Date: Tue, 28 Mar 2017 19:53:54 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Julian Elischer <julian@vps1.elischer.org> Cc: Bruce Evans <brde@optusnet.com.au>, Warner Losh <imp@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r316064 - head/sys/boot/i386/boot2 Message-ID: <20170328192226.F1710@besplex.bde.org> In-Reply-To: <alpine.BSF.2.20.1703272315260.73707@vps1.elischer.org> References: <201703272253.v2RMra2L032487@repo.freebsd.org> <20170328141213.T927@besplex.bde.org> <alpine.BSF.2.20.1703272315260.73707@vps1.elischer.org>
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On Mon, 27 Mar 2017, Julian Elischer wrote: > On Tue, 28 Mar 2017, Bruce Evans wrote: > > [...] > >> they have to fit below 640K and a few multiples of 64K are already >> used for buffers). The limit on 8K is mainly a historical mistake. >> A limit of 7.5K simplified booting from 15-sector floppies. 18-sector > > My memory says that the limit of 7.5K is becuase there was only 8k left free > at the front of UFS1 and one sector was used for the boot0 code. That is only a limit if the boot code is in the ffs partition. This causes other problems. It was the default to start the 'a' partition at offset 0, but that was changed 10-15 years ago. I can't find exactly where it is changed. I use an offset of 8192 sectors or 4M on new and repartitioned hard disks. This is again affected by the existence of floppy disks. Floppy disks are usually not partitioned, and don't have space to spare for large boot blocks. Some version of the boot code has to work on small media, and FreeBSD uses the same boot code for all media. This allowed FreeBSD-1 to have a single boot.flp where IIRC Linux had about 100 variations. Small media is not as small as it used to be. Bruce
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