Motorola M68000 User's Guide download pdf (Page 135)

Llnhed Lists—A Programming Example 121

INSR: JSR NEWLINE

CLR.B (AO)

CMP.B #CR,D0

BNE INSR1

CLR.B -1(AO)

INSR1: MOVEM.L (SP)+,A0/D0-D1

RTS

OUTPUT A NEWLINE

ADD A NULL TO STRING

LAST CHAR A CR?

YES, PUT NULL THERE TOO

RESTORE REGISTERS

Note that the method used to handle an overflow count is different from

that used to handle a string terminated with a carriage return. If the

carriage return is entered, it must not appear in the string. The second

CMP.B instruction handles this check. If the last character entered was a

carriage return, it is overwritten with a null byte.

At this point we have enough basic subroutines to start manipulating

the linked lists themselves. We will need two types of list insertions; one

that inserts a node at the head of a list, and one that inserts a node in

alphabetical order. To insert at the head of the list is very simple. We

merely obtain the pointer to the first node from the header. You can think

of the header as a special node that has no data and is always located in a

known place. For example, the header for the free list is always located

at the memory location FLIST. We set the forward pointer in the node

we are inserting to point to the node that was pointed to by the header.

This may be a null pointer, but we don’t care. The only other piece of

business is to set the header to point to the node we are inserting. Here

is how it’s done:

* INSERT - INSERT A NODE AT THE HEAD OF A LIST.

* AO -> NODE

* A1 -> HEADER OF LIST

****************************************************************

INSERT: MOVE.L (A1),(A0) SET UP LINK IN NEW NODE

MOVE.L A0,(A1) SET HEADER TO -> NEW NODE

RTS

Figure 10 shows this operation.

Insertion of a node in alphabetical order is considerably more difficult.

We must chain down the linked list until we find the proper insertion

point. This is determined by a string comparison using STRCMP. We

then have to link this node into the list by breaking the previous link and

setting the proper pointers. Refer to Figure 11.

INSERTA - INSERT A NODE ALPHABETICALLY.

AO -> NODE

A1 -> HEADER OF LIST

INSERTA:MOVEM.L A0-A2,-(SP) SAVE REGISTERS

ADDQ.L #4/AO AO -> NEW NODE DATA

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Motorola M68000 User's Guide Page 135

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