Date: Fri, 15 Dec 1995 16:30:14 -0800 From: David Greenman <davidg@Root.COM> To: Bruce Evans <bde@zeta.org.au> Cc: CVS-committers@freefall.freebsd.org, cvs-sys@freefall.freebsd.org Subject: Re: cvs commit: src/sys/i386/isa if_ed.c Message-ID: <199512160030.QAA02036@corbin.Root.COM> In-Reply-To: Your message of "Sat, 16 Dec 95 10:43:56 %2B1100." <199512152343.KAA04105@godzilla.zeta.org.au>
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>>>In this case, I think the casts only work because the hardware ignores >>>some bits in the address (bits 14-15 of 0-31 for 16K cards and bits >>>13-15 for 8K cards?). Conversions of the form >>> >>> (int)(buf - base) >>> ((int)buf) & 0x3fff >>> >>>would be more obviously correct, but might be slower. > >> That's not quite correct. The offset within the NIC memory segment is a 16 >>bit number but is written out a byte at a time. The fact that only the lower >>two bytes are written out *implies* a masking of the upper bits. This is both >>intended and correct. I chose to fix the problem the way I did to avoid > >Yes, the top 16 bits are masked by not writing out the bytes for them. >Bits 14-15 aren't masked. Consider a board with 16K of memory address >0xd4000. Then address 0xd4000 corresponds to offset 0 but offset 0x4000 >is written out. This apparently works because bits 14-15 are ignored by >the h/w. It wouldn't work for a board with 32K of memory at address >0xd4000 but this is unlikely to be supported since it would take a gate >or two more to decode. It wouldn't work if the board used bits 14-15 >for something else. To determine the correctnes of the cast, you have >to look at the manuals for all supported boards (a growing list :-) and >and see if those bits are specified as "don't care". No, you apparantly don't understand what's happening in there. In the PIO case, the "base" is 0 and the offset is just an offset relative to zero. The upper bits will never be non-zero. -DG
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