Specifications

DMA Controller (V40)
OVERVIEW
This chapter describes the Direct Memory Access Control Unit
(DCU) and provides register descriptions.
The DCU is a programmable peripheral device used to direct high
speed data transfers between the ZT 8832 and SBX expansion module
I/O. The approximate data rate between the expansion module and
local RAM using standard input and output instructions is 235 Kbytes
per second. Using DMA, the data transfer rate is increased to approxi-
mately 1.3 Mbytes per second, for a performance increase of over 80
percent. DMA transfers to dual port RAM are slightly slower because
of arbitration time. If no dual port RAM accesses are made by the
STD bus CPU during the DMA operation, the data transfer rate is
approximately 1.1 Kbytes per second.
There are two primary reasons for this performance increase. The first
is that the DCU requires no instruction fetches to perform the data
transfer. All address manipulations are done with hardware internal to
the DCU, as opposed to the CPU having to fetch and execute the
address adjustment instructions. The second reason is that the DCU
transfers the data in one CPU machine cycle, rather than two. The
DCU allows data to travel from the source device through to the
destination device without temporarily storing it in a register, as is
done with the CPU.
The DCU has the following features:
• 1 Mbyte of direct memory addressability
• Single, demand, and block data transfers
• Autoinitialization
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