Motorola CPU32 Bedienungsanleitung PDF herunterladen (Seite 70)

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Addressing Capabilities

MOTOROLA M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL 2-29

To implement stack growth from low memory to high memory, use (An) + to push data on

the stack and -(An) to pull data from the stack. After either a push or pull operation, the

address register points to the next available space on the stack. .

2.6.2 Queues

The user can implement queues, groups of information waiting to be processed, with the

address register indirect with postincrement or predecrement addressing modes. Using a

pair of address registers, the user implements a queue that ﬁlls either from high memory to

low memory or from low memory to high memory. Two registers are used because the

queues get pushed from one end and pulled from the other. One address register contains

the put pointer; the other register the get pointer. To implement growth of the queue from low

memory to high memory, use the put address register to put data into the queue and the get

address register to get data from the queue.

After a put operation, the put address register points to the next available space in the

queue; the unchanged get address register points to the next item to be removed from the

queue. After a get operation, the get address register points to the next item to be removed

from the queue; the unchanged put address register points to the next available space in the

queue. .

To implement the queue as a circular buffer, the relevant address register should be checked

and adjusted. If necessary, do this before performing the put or get operation. Subtracting

the buffer length (in bytes) from the register adjusts the address register. To implement

growth of the queue from high memory to low memory, use the put address register indirect

to put data into the queue and get address register indirect to get data from the queue.

BOTTOM OF STACK

LOW MEMORY

TOP OF STACK

(FREE)

HIGH MEMORY

GET (Am) +

PUT (An) +

HIGH MEMORY

LOW MEMORY

(FREE)

LAST GET (FREE)

NEXT GET

LAST PUT

1 2 ... 65 66 67 68 69 70 71 72 73 74 75 ... 645 646

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