4-5
I/O System
J Class Technical Reference
Parallel DMA Register Map
Table 4–2 shows a parallel DMA register map.
Table 4–2. Parallel DMA Register Map
DMA Controller Register Map
Address
Type
Size
(Bytes)
Description
*
drst+000
write only
1
DMA Reset Register
**
dma+000
read/write
1
Current Address Register
dma+001
read/write
1
Current Count register
dma+008
read only
1
Status Register
dma+00A
write only
1
Write single mask bit
dma+00B
write only
1
Mode register
dma+00C
write only
1
Clear byte pointer
dma+00D
write only
1
Master Clear
dma+00E
write only
1
Clear Mask register
dma+00F
read/write
1
Mask register
dma+010
read/write
1
FIFO limit register (not used)
dma+087
read/write
1
Current Address low page register
dma+401
read/write
1
High Current Count register
dma+40A
read/write
1
Interrupt Pending register
dma+487
read/write
1
Current Address High Page register
*
drs = 0XFFD03000
**
dma = 0XFFD01000
How Parallel Port DMA Works
The J Class Parallel Port DMA controller transfers data from memory to the parallel port without
disturbing the CPU until the transfer sequence is complete. To start a sequence, the DMA channel needs
to have a beginning address and byte count placed into the proper registers. Given that the mode (read or
write) is set up properly, DMA will start once the mask bit is reset. After the sequence is complete, an
interrupt will happen, the mask bit will be set, and the address and count registers will be at their final
value. This controller does not support chaining, so after each sequence the count and address registers
need to be reinitialized to their starting values.
The DMA controller does transactions by arbitrating for the bus, reading one 32-bit word, giving up the
bus, and then handshaking each needed byte out to the parallel device. The DMA controller never
produces multi–word GSC transactions or writes to memory.
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