Zilog Z08470 Uživatelský manuál stáhnout pdf (Strana 67)

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UM008101-0601 Direct Memory Access

Figure 19. Basic Functions of the Z80 DMA

Modes of Operation

Within any class of operation, the Z80 DMA can be programmed to operate

in one of three Transfer and/or, Search modes:

Byte Mode

Data operations are performed one byte at a time. Between each byte oper-

ation the system bus is released to the CPU. The bus is requested again for

each succeeding byte operation. This is also sometimes called Single mode

or byte-at-a-time mode.

Burst Mode

Data operations continue until a port’s Ready line to the DMA goes

inactive. The DMA then stops (releases the system bus) after completing its

current byte operation. This is also called demand mode.

Continuous Mode

Data operations continue until the end of the programmed block of data or a

stop-on-match condition is reached before the system bus is released. If a

DMA I

I/O

1. Transfer Memory-to-I/O (optional search)

2. Transfer I/O-to-Memory (optional search)

3. Transfer Memory-to-Memory (optional search)

4. Transfer I/O-to-I/O (optional search)

5. Search Memory

6. Search I/O

Memory

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