cliffhanger@cup.portal.com (Cliff C Heyer) (05/31/90)
I'm wanting to check my line of thinking... Compared to the 680X0 machines, I found myself thinking that UNIX on the 80X86 architecture might be a bit more complicated because the ROM and VRAM located above 640K. You don't have a clear linear address space even with a 386 in protected mode because the PC architecture hard wires this memory into certain locations. Is this true or do 680X0 machines also have VRAM and ROM at certain memory locations? Also is there any truth to the idea that UNIX is more cumbersome on PC platforms compared to minicomputer-type platforms that have no ROM or VRAM in their address space? Also does UNIX on a PC use the BIOS? Cliff
kaleb@mars.jpl.nasa.gov (Kaleb Keithley) (05/31/90)
In article <30344@cup.portal.com> cliffhanger@cup.portal.com (Cliff C Heyer) writes: > >I'm wanting to check my line of thinking... > >Compared to the 680X0 machines, I found myself >thinking that UNIX on the 80X86 architecture >might be a bit more complicated because the >ROM and VRAM located above 640K. You don't >have a clear linear address space even with >a 386 in protected mode because the PC >architecture hard wires this memory into >certain locations. > >Is this true or do 680X0 machines also >have VRAM and ROM at certain memory >locations? Not only is VRAM and ROM in certain locations on 680x0, but I/O is memory mapped, compared to 80x86 which has a separate I/O bus. While DOS expects ROM and VRAM to be in certain locations, and these are "hard-wired" into certain locations, the 386 architecture allows a great deal of flexibility, the ROM and VRAM can me mapped somewhere else, allowing a contiguous memory segment. QEMM (for DOS) does just this (memory map manipulation that is, not create contiguous memory.) Most(?) 386 PCs are populated with 1MB memory chips, which would overlay the ROM and VRAM locations if it weren't for this ability. >Also does UNIX on a PC use the BIOS? It has been my experience (with ESIX) that UNIX does not use the BIOS. I can only surmise that since the BIOS must work with DOS, it probably does not use 386 code, and probably is not re-entrant; both of which may be a factor for UNIX. Plus UNIX probably has/wants a different calling sequence than that used by the BIOS. Since I don't have kernel source, it's hard to provide a definitive answer to this; perhaps someone within the works at ISC, SCO, or ESIX would care to confirm or deny this? kaleb@thyme.jpl.nasa.gov Jet Propeller Labs Kaleb Keithley
johnl@esegue.segue.boston.ma.us (John R. Levine) (06/01/90)
In article <3900@jato.Jpl.Nasa.Gov> kaleb@mars.UUCP (Kaleb Keithley) writes: >In article <30344@cup.portal.com> cliffhanger@cup.portal.com (Cliff C Heyer) writes: >>Compared to the 680X0 machines, I found myself >>thinking that UNIX on the 80X86 architecture >>might be a bit more complicated because the >>ROM and VRAM located above 640K. >Not only is VRAM and ROM in certain locations on 680x0, but I/O is memory >mapped, compared to 80x86 which has a separate I/O bus. All 386 unices page, which means that they treat RAM as a pool of 4K pages, the real addresses of the pages being nearly irrelevant. The hole in the address space causes a little extra startup complication as the kernel makes the initial free memory list, but as architectural warts go, it's not a big one. Incidentally, 86 machines have a separate I/O space but not a separate I/O bus. There's only one bus, with a line that indicates whether an address should be treated as an I/O or memory address. >>Also does UNIX on a PC use the BIOS? It can't, the BIOS code runs in real mode and unices run in protected mode. The PS/2 series has an ABIOS which runs in 286 protected mode, but I haven't heard of anyone who uses it other than IBM OS/2. Given that AT clones don't have an ABIOS, any protected mode system has to be able to run without an ABIOS, so most of them don't bother to use it even if it's there. -- John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 864 9650 johnl@esegue.segue.boston.ma.us, {ima|lotus|spdcc}!esegue!johnl Marlon Brando and Doris Day were born on the same day.