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Z80 Datasheet(PDF) 141 Page - Zilog, Inc. |
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Z80 Datasheet(HTML) 141 Page - Zilog, Inc. |
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141 / 330 page  < %27 2GTKRJGTCNU 7UGT /CPWCN UM008101-0601 Direct Memory Access 5. ENABLE AFTER RETI command 6. ENABLE DMA command • • • 7. RETI instruction Interrupts at end-of-block, for example, might occur when reading a floppy disk. If the disk transfers 128-byte records, the DMA can be made to interrupt at the end of each record to notify the CPU of its completion. Then the CPU can read the destination (memory) address counter to find the last memory location filled. See Table 12 for address-counter contents. A service routine for continuing inputs to contiguous locations of memory typically contains the CONTINUE, REINITIALIZE STATUS BYTE ,and ENABLE DMA commands before the CPU’s return from interrupt. A service routine for shutting down the DMA after the record arrives typically includes DISABLE DMA and REINITIALIZE STATUS BYTE commands. If the DMA transfer is started by an interrupt from another device, the service routine for that other device includes an ENABLE DMA command written to the DMA’s port address. Interrupts on byte match (a search or transfer/search operation) can be implemented so that any ending byte, error indicator, or other character causes the interrupt. This procedure frees the CPU from looking for these characters in a stream of data, increases throughput, and reduces CPU software complexity. For example, the DMA might search for end-of-text (EXT) characters or carriage returns in a communications environment and interrupt the CPU only when the complete message frame has arrived. The service routines for this would be very much like those for interrupts on end-of-block. Interrupts on Ready are somewhat different. First, the DMA cannot be the bus master before the interrupt because the CPU only senses interrupts when the CPU is the bus master (the other types of interrupts are not |
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