abcd.md

ABCD - Add decimal with extend

Operation

[destination]₁₀ ← [source]₁₀ + [destination]₁₀ + [X]

Syntax

ABCD Dy,Dx
ABCD -(Ay),-(Ax)

Attributes

Size byte

Description

Add the source operand to the destination operand along with the extend bit, and store the result in the destination location. The addition is performed using BCD arithmetic. The only legal addressing modes are data register direct and memory to memory with address register indirect using pre-decrementing.

Application

The ABCD instruction is used in chain arithmetic to add together strings of BCD digits. Consider the addition of two nine-digit numbers. Note that the strings are stored so that the least-significant digit is at the high address.

        LEA Number1,A0      ;A0 points at first string
        LEA Number2,A1      ;A1 points at second string
        MOVE #8,D0          ;Nine digits to add
        MOVE #$04,CCR       ;Clear X-bit and Z-bit of the CCR
LOOP    ABCD -(A0),-(A1)    ;Add a pair of digits
        DBRA D0,LOOP        ;Repeat until 9 digits added

Condition codes

X	N	Z	V	C
*	U	*	U	*

The Z-bit is cleared if the result is non-zero, and left unchanged otherwise. The Z-bit is normally set by the programmer before the BCD operation, and can be used to test for zero after a chain of multiple-precision operations. The C-bit is set if a decimal carry is generated.

From MOTOROLA M68000 FAMILY Programmer's reference manual. Copyright 1992 by Motorola Inc./NXP. Adapted with permission.