit
is
set, the interrupt
is
latched and
is
processed
as
soon
as
the interrupt
bit
is
cleared.
4.5.3 Negative (N)
When set, this bit indicates that the result of the last arithmetic, logical, or data manipulation was
negative (bit 7
in
the result
is
a logic one).
4.5.4
Zero (Z)
When set, this bit indicates that the result of the last arithmetic, logical, or data manipulation was
zero.
4.5.5 Carry/Borrow (C)
When set, this bit indicates that a carry or borrow out of the arithmetic logical unit (ALU) occurred
during the last arithmetic operation. This bit
is
also affected during bit test and branch instructions
plus shifts and rotates.
4-2
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