Motorola HC12 User Manual

User’s Manual

MCUez HC12 Assembler

140

Assembler Syntax

MOTOROLA

7.4.3.12  Indexed, Post-Decrement

This addressing mode allows the user to decrement the base register by a
specified value after indexing takes place. The content of the base register is
read and then decremented by the specified value.

Valid range for a pre-decrement value is [1...8]. The base index register may be
X, Y, or SP.

Example:

ORG $1000

CST_TBL:

DC.B $5, $10, $18, $20, $28, $30

END_TBL:

DC.B $0

main:

CLRA

CLRB

LDX #$END_TBL

loop:

ADDD 2,X-

CPX   #CST_TBL

BNE   loop

Base register X is loaded with the address of the element following the table

CST_TBL

 ($1006).

The value at address $1006 ($0) is added to register D. Register X is
decremented by 2 (its value is $1004).

Register X is not equal to the address of

CST_TBL

, so the value at address

$1004 is added to D; and X is decremented by 2 again (its value is now $1002).

This loop is repeated as long as register X did not reach the beginning of the
table

CST_TBL

 ($1000).