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VMEchip2
2
2-52
A maximum of 4GB of data may be transferred with one DMAC
command. Larger transfers can be accomplished using the command
chaining mode. In command chaining mode, a singly-linked list of
commands is built in local memory and the table address register in the
DMAC is programmed with the starting address of the list of commands.
The DMAC control register is programmed and the DMAC is enabled. The
DMAC executes commands from the list until all commands are executed
or an error is detected. When the DMAC stops, the status bits are set in the
DMAC status register and an interrupt is sent to the local bus interrupter.
If the DMAC interrupt is enabled in the local bus interrupter, the local bus
is interrupted. When the DMAC finishes processing a command in the list,
and interrupts are enabled for that command, the DMAC sends an interrupt
to the local bus interrupter. If the DMAC interrupt is enabled in the local
bus interrupter, the local bus is interrupted.
The DMAC control is divided into two registers. The first register is only
accessible by the processor. The second register can be loaded by the
processor in direct mode and by the DMAC in command chaining mode.
Once the DMAC is enabled, the counter and control registers should not
be modified by software. When you use the command chaining mode, the
list of commands must be in local 32-bit memory and the entries must be
quad-byte aligned.
A DMAC command list includes one or more DMAC command packets.
A DMAC command packet includes a control word that defines the
VMEbus AM code, the VMEbus transfer size, the VMEbus transfer
method, the DMA transfer direction, the VMEbus and local bus address
counter operation, and the local bus snoop operation. The format of the
control word is the same as the lower 16 bits of the control register. The
command packet also includes a local bus address, a VMEbus address, a
byte count, and a pointer to the next command packet in the list. The end
of a command is indicated by setting bit 0 or 1 of the next command
address.
Table 2-3
shows the command packet format.
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