Motorola HC12 Manuale Utente

User’s Manual

MCUez HC12 Assembler

138

Assembler Syntax

MOTOROLA

7.4.3.10  Indexed, Pre-Decrement

This addressing mode allows the user to decrement the base register by a
specified value before indexing takes place. The base register is decremented by
the specified value and the content of the modified base register is referenced in
the instruction.

The 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 1,-X

CPX   #CST_TBL

BNE   loop

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

CST_TBL

 ($1006).

Register X is decremented by 1 (its value is $1005) and the value at this address
($30) is added to register D.

X is not equal to the address of

CST_TBL

, so it is decremented again and the

content of address $1004 is added to register D.

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

CST_TBL

 ($1000).