The 640K memory limit of MS-DOS(xtof.info)
xtof.info
The 640K memory limit of MS-DOS
https://www.xtof.info/blog/?p=985
16 comments
"Those were the days."
No they were not. I wasted a huge amount of time getting this to work. I created a golden Novell IPX and IP boot disc. It was published on Cool Solutions so it must have been good. No it wasn't.
No they were not. I wasted a huge amount of time getting this to work. I created a golden Novell IPX and IP boot disc. It was published on Cool Solutions so it must have been good. No it wasn't.
> I wasted a huge amount of time getting this to work.
The more things change the more they stay the same I guess. As Rick Moranis once put it, "Fuck! Even in the future nothing works!".
The more things change the more they stay the same I guess. As Rick Moranis once put it, "Fuck! Even in the future nothing works!".
And you learned a lot of things while doing that... So is it really time wasted?
I did the same. I had smartdrv, and I used as necessary the option that gave an alternative UMB/conventional tradeoff, possibly giving more room for some other driver that was loaded later. I used mscdex, of course, and fiddled with its buffer settings to get the best memory/performance tradeoff. I tried more than ten mouse drivers before finally finding the one with the best EXE size/resident size/acceleration curve tradeoff, so I could play Doom without going mad while still being able to load games that needed 600+K conventional memory. I did all the shit, that I now can't even remember, to get IPX and TCP/IP running, which included whatever thing you did to merge your card driver and the other thing (LSL? ODI? I dunno...) into one EXE or COM or whatever it was. There was some other stuff too, I'm sure of it - even part from that EMM386 option that put the EMS page frame at the end of the HMA, to minimize fragmentation - because I knew all the tricks, deployed them all as required, and in the end my PC had something like 620K conventional memory free with just ordinary HIMEM.SYS and EMM386.EXE. No QEMM necessary.
What use is any of this these days? None, that's what. None at all. Even at the time, most of the good games used DOS4GW, so you only needed 500K free. You'd have to be a proper completist to bother with this stuff. I guess I don't regret it, because it was more fun than trying to install fucking Linux, but overall it really was time wasted.
(One strange thing that's stuck: 25 years on, I still use the US keyboard layout. I never bothered loading KEYB.COM, because that took up nearly 30K, and it hardly seemed worth it when most of the keys were the same... and one day I suppose the wind changed, because here I still am.)
What use is any of this these days? None, that's what. None at all. Even at the time, most of the good games used DOS4GW, so you only needed 500K free. You'd have to be a proper completist to bother with this stuff. I guess I don't regret it, because it was more fun than trying to install fucking Linux, but overall it really was time wasted.
(One strange thing that's stuck: 25 years on, I still use the US keyboard layout. I never bothered loading KEYB.COM, because that took up nearly 30K, and it hardly seemed worth it when most of the keys were the same... and one day I suppose the wind changed, because here I still am.)
> Then I learned about boot disks. And with DOS4/GW all problems were gone.
Actually, I think MS-DOS 6.0 added CONFIG.SYS boot menus, which made it unnecessary to mess around with boot disks (but it didn't help with your dad discovering that you have been messing with the CONFIG.SYS).
Actually, I think MS-DOS 6.0 added CONFIG.SYS boot menus, which made it unnecessary to mess around with boot disks (but it didn't help with your dad discovering that you have been messing with the CONFIG.SYS).
Boot menus were great, I remember having many of them optimized for different games.
Also MEMMAKER.EXE, I spent a lot of time playing with that tool squeezing out the last byte of memory :)
Hah, I love this. You've reminded me of something I completely forgot it existed. Yet, at that point in time, it was almost everything daily computer usage revolved around.
"Actually, I think MS-DOS 6.0 added CONFIG.SYS"
No, config.sys was a thing from day 1. Getting the bloody memory setup on MS stuff right was a nightmare and wasted a lot of time.
No, config.sys was a thing from day 1. Getting the bloody memory setup on MS stuff right was a nightmare and wasted a lot of time.
"...added config.sys boot menus", is what the parent comment says
I have an idea about what I'm on about:
https://www.novell.com/coolsolutions/tools/14053.html
(I'm Jon)
https://www.novell.com/coolsolutions/tools/14053.html
(I'm Jon)
Yeah no I'm sure you do, but you didn't read the comment, he's talking about the native INI-style syntax for boot menus baked into the DOS 6.0 kernel that allows you to specify multiple different CONFIG.SYS sections, and lets you pass an option to your AUTOEXEC.BAT containing the config name so it can finish the job off.
A must for anyone who ran a different config for different scenarios. I often ran without CD-ROM drivers to get a couple K more memory for the pickiest programs.
A must for anyone who ran a different config for different scenarios. I often ran without CD-ROM drivers to get a couple K more memory for the pickiest programs.
But not in this case, the menu system was not there on day 1.
I remember the sound card and the things one had to do to get it working. All the IRQ(?) settings and IRQ is already assigned errors. On top of that DOS games came with their own sound drivers and there were only two options adlib and sound blaster pro (again IIRC).
I think configuring the game and making it work perfectly was more fun and rewarding than playing the game itself sometimes.
