Specifications

4.4.3 DMA Controller
The V40 CPU has four independent DMA channels. They provide full 2O-bit address regis-
ters and 16-bit byte count registers. In addition the V40's DMA controller features:
• Three types of transfer modes: single byte, demand, and block.
• Two types of bus access priority modes: release and hold.
• Two types of channel access priority: fIXed and rotating
• Automatic channel reinitializing.
The modes and methods used to program the V40's
DMA controller are described in detail in
the V40's
User's Manual.
Although
the V40 DMA controller's 2O-bit address registers are useful in directly accessing
the system's full memory space, this and other slight differences from a standard PC's 8237A
DMA controller would tend to result in incompatibilities with PC software that attempts to
directly access the DMA controller's internal registers.
To make the Little Board/PC's DMA "compatible", special hardware (in ASICl) has been
included, which masks the differences between the V40's DMA controller and the 8237A
DMA controller used on a standard PC. This "8237A emulation" is completely transparent to
software, with
the result that software that runs on PC's and compatibles can run without
modification
on the Little Board/PC.
8237 A EmulatiOn
To accommodate the board's 8237A emulation. the V40's DMA controller registers are actu-
ally located at unique I/O port addresses, rather than at the I/O addresses normally occupied
by the
PC's 8237A DMA controller (see Chapter 5). The 8237A emulation logic recognizes
CPU accesses to the DMA controller and issues an NMI (non-maskable interrupt). The
board's PC compatible ROM-BIOS detects when NMI's are generated by access to the DMA
controller (as opposed to other NMI sources) and converts the 8237A-like DMA controller
software sequences into proper V40
DMA controller accesses.
DMA Channel Mapping
Because the V40's DMA Channel 4 shares several pins with its internal serial port, the two
functions cannot both
be used simultaneously. On the Little Board/PC, the serial port option
rather than the
DMA Channel 4 option is implemented. Therefore only channels 0, I, and 2
of the V40's DMA controller are available for use in system I/O. Similarly, the standard PC's
8237A DMA controller provides four DMA channels, but only three of these are available for
use in system
I/O because DMA channel 0 is used for DRAM refresh.
Consequently,
to provide PC DMA channel compatibility, the Little Board/PC's 8237A
emulation logic and software perform a second function. which
is to remap the three available
V40
DMA channels (0,1, and 2) into the three required PC DMA channel assignments (I, 2,
and 3). This is illustrated in Table 4-1.
4.£