Page 93 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

70 Chapter 3 Addressing Modes

For example,
LDAA <L,PCR

can load any word into A that can be reached by adding an 8-bit signed number to the
program counter. (Recall that the PC is pointing to the next instruction just below the
LDAA instruction when the effective address L is calculated.) The instruction
LDAA >L,PCR
can be used to access words that are farther away than -128 to + 127 locations from the
address of the next instruction; it adds a 16-bit offset to the current value of the program
counter to get the effective address. Although the machine coding of relative addressed
instructions is the same as that of index addressed instructions, do not dwell too much on
that similarity because the offset put in the machine code is determined differently.
Program counter relative indirect addressing can be used to access locations such as
I/O ports as in
LDAA [L,PCR]
Assuming that L is 18 bytes below this instruction, the machine code is given by

where $A6 is the opcode byte for any LDAA index mode; the post byte $FB indicates
indirect index addressing with 16-bit offset, but using the program counter as the "index
register", and the last two bytes are added to the program counter. The indirect address
($12 in the example above) is in a location relative to the program. If the program is
loaded into a different location, the offset $12 is still used to get the indirect address.
Such use of relative and indirect relative addressing lets the program have one location
and only one location where a value is stored, so that a downloaded file can insert the
value in one place to run the program anywhere it is stored.
Branch and long branch instructions do not need the ",PCR" symbol in the
instruction because they only use relative addressing with 16-bit relative offsets.
However, the BSR L, having an 8-bit offset, doesn't have a corresponding long branch
to subroutine. But JSR L,PCR is a 16-bit position independent subroutine call that
has the same effect as the missing LBSR L.
A 16-bit position independent indirect subroutine call, JSR [ L, PCR ], can jump
to a subroutine whose address is in a "jump table," as discussed in a problem at the end
of this chapter. Such jump tables make it possible to write parts of a long program in
pieces called sections and compile and write each section in EEPROM at different times.
Jumps to subroutine in a different section can be made to go through a jump table rather
than going directly to the subroutine. Then when a section is rewritten and its
subroutines appear in different places, only that section's jump table needs to be
rewritten, not all the code that jumps to subroutines in that section. The jump table can
be in EEPROM at the beginning of the section, or in RAM, to be loaded at run time.