Motorola MC68030 User Manual page 86

•
2-40
To implement the queue as a circular buffer, the relevant address register
should be checked and adjusted, if necessary, before performing the "put"
or "get" operation. The address register is adjusted by subtracting the buffer
length (in bytes) from the register.
To implement growth of the queue from high to low memory, use:
- (An) to put data into the queue,
- (Am) to get data from the queue.
After a "put" operation, the "put" address register points to the last item
placed in the queue, and the unchanged "get" address register points to the
last item removed from the queue. After a "get" operation, the "get" address
register points to the last item removed from the queue, and the unchanged
"put" address register points to the last item placed in the queue. This is
illustrated as:
LOW MEMORY
(FREE)
PUT -(An)
--..
LAST PUT
7
•
7
L
•
L
•
NEXT GET
GET -(Am)
--.
LAST GET (FREE)
HIGH MEMORY
To implement the queue as a circular buffer, the "get" or "put" operation
should be performed first, and then the relevant address register should be
checked and adjusted, if necessary. The address register is adjusted by adding
the buffer length (in bytes) to the register contents.
MC68030 USER'S MANUAL
MOTOROLA