WINOOWS
BLOCK MODE ADDRESS ING
As mentioned earlier, in a block mode transfer the IP determines the displacement of a transf€r inbo the windowed object b¥ means of its on-chip displacement counter. Unlike random mode, then, the object displacement is independent of the subrange displacement. This gives rise to two addressing techniques that may be used by the Peripheral Subsystem in block mode: swept and source/sink.
In swept addressing, the Peripheral-Subsystem bus master driving the transfer operation "sweeps" serially (fran 100 addresses bo high) through a block of addresses in the windowed subrange. That is, the address references will be n, n+l, n+2 ••• or n, n+2, n+4 ••• for 8- and l6-bit Peripheral Subsystem buses respectively. The range of PS addresses swept is equal bo the number of bytes transferred, so the subrange must be at least as large as the number of bytes transferred. Figure 3-5 illustrates swept addressing in a block mode write operation.
In source/sink addressing, the master driving the transfer repeatedly addresses a single location in the windowed subrange. For a read operation, this single (byte or double-byte) location acts as a data source; for a write operation, the location serves as a data sink. By permitting the transfer of large blocks (up to 64K bytes) of data through a single location, source/sink addressing conserves "subrange space." To transfer 32K bytes in randan mode requires setting up a 32K byte subrange, leaving only half of the IP's range available for concurrent use with other windows. Only a byte or Cbuble-~teof the range is needed bo perform the same transfer in block mode using source/sink addressing. Figure 3-6 shows hoo source addressing works in a block mode read operation.
Note that the IP has no knowledge of the addressing technique used in a block mode transfer. It s~ly considers any address reference in window 0' s subrange as a signal to transfer the next b¥te or double-byte.
