Page 87 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

64 Chapter 3 Addressing Modes

loads the number contained in location $3012 + $843 = $3855 into accumulator A. The
post byte for this 16-bit offset mode (see Figure 3.2) has ones in bits 7 to 5, the index
register in bits 4 and 3 (00 is X, 01 is Y, 10 is SP, and 11 is PC), and 010 in bits 2 to
0. The machine code is given by

where $A6 is the opcode byte for any index option with LDAA, $E2 is the the post
byte, and $3012 is the 16-bit two's-complement offset. The saved offset is added to the
index register to get the effective address, as illustrated by Figure 3.4.
In short, addresses requiring several accesses are kept in index registers, if possible,
and utilize the more efficient index addressing. Shorter offsets produce more efficient
programs and can be used if the index register value is close to the effective addresses that
will be used. But while negative offsets are mechanically as usable as positive offsets to
more often use shorter offsets, positive offsets are often preferred for clarity.
The 5-, 9-, and 16-bit offset index addressing modes are useful for reading data out of
a vector. Suppose a 10-element vector of 8-bit items has element 0 at $843, element 1 at
$844, element 2 at $845, and so on. Then if X has $843, then

LDAA 2,X
puts element 2 into accumulator A. Suppose now that a 10-element vector of 8-bit items
has element 0 at $872, element 1 at $873, element 2 at $874, and so on. Then if X has
$872, this instruction still gets element 2 out of the vector. This instruction uses the
efficient 5-bit offset mode. The following instruction gets element i from the vector
beginning at $843 into accumulator A, where the vector index i is in index register X:
LDAA $843,X
This instruction uses the less efficient 16-bit offset mode, but it lets the variable index
be in the X index register.
Index registers can be either autoincremented or autodecremented before or after
being used in effective address calculations. It is denoted by a delta value between 1 and

Figure 3.5. Autoincrement Address Calculation