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080 manufacture's diagnostic checkpoint
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081H-08fH DMA page registers.
On the AT, all 8 bits of the Page registers are
used. They become the high 8-bits of a 24-bit address space (with the
low 16-bits being set in a channel's base/current address register).
The page size is 128K (64K words) so DMA transfers must not cross a 128K
boundary (eg, address 2000:0, 4000:0, 6000:0, etc.)
081H Channel 2 page register (address bits 16-23)
082H Channel 3 page register (address bits 16-23)
083H Channel 1 page register (address bits 16-23)
087H Channel 0 page register (address bits 16-23)
089H Channel 6 page register (address bits 17-23)
08bH Channel 5 page register (address bits 17-23)
08aH Channel 7 page register (address bits 17-23)
08fH refresh
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