When you use an IBM SDLC adapter for an X.25 link, a Direct Memory Access
(DMA) error appears in the connection trace in response to the SET LINK
This warning message is being displayed because the SDLC and X.25 link
services always ask the adapter driver to use DMA. If the driver has no DMA
support, or can't provide full duplex DMA, it returns a warning to the link
service in the form of a return code to the SET LINK CHARACTERISTICS
command. The link services log this return code at level 6 and continue.
IBM SDLC adapters have only one DMA channel. X.25 is a full duplex
protocol, which means that data is normally being both transmitted and
received at the same time. This would require two DMA channels on the
adapter to use it in DMA mode. Therefore, since DMA can't be used, the
driver falls back to using character interrupts when the protocol requires
simultaneous send/receive. In practice, per-character interrupts can be
sustained at 9600 baud, so this warning does not necessarily indicate a
Direct Memory Access (DMA) is a hardware feature that permits adapters to
write directly to memory rather than requesting that the CPU transfer the
data. A non-DMA adapter must interrupt the processor with each byte of
information, whereas a DMA adapter places the information in a predefined
memory location and interrupts the processor when a complete block has been
transferred. An adapter operating in DMA mode is less demanding on the CPU.
In the case of SDLC connections, a PC AT type machine can support DMA to
the SDLC adapter, or to one of the MPCA adapters. The PS/2 can support DMA
to both adapters.