I think configuring the game and making it work perfectly was more fun and rewarding than playing the game itself sometimes.
One day I decided to fool around with memory managers (out of boredom). I managed to fubar the MBR or something similar. Free trip to the computer shop, 200$ for restore and backup tape drive (the shop owner made himself a nice sell on a HP Colorado T1000 [still working]).
Similarly I once blew away the partition table and had to manually reconstruct it with the only available tool, MSDOS debug.com and a copy of Peter Norton's book.
I like that you can get in trouble with your dad for this. Smart and naughty at the same time.
Ah, the good old days, where this worked. Amazingly, it even worked later in DESQview.
jmp word 0xffff:0000
jmp word 0xffff:0000
[deleted]
"A solution might have been to switch back and forth from Protected Mode. But entering Protected Mode on the 286 is a one way ticket: if you want to switch back to Real Mode you have to reset the CPU! Bill Gates, who understood very early all the implications, is said to have called the 286 a “brain dead chip” for that reason."
But Extenders like DOS/16M[0] did this transparently fast by having a real-mode process handle DOS interactions.
Given the proliferation of IBM "compatible" BIOSes in PC clones of that era (many not all that "compatible"), DOS/16M did a fantastic job of running on almost all of them. I worked with it a lot when Informix was first ported to extended-memory DOS usage using DOS/16M. There was a large deployment for a branch of the US armed forces using the product.
[0] http://www.tenberry.com/dos16m/faq/basics.html
When DOS/16M was first released, the company name was Rational Systems.
But Extenders like DOS/16M[0] did this transparently fast by having a real-mode process handle DOS interactions.
Given the proliferation of IBM "compatible" BIOSes in PC clones of that era (many not all that "compatible"), DOS/16M did a fantastic job of running on almost all of them. I worked with it a lot when Informix was first ported to extended-memory DOS usage using DOS/16M. There was a large deployment for a branch of the US armed forces using the product.
[0] http://www.tenberry.com/dos16m/faq/basics.html
When DOS/16M was first released, the company name was Rational Systems.
> But Extenders like DOS/16M[0] did this transparently fast
Transparent, yes. Fast, no. To get back to real mode they had to reset the CPU.
There were a couple ways to do this. There was the triple fault technique that phicoh mentioned. I'm not sure how portable that was. A triple fault shuts down the processor. Whether or not that reboots or just sits there halted would depend on the motherboard design.
The official way on the PC AT was to ask the keyboard controller to do it. On the PC AT, the keyboard controller was connected to the reset signal, and had a command to tell it to reset the computer. (The keyboard controller was also used to control the A20 line, to workaround another 286 annoyance).
AT compatibles generally included that behavior in their keyboard controllers for compatibility.
Transparent, yes. Fast, no. To get back to real mode they had to reset the CPU.
There were a couple ways to do this. There was the triple fault technique that phicoh mentioned. I'm not sure how portable that was. A triple fault shuts down the processor. Whether or not that reboots or just sits there halted would depend on the motherboard design.
The official way on the PC AT was to ask the keyboard controller to do it. On the PC AT, the keyboard controller was connected to the reset signal, and had a command to tell it to reset the computer. (The keyboard controller was also used to control the A20 line, to workaround another 286 annoyance).
AT compatibles generally included that behavior in their keyboard controllers for compatibility.
Transparent, yes. Fast, no. To get back to real mode they had to reset the CPU
A modern adware-laden web page on gigabit-speed Internet is dog slow in comparison.
On a 286 you do not have access to real mode when in protected mode. You need a 386 for that.
A crazy trick that did work is to cause a triple fault in protected mode. That would reset the 286. Then there was a little bit of support from the BIOS. You could tell the BIOS to not run the power on self tests and load the MBR from disk. Instead the BIOS would jump to a specific memory location.
A crazy trick that did work is to cause a triple fault in protected mode. That would reset the 286. Then there was a little bit of support from the BIOS. You could tell the BIOS to not run the power on self tests and load the MBR from disk. Instead the BIOS would jump to a specific memory location.
> On a 286 you do not have access to real mode when in protected mode. You need a 386 for that.
I think that you are mistaken: I am rather sure that the undocumented LOADALL instruction could make something like that possible:
> https://en.wikipedia.org/wiki/LOADALL
I think that you are mistaken: I am rather sure that the undocumented LOADALL instruction could make something like that possible:
> https://en.wikipedia.org/wiki/LOADALL
Update: Indeed it is possible to use LOADALL to switch from protected mode to real mode:
> http://www.os2museum.com/wp/loadall-strikes-again/
Article found via https://retrocomputing.stackexchange.com/a/220/4889
> http://www.os2museum.com/wp/loadall-strikes-again/
Article found via https://retrocomputing.stackexchange.com/a/220/4889
No it is not, the article was later corrected.
Correct link BTW: http://www.os2museum.com/wp/will-the-real-real-mode-please-s...
If I understand it correctly it uses Virtual Mode which was added in 386. In FAQ question about speed it also mentions 386 onward[1]. I don't think DOS/16M contradicts anything said there.
