Page 86 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

3.2 Post Byte Index Addressing Modes 63

Figure 33, Program Segment to Add Two Bytes Using Vector Indexing
$A6 is the opcode byte for any LDAA index mode, and $01 is the post byte. The saved
offset is sign extended and added to the index register (see Figure 3.4).
The program segment in Figure 3.3 adds the word at $844 to the word at $845,
putting the sum in $846. No instruction's execution changes the contents of X.
The 9-bit option will be used when the offset is between -256 and +255 or when
the offset is between -16 and +15 and a "<" symbol, as it is used in the page zero mode,
is written preceding the offset. The instruction
LDAA <$11,X
loads the number contained in location $11+ $843 = $854 into accumulator A. The post
byte for this 9-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), a zero in bits 1 and 2, and the
sign bit of the offset in bit 0. Like the 5-bit offset case, the saved offset is sign extended
and added to the index register to get the effective address, as illustrated by Figure 3.4.
The machine code is

where $A6 is the opcode byte for any index option with LDAA, $EO is the post byte,
and bit 0 of the post byte and the next byte $11 are the offset.
When a larger offset is needed, the full 16-bit offset option can be used. The 16-bit
option will be used when the offset is outside the range -256 and +255 or when the
offset is in this range and a ">" symbol, as it is used in the direct mode, precedes the
offset. The instruction
LDAA >$3012,X

Figure 3.4. Offset Calculation