HCS12 datasheet(37/452 Pages) FREESCALE

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S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

3.9.1 5-Bit Constant Offset Indexed Addressing

This indexed addressing mode uses a 5-bit signed offset which is included

in the instruction postbyte. This short offset is added to the base index

location that will be affected by the instruction. This gives a range of –16

through +15 from the value in the base index register. Although other

indexed addressing modes allow 9- or 16-bit offsets, those modes also

require additional extension bytes in the instruction for this extra

information. The majority of indexed instructions in real programs use

offsets that fit in the shortest 5-bit form of indexed addressing.

Examples:

LDAA

0,X

STAB

–

8,Y

For these examples, assume X has a value of $1000 and Y has a value of

$2000 before execution. The 5-bit constant offset mode does not change

the value in the index register, so X will still be $1000 and Y will still be $2000

after execution of these instructions. In the first example, A will be loaded

with the value from address $1000. In the second example, the value from

the B accumulator will be stored at address $1FF8 ($2000 –$8).

3.9.2 9-Bit Constant Offset Indexed Addressing

This indexed addressing mode uses a 9-bit signed offset which is added to

the base index register (X, Y, SP, or PC) to form the effective address of the

memory location affected by the instruction. This gives a range of –256

through +255 from the value in the base index register. The most significant

bit (sign bit) of the offset is included in the instruction postbyte and the

remaining eight bits are provided as an extension byte after the instruction

postbyte in the instruction flow.

Examples:

LDAA

$FF,X

LDAB

–

20,Y

For these examples, assume X is $1000 and Y is $2000 before execution of

these instructions.

NOTE:

These instructions do not alter the index registers so they will still be $1000

and $2000, respectively, after the instructions.

The first instruction will load A with the value from address $10FF and the

second instruction will load B with the value from address $1FEC.

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