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I/O Handling
147
2. If indirect addressing is used in a JMP or JSM (including any jumps to external symbols
or symbols more than 512 words away from the current instruction, both of which have
implied indirect addressing), then the most significant bit must be set in the address. For
example, instead of -
in an ISH the procedure must be -
3SM (:Sub+100000B),I
Direct Memory Access
(DMA)
Direct memory access (DMA) is a means to exchange entire blocks of data between memory
and peripherals. A block is a series of consecutive memory locations. Once started, the process
is automatic; it is done under processor control, regulated by the interface. Since only the
98032A interface supports DMA, the following discussion is in terms of that interface.
To the peripheral, the DMA operation appears as programmed I/O. The transfer, however, is
actually performed by special DMA hardware. Information regarding the transfer is stored in
the DMA registers for the DMA hardware to use. This information is the select code, the initial
memory location, and the number -of words to be transferred. The memory location register
and the count register are successively adjusted after each word transferred until the transfer is
complete. Upon completion of the transfer, the interface and the DMA hardware stop
automatically.
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