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Z80 Datasheet(PDF) 125 Page - Zilog, Inc. |
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Z80 Datasheet(HTML) 125 Page - Zilog, Inc. |
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125 / 330 page  < %27 2GTKRJGTCNU 7UGT /CPWCN UM008101-0601 Direct Memory Access cates that it is never loaded. This special situation is discussed in a later section entitled “Fixed-Address Destination Ports.” If the DMA is in an inactive state (Table 15) when the LOAD command is written, another DMA control byte must precede the LOAD. Any other command, such as DISABLE DMA, serves this purpose. Because LOAD unforces a Forced-Ready condition, the LOAD must precede a FORCE READY command when the latter is used. Continue (D3) This command clears the byte counter to zero but leaves the address counters of both ports with their current contents. Transfers or searches continue from where they left off after an ENABLE DMA command, although the byte count starts over. The CONTINUE command is typically used to transfer several blocks to consecutive locations in memory when it is desirable to know when each block has finished transferring. Specifically, an interrupt at the end of each block may be needed. Use this command rather than a LOAD command to transfer the next block after the interrupt. A new block length can be entered in WR0 in conjunction with the CONTINUE command. If the DMA is in an inactive state (Figure 15) when the CONTINUE command is written, another DMA control byte must precede the CONTINUE. Any other command, such as DISABLE DMA, serves this purpose. Disable Interrupts (AF) The command is used in non-Z80 CPU environments to simulate the Z80 CPU’s automatic interrupt acknowledge to the DMA. When the DMA interrupts a non-Z80 CPU, the CPU writes a DISABLE INTERRUPTS to the DMA early in the service routine. This allows the INT line to go inactive but prevents the DMA from sending subsequent interrupts while |
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