SECTION 4
PROGRAMMABLE
REGISTERS
The M6805 Family
CPU
has five registers available
to
the programmer. They
are
shown
in
Figure
4-1
and are explained in the following paragraphs.
7
0
I
A
I Accumulator
0
I
X
I Index Register
11
8 7
0
I
PCH
1
PCl
1
Program Counter
11
54
0
101010101011111
SP
I
Stack Pointer
4 0
Condition Code Register
'--r-...L....r--'--r....L....,r-'-~
4.1
ACCUMULATOR (A)
Carry / Borrow
Zero
L----Negative
L-----Interrupt
Mask
L....------Half
Carry
Figure
4-1.
Programming
Model
The accumulator
is
a general purpose 8-bit register
used
to hold operands
and
results
of
arithmetic
calculations or data manipulations.
4.2 INDEX REGISTER (X)
The index register
is
an
8-bit register used for the indexed addressing mode.
It
contains
an
8-bit
value that may
be
added
to
an
8-
or 16-bit immediate value to create
an
effective address. The index
register may
also
be
used
as
a temporary storage
area.
4.3 PROGRAM COUNTER (PC)
The program counter
is
a 12-bit register that contains the address
of
the next bye to
be
fetched.
4-1
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