Page 103 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

80 Chapter 3 Addressing Modes

it is not clear whether the load is executed before the + or after the +. Note that if the
latter is true, the + would have no effect on the instruction. Indeed, in the 6812, the + is
carried out before the operation; in this case a load, so that
LDX 2,X+
is the same as
LDX 2,X-

For any load instruction involving the same index register for the location of the
operand and the location of the result, the general rule is that postincrementing has no
effect on the instruction. However, the fact that the postincrementing is carried out before
the operation produces an unexpected result in the store counterpart of the load
instruction just discussed. For example, with

STX 2,X+
suppose that X initially contains 373. After the instruction is executed, one will find that
X becomes 375, and 375 has been stored in locations 373 and 374. We conclude this
discussion by noting that predecrementing has none of these ambiguities. For example,
if X initially contains 373 before the instruction
STX 2,-X

is executed, then 371 will be stored in locations 371 and 372.
There is often considerable confusion about LDX (direct), LDX #, and LEAX.
Consider the following examples, assuming location $820 stores $1234.

LDX $820
will load $1234 into X. Direct addressing loads the data located at the instruction's
address. However, immediate addressing loads part of the instruction into the register, as

LDX #$820
will load $820 into X. Sometimes, immediate addressing is used to load an address into
memory so that pointer addressing (index addressing with zero offset) can access the data:
LDX #$820
LDX 0,X
will eventually load $1234 into X. Also, the LEAX instruction loads the effective address
into an index register. When it is used with program counter relative addressing, it has
the same effect as LDX # but is position independent.

LEAX $820,PCR
LDX 0,X

will eventually load $1234 into X. But LEAX can be used with other addressing modes
for other effects; for instance LEAX 5, X adds 5 to X, and LEAX D, X adds D to X.