Motorola M68000 User's Guide download pdf (Page 132)

118 Assembly Language Programming for the 68000 Family

in the correct location. There are many ways to obtain the storage for

a node. One way is to maintain a linked list of free nodes. All we have

to do is to initialize this linked list by chaining the nodes together. For

our example we will have nodes of size NDATA. This represents a node

of size NDATA+4, where NDATA is the size of the data in the node.

A header, FLIST, will be maintained that will point to the pool of free

nodes. Assuming the total number of nodes is equal to the value of symbol

NNODES the following subroutine, INIT, will initialize a free list of

nodes:

NDATAs EQU 10 10 BYTES OP DATA (MUST BE EVEN NO.)

NSIZE: EQU NDATA+4 DATA + POINTER

NNODES: EQU 100 NUMBER OF NODES

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

* INIT - INITIALIZE THE FREE LIST

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

INIT:

MOVEM.L

D1/A0-A1,-(SP)

SAVE REGS

LEA

NODES,A0

A0 -> NODE POOL

LEA

FLIST,A1

Al -> HEADER OF FREE LIST

MOVE.W

#NNODES-l,D1 SET UP FOR LOOP

INIT1:

MOVE.L

A0,(Al)

SET -> TO NEXT NODE

MOVEA.L A0 ,A1

SET UP FOR NEXT NDOE

ADDA.L

#NSIZE,A0

A0 -> NEXT NODE

DBRA

Dl ,INIT1

LOOP UNTIL DONE

CLR.L (Al)

PLACE NULL PTR IN LAST NODE

MOVEM.L

(SP)+,D0/A0-A1

RESTORE REGS

RTS

DATA

FLIST: DS.L

-> FREE LIST

NODES:

DS.B

NSIZE*NNODES

NODE POOL

The CLR.L instruction places a zero value in the very last pointer. A

value of zero is used to represent the NULL pointer. We can never have

a node at address zero for reasons that will be covered in Chapter 12.

Locations starting at zero are special reserved locations on the 68000.

Our next step is to write a subroutine that will obtain a free node from

this free list. Here is how we do it:

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

* GETFREE - RETURNS PTR TO FREE NODE IN A0.

* IF NO NODES LEFT, NULL IS RETURNED

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

GETFREE:

MOVEA.L FLIST,A0

CMPA.L #0,A0

BEQ GETFRET

MOVE.L (A0),FLIST

GETFRET:RTS

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

GET HEAD PTR

NULL?

YES, JUST RETURN

NO, SET NEW HEAD

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Motorola M68000 User's Guide Page 132

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