md89mch@cc.brunel.ac.uk (Martin Howe) (05/25/90)
In article <1990May22.203656.1091@xavax.com> alvitar@xavax.com (Phillip Harbison) writes: >Binary compatibility might be a worthwhile goal, since it would allow >the software from the previous generation to run on the new machine. >However, none of the 16-bit processors maintained binary compatibility, >so what is gained by propagating an 8080-like architecture? >Nothing, in my opinion. ^^^^^^^^^^^^^^^^^^^^^^ I agree; the problems it brought are worse than the benefits which, it is apocryphally held, are that it made code conversion easier (8080/5 -> 8086). There were automatic translators for this. YUKK ! In article <54810@microsoft.UUCP> markha@microsoft.UUCP (Mark HAHN) writes: >segmentation: >it's a problem because: (1) loading a segment register takes ~10 times >as long as a 'normal' register load, even across all family members. Granted, but with 2**32 byte segments it should not be as necessary to change segments. It's a problem on the 286 and below, though, and it doesn't help with supervisor calls (change of CS and maybe everything else). >the 286: it also seems just plain perverse to stick the ring number >(and a tag bit) in the low bits of each selector; maybe Intel was waging a war >against data larger than 64K. Descriptors are 8-byte objects. Hence address within [LG]DT of descriptor associated with segment register value nnnn is (nnnn & $FFF8) - no shifting or adding ! The "tag bit", officially called Ti selects GDT or LDT - table index. >part of me mourns the passing of segments, because it's obvious that >they are a big part of the influence the 432 exerted. (of course, >this only applies to the chips that have protection, the 286 and sucessors. >I wonder if anyone will ever attempt to do an object/capability system >with the heaps of old 286's out there. if someone finds a niche, the >hardware is incredibly cheap. the thought of hundreds of AT motherboards >wired together brings tears to my eyes, though, both ways! I would like to have seen some sort of capability based x86 (x > 4) chip using segmentation as a basis. However, you'd probably need 32-bit segment names and it would clutter up the 86-chips even more. Hence the i960XA, which seems to have died. Oh well. -- - /| . . JCXZ ! MOVSB ! SGDT ! iAPX ! | There are many kinds of revenge,but \`O.O' . Martin Howe, Microelectronics| none so sweet as Cats revenge ! ={___}= System Design MSc, Brunel U. | - Hodge ` U ' Any unattributed opinions are mine -- Brunel U. can't afford them.