Page 384 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

12,4 The 68300 Series 361

Table 12.4. Addressing Modes for the 68300 Series

The addressing modes listed in Table 12.4 can be used with almost all instructions.
We will note those that are essentially the same as 6812 modes first, and then we will
examine those that are significantly different.
Moving a byte, or word, into a 32-bit data register will result in replacing only the
low byte, or word, in the register, leaving the other bits of the register untouched.
Moving a word to an address register results in filling the high sixteen bits with the sign
bit of the word that was moved, and moving a byte to or from an address register is not
permitted. Immediate addressing can provide 16- or 32-bit operands. (Long) direct
addressing uses a 32-bit address and can therefore address any word in memory. Pointer,
postincrement, and predecrement are the same as in the 6812; a postincrement read of a
word increments the pointer by 2, and a postincrement read of a long word increments the
pointer by 4. Predecrement works similarly and write works similarly. Your experience
with the 6812 should facilitate learning these modes.
The remaining modes consistently use sign extension to expand 8- or 16-bit
instruction offsets to 32 bits before using them in address calculations. (Short) direct
addressing is somewhat like direct page addressing, requiring a short 16-bit instruction
offset; but, using sign extension, it can access locations 0 to $7FFF and $FFFF8000 to
$FFFFFFFF. Similarly, index addressing uses sign extension, so if AO contains
$10000000, AO index addressing accesses locations $QFFF8000 to $10007FFFF.
In Tables 12.5 and 12.6, Dn is any data register; An or Am is an address register;
Xn is any An or Dn; nnnnnnnn is any 32-bit number; nnnn is any 16-bit number; nn is
any 8-bit number; and n is a number 0 to 7. rrrr is any 16-bit offset for which PC+rrrr =
LI; and IT is any 8-bit offset for which PC + rr = LI. Re —SFC, DFC, VBR, or USP—