Specifications

TURBOchannel Bus Support
12.2 TURBOchannel on DEC 3000 Model 500
12.2.3.3 DEC 3000 Model 500 TURBOchannel DMA
This section summarizes and describes the OpenVMS AXP routines that support
DEC 3000 Model 500 map register usage.
The TURBOchannel architecture defines a 34 bit DMA address, however, DEC
3000 Model 500/TURBOchannel implements a 30 bit DMA address. DEC 3000
Model 500 restricts DMA bursts on the TURBOchannel to be 128 longwords or
less. A DMA burst must not crossa2KBaddress boundary. DEC 3000 Model
500 implements a set of map registers (also called a Scatter/Gather map) that
can be used to direct DMA transfers to anywhere in DEC 3000 Model 500 system
memory. Use of the Scatter/Gather map for DMA can be enabled on a per-slot
basis (described later). DEC 3000 Model 500 refers to ‘‘physical DMA’’ as DMA
that does not use the Scatter/Gather map, and ‘‘virtual DMA’’ as DMA that uses
mapping information from the Scatter/Gather map.
12.2.3.4 Physical DMA
In physical DMA, the TURBOchannel option emits a 30 bit DMA address on the
TURBOchannel (it may emit more bits, but DEC 3000 Model 500/TURBOchannel
only implements 30 bits). This address is passed directly to system memory.
The pages of the system memory buffer to which the DMA is directed must be
physically contiguous. As mentioned, an individual DMA burst may not exceed
128 longwords in length, and may not crossa2KBaddress boundary.
12.2.3.5 Virtual DMA
In virtual DMA, part of the DMA address from the TURBOchannel option is used
to access the Scatter/Gather map. The Scatter/Gather map entry provides the
‘‘physical page number’’, which is appended to the byte offset from the option
DMA address to form the address that is sent to system memory. The format of
a Scatter/Gather map entry and a diagram of the address translation is shown in
Figure 12–8.
Figure 12–8 Scatter/Gather Map Entry
0
ZK−6719A−GE
23 22 21 20 4 3
unusedV F P physical page number
Bit 23 − Valid bit, indicates entry is valid
Bit 22 − Funny bit, used by diagnostics
Bit 21 − Parity bit, protects entire entry
Bit 20:4 − Physical Page Number
Scatter/Gather Map entry
The Scatter/Gather Map is used to translate a TURBOchannel DMA address as
shown in Figure 12–9.
12–9