Motorola MVME172 manual No Address Increment DMA Transfers

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VMEchip2

The DMAC also supports command chaining through the use of a singly- linked list built in local memory. Each entry in the list includes a VMEbus address, a local bus address, a byte count, a control word, and a pointer to the next entry. When the command chaining mode is enabled, the DMAC reads and executes commands from the list in local memory until all commands are executed.

The DMAC can be programmed to send an interrupt request to the local bus interrupter when any specific table entry has completed. In addition the DMAC always sends an interrupt request at the normal completion of a request or when an error is detected. If the DMAC interrupt is enabled in the DMAC, the local bus is interrupted.

To allow increased flexibility in managing the bus tenure to optimize bus usage as required by the system configuration, the chip contains control bits that allow the DMAC time on and off the bus to be programmed. Using these control bits, software can instruct the DMA Controller to acquire the bus, maintain mastership for a specific amount of time, and then, after relinquishing it, refrain from requesting it for another specific amount of time.

No Address Increment DMA Transfers

During normal memory-to-memory DMA transfers, the DMA controller is programmed to increment the local bus and VMEbus address. This allows a block of data to be transferred between VMEbus memory and local bus memory. In some applications, it may be desirable to transfer a block of data from local bus memory to a single VMEbus address. This single VMEbus address may be a FIFO or similar type device which can accept a large amount of data but only appears at single VMEbus address. The DMA controller provides support for these devices by allowing transfers without incrementing the VMEbus address. The DMA controller also allows DMA transfers without incrementing the local bus address, however the MVME172 does not have any onboard devices that benefit from not incrementing the local bus address.

The transfer mode on the VMEbus may be D16, D16/BLT, D32, D32/BLT or D64/BLT. When the no increment address mode is selected, some of the VMEbus address lines and local bus address lines continue to increment in some modes. This is required to support the various port sizes and to allow

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