Motorola M68000 Uživatelská příručka stáhnout pdf (Strana 76)

62 Assembly Language Programming for the 68000 Family

MOVE.L #10,DO

NEXT: JSR OUTDEC

JSR NEWLINE

SUB.L #1,D0

BNE NEXT

The programmer may also want to determine if an arithmetic opera

tion results in a positive or a negative value. The negative bit (N) will be

set to a binary 1 for all negative numbers (note that zero is always a pos

itive number in two’s complement). The negative bit is set to the same

value as the sign bit of the result of the operation. The BMI (branch on

minus) and BPL (branch on plus) instructions will test for negative and

positive numbers respectively.

The MOVE instruction always sets the negative and zero bits in the

CCR just as the arithmetic instructions do. However, the programmer

sometimes desires to determine if a value is zero or negative, but hasn’t

moved it or used it in arithmetic. A special instruction, TST, is provided

for just such a purpose. The general form is

TST[.<size>] <ea>

The following instructions test register D5 and memory location COUNT.

TST.L D5

TST.L COUNT

Of course, the TST instruction must be followed with an appropriate

conditional branch.

The Extend Bit

The extend bit (X) is normally a copy of the carry bit. It is used for

performing multiple precision arithmetic, which will be covered in detail

in Chapter 11. However, you should be aware that some instructions do

not affect the extend bit. The most notable of these instructions is the

MOVE instruction. MOVE conditionally sets or clears N and Z, clears V

and C, but doesn’t change the current value of X. You won’t be using the

extend bit at this time, so this fact shouldn’t cause concern.

Com parisons

An essential task in any programming language is the comparison of

two numbers, or any data items that are being represented by numerical

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Motorola M68000 Uživatelská příručka Strana 76