Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 74)

60 Assembly Language Programming for the 68000 Family

have the JMP instruction actually jump to the target label. Here's how

it’s done.

BCS CLOSE

--------------

< 32K BYTES I

CLOSE: JMP FARWAY

-------------

• I

> 32K BYTES I

PARWAY: .

--------------

We can also do this:

•

BCS CLOSE

BRA NEXT

CLOSE: JMP PARWAY

This second approach has the advantage that we know the JMP is close

enough, and we have a definite place for it to go without interfering

with other instructions. The BRA instruction is an unconditional branch.

It always branches, but it suffers from the same limited range as the

conditional branches. The solution to this problem can be made even

simpler if the condition for branching is reversed. In other words, if

we could perform a conditional branch when the carry bit was not set,

rather than set, then we could use the conditional branch to go to the next

sequential instruction in the program. We are in luck! The 68000 provides

complementary instructions for all conditional branches. We can always

find an appropriate conditional branch that branches when the condition

we wish to test is not true. In the case of the BCS instruction, the BCC

(branch on carry clear) is the one to use. Our program can now be written:

BCC NEXT

JMP FARWAY

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Motorola M68000 Betriebsanweisung Seite 74