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20 CHAPTER 2. INTRODUCTION TO LOOPING

SWI

Verify that CALL C100 and CALL C200 produces the same output. Explain

why this is so, and how and why the loop terminates.

2.4.2 One, two! One, two! And through and through ...

Marching through memory

Often loops are combined with marching through memory and operating on con-

secutive memory location. In this case, the X (and/or Y) register is initialized

to a starting memory address. Inside the loop, memory is accessed using IND,X

addressing mode. This operates on memory whose address is computed using the

value in X register. At the bottom of the loop, X is incremented, so that next

time around the loop, the operation is performed on the next memory location

The following program shows prints using OUTA the values stored in 9 consecu-

tive locations starting from location $D000. Note we are using FCB which is the

assembly language equivalent of memory modify.

ORG $C300

LDAB #$09

LDX #$D000 *Don’t forget the #

VORPAL TSTB

BEQ SWORD

LDAA 0,X

JSR OUTA

INX

DECB

BRA VORPAL

SWORD SWI

; FOLLOWING SAME AS MM D000 FOLLOWED BY: 55 6F 66 4D 2D 44 62 72 6E

In some cases the memory has to be accessed in the reverse order. In this case, X starts at

the end, and at the bottom of the loop, X is decremented.

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 100 101

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Motorola Axiom CMD11E1 Uživatelská příručka Strana 20