Path: utzoo!attcan!telly!moore!eastern!egsgate!Uucp From: peter.da.silva.Of.250/401@p402.f401.n250.z1.FidoNet.Org (peter da silva Of 250/401) Newsgroups: comp.arch Subject: Re: Segmented Architectures ( formerly Re: 48-bit computers) Message-ID: <671521507.0@egsgate.FidoNet.Org> Date: 8 Apr 91 20:54:10 GMT Sender: Uucp@egsgate.FidoNet.Org Lines: 100 In article <1991Apr06.030330.1533@kithrup.COM>, sef@kithrup.COM (Sean Eric Fagan) writes: > In article peter@ficc.ferranti.com (Peter da Silva) writes: > >In article <1991Apr04.230953.15294@kithrup.COM>, sef@kithrup.COM (Sean Eric Fagan) writes: > >No, I didn't miss the point. If I have a 48 bit wide VM address and can't > >operate on any object larger than a 32 bit wide pointer can address, then > >it's a problem. > Why is everybody harping on this 48 bits wide? This subject started with the idea of using segments to expand the address space of 32-bit computers. Many of us are still thinking along those terms. > I actually never said > anything about how wide the address space, except that I implied that it was > at least 32 bits. In fact, given the machine I proposed, it would still > work quite well with only a 32-bit address space. In fact, it would only work well with a 32-bit address space (or maybe 33 or 34 bits). Once you get much more address space available to a program than you can stick into a single object you will run into problems: that's the lesson of the 8086. The reverse situation is not a big deal: that's the lesson of the 68000. > And that's the difference > between the flast and the segmented: using the segmented version (which, as > I pointed out two articles ago, can imitate, if slowly, a flat-address-space > machine), I can hide the fact that I only have 32-address bits, virtual or > otherwise. My Amigas both have a flat address space, but one has 32 address bits and the other 24. Apart from some trash software written by Microsoft, there is no difference that the program has to deal with. > Please read that agin, peter. You assumed, incorrectly, that I had more > than 32-bit for addressing. You assumed that, if I cannot allocate a 5Gbyte > object, things are broken. So what's the advantage to segments? > >Now, here's the important question: why is the 64K object size limitation > >in the IBM-PC a problem? > Peter: not all people who run into memory problems need more than 4Gbytes > for a single object. And not all people who run into memory problems need more than 64K for a single object: that's one reason why ints in most IBM-PC C compilers are only 16 bits wide. For people who don't, they will never now that the machine can access more than 64K, assuming, of course, it can. Since ints are only 16 bits, there is no way to specify the size of an object longer than 64K (that's why I was talking abouE cz8V9.RTuDnV GDMK]xM1ZtD%VMwa:TQM0``CO$aVK >4w|vnNQQ-X60$QB[Be$V {GDUwfV0D*6/12i1iYF\(2q|LusFQwXj${D#Ou4&7t8`a8TAIueq Y&W|7I,;T2P@PpM[]hNq?Mq*xf(}TAwHAQPD aTi Qzh*%RM#34P Z2Bv P u(!O=!$d%iTd9T`r Jl@C4Phz C]V0M=eJK\dn)$ Qa WX3M hP>pR2;2 1 N:H\fRD-$dnMY c3X;B=J}C yr&5DMh 0r} /? g$4dM $U?zBp = BX3f`XV+]D! s, s!Np/EbLg0Ie[BJ88 0X Ws&1Ch$uWbL7[ Q%4#4Bo" "& dwJ!->F`pRbRR6z=*Hj!Jh^ C1I"lxH4"+#StHTcrPPU; 3U. G\WD 9.s6iUc?%8%A}^f3s5WA7yjC@AhZl/ ;E/$h5.8GC]k =-X'mS haaZ4,e6*V|Oy S;ZhaF:^ G#mUpe-|J95Xt\ ZM:ZyT4\H? a/T,E'HH\wd`y A8u`dBO)Q+C3E D$8 BU\P7~Q#9T0d=-E!jRd}uB3:QcrE<1jY'1#,X!Q$1W_6C"6C`\T Wdp+A~A$#XkUFA. 3E[Cw%-'H#YWqeQ"7D[U0 &#or,asj2(@%@s-Fq]Xg:43JP R1S(QS' R*)go TmIT=uG#ZUE 3x{6 &dLeq!Xup "MvpP`\c[qB9,%$nrasS<8s a(36+rn@uc\xTH6.t(t9'HQ!vDc`.(R$ x%)pr3p '@EbU26_! ~6;#1gYkO `}2nm1z+t}T@Ux!\rA`P_P'`}d@ ^% |.pP711 73 2pDR1@1tU ^'}gq{8740#ka2yp"@m8 E-P.p(@)0BPT]p gO`>4EpSPQbIz12(q0`94 @AhdRR>j`c &s1&ez&8"o