8086 (was Re: more talking to the press.)

Am 14 Nov 2003 5:45 meinte Eric Smith: So MMU is still external, like it could be to _any_ CPU, and not realy an architectural feature. Depends what you mean wit 'dealing' if it's load them with the proper handle, then yes, think of it as a hardware shortcut to a set of OS calls. For example if you have two data blocks and want to copy data between them, you may eiter map them in in sequence via an OS call, and do a double move, or map both in (via OS calls) and then copy ... of course after recalculateing the addresses, since the blocks are now moved within the address space ... of course you may always use some kind of base and offset calcualtion to get an effective address ... all in additional instructions to be programmed - just to get the same result? Aw, come on. /360 with virtual extension - don't ask me about the model, the similar Siemens version has been the 4004-220 Now your mixing up design and implementation. The design of segments operates with 64K possible segments with 64K max size each, while the implementation on the 8086 did only use a 20 Bit address bus and thus only allowed 16 independent _maxed_out_ segments. This is why I said Mac, and not 68k, it's the Mac memory management that uses so called 'Handles' to refer to relocable blocks in Heap. While they are in fact just a pointer to a pointer, no Mac programm should (and can) assume anything from just looking at the handles content. And exactly the same view a programm should maintain at segment values. See above. Of course if one always looks at the exact hardware implementation, and if one always does the stupid segment x 16 + ofset calcualtion, one will never get the abstract view. This sometimes is in an operating system made to fit the 8086 an extreme reare case and only needed within Memory management (*1), but 99.999% of all programmers will never have to think about it. I did a whole lot of DOS and Windows programming, 100% assembly, and have never ever needed to compare pointers (*2). So, if these pointer compares have not been uncommon, I maybe lived in a different world... an island of happyness (Munich is spacial, but not that special :) Gruss H. *1 I wrote once a little reale time system under DOS (in fact it was the other way around, I just used DOS as loader, then took over and let it run as a task within my system :). Memory management was the core to it, and the segments where the key. It was simple, elegant and extremly fast - basicly it's been a little application and dial in server for a mainframe system, I was able to handle 64 19,200 Bd lines, 4 mainframe connections and a bunch of printer ques simultanious on an 8 MHz 286 (It was also a real mode application, so only 8086 code, none of the 286 features where used, since the initial development has been on a 186 machine) and still sitting around idle most of the time. On a 12 MHz 386 I could even run the app in 100% trace mode via the integrated self trace (I some of the features of the 386 are extremly nice) - I was looking for a memory problem, where I suspected a segment problem (jep, even I did distrust them), just when traceing itself and monitoring each segment related operation, the problem did not happen, so I let the production machines just trace themselves all the time - after weeks of additional investigation (inofficial, since the Problem was closed) I found out that a timeing for a third party hardware was just randomly not working with 12 MHz ISA bus speed :) *2 The rule is quite simple, if the segment (aka handle) is not the same, they are not equal. -- VCF Europa 5.0 am 01./02. Mai 2004 in Muenchen http://www.vcfe.org/