[1] http://www.tenberry.com/dos16m/faq/basics.html#10
[1] http://www.tenberry.com/dos16m/faq/basics.html#10
And virtual 8086 mode is still supported on modern Linux kernels! (Only 32-bit kernels, though. AMD decided to prevent it from being used by a 64-bit kernel.)
v8086 mode, like many aspects of the x86 architecture, is a big mess.
v8086 mode, like many aspects of the x86 architecture, is a big mess.
I used DOS/16M extensively on 286 (one specific example was my home Zenith AT at the time, a 80286 at 6MHz with 0 wait states). This was with all but 512K on an Intel AboveBoard in the backplane.
"Windows 95 made those memory headaches a thing of the past as MS-DOS became progressively irrelevant."
Hmmm... The key word is "progressively". Windows 95 was still an extender started from DOS and using its file access subsystem.
I clearly remember I made some tweaks so I started the computer with DOS and could choose between launching Windows 3 or launching Windows 95 from the command line. And after shuting down Windows, it returned to DOS.
The default was to launch Windows 95 (from autoexec.bat?) and powering down the machine when shutting down 95, hiding the fact that DOS was still there.
One note: every time I have told that story, someone insisted that 95 wasn't just eye candy. Of course it wasn't, it switched to protected mode so DOS wasn't active. But the file subsystem was not different and it showed.
Hmmm... The key word is "progressively". Windows 95 was still an extender started from DOS and using its file access subsystem.
I clearly remember I made some tweaks so I started the computer with DOS and could choose between launching Windows 3 or launching Windows 95 from the command line. And after shuting down Windows, it returned to DOS.
The default was to launch Windows 95 (from autoexec.bat?) and powering down the machine when shutting down 95, hiding the fact that DOS was still there.
One note: every time I have told that story, someone insisted that 95 wasn't just eye candy. Of course it wasn't, it switched to protected mode so DOS wasn't active. But the file subsystem was not different and it showed.
Windows 95 used "32-bit file access" (an option in Windows 3.1) by default. Which meant it had its own file drivers and was, in effect, a full 32-bit OS chain-loaded from DOS. However, in order to keep the "DOS box" working and make sure DOS knew about all the changes to the file system, Windows 95 had to update the DOS structures in memory also. Which made it an even bigger mess of hacks than we realized.
> Windows 95 was still an extender started from DOS and using its file access subsystem.
True, but sort of beside the point for this article. Using DOS as the equivalent of a bootloader and being too lazy[1] to write a proper block device layer doesn't really change the fact that win32 applications were run in 386 protected mode with their own 4G memory space.
[1] It's not actually insane in hindsight. Hard drive integration in the early 90's was a huge mess, with vendors putting all sort of crazy junk on their boards and gluing it together with BIOS hackery. The days of pervasive standards-compliant ATA were still in the future.
True, but sort of beside the point for this article. Using DOS as the equivalent of a bootloader and being too lazy[1] to write a proper block device layer doesn't really change the fact that win32 applications were run in 386 protected mode with their own 4G memory space.
[1] It's not actually insane in hindsight. Hard drive integration in the early 90's was a huge mess, with vendors putting all sort of crazy junk on their boards and gluing it together with BIOS hackery. The days of pervasive standards-compliant ATA were still in the future.
> file subsystem was not different
Windows 95 used its own 32-bit file system code whenever possible:
https://en.wikipedia.org/wiki/32-bit_file_access
It had to fall back to call BIOS only in special cases, as a backward compatibility for some older hardware/drivers. So even if you would launch it from DOS, it would for most newer hardware be just a bootstrap phase.
Windows 95 used its own 32-bit file system code whenever possible:
https://en.wikipedia.org/wiki/32-bit_file_access
It had to fall back to call BIOS only in special cases, as a backward compatibility for some older hardware/drivers. So even if you would launch it from DOS, it would for most newer hardware be just a bootstrap phase.
My memories about that are blurry but I remember there was some general problem with file access that persisted until XP adopted NT core for consumer versions. Maybe something about concurrency, not sure, sorry.
Windows 95 didn't have concurrency problems different from NT, unless you used it on the old hardware that depended on DOS drivers. The main benefits NT brought were various protection mechanisms, which is convenient for programmers, but 95 surely wasn't slower, especially with less RAM, the major factor that slowed the NT adoption was its significantly higher RAM needs. Which translated in visibly more expensive computers needed. NT was also slower for all graphical routines for a while:
https://en.wikipedia.org/wiki/Architecture_of_Windows_NT
"The Windows NT 3.x series of releases had placed the GDI component in the user-mode Client/Server Runtime Subsystem, but this was moved into kernel mode with Windows NT 4.0 to improve graphics performance.[23]"
Note NT 4.0 appeared a year after Windows 95. The programmers had the benefit of using Windows NT even before 95 appeared, but the advantages of 95 were clear for the "normal users" who didn't think they needed the "protection" aspects: the most popular software of the time (e.g. Microsoft Word) was unstable enough to, from time to time, crash even on NT. The more stable Word versions appeared only after the famous Gates' 2002 memo started to be applied to the Microsoft Office products:
https://en.wikipedia.org/wiki/Trustworthy_computing#Microsof...
Windows 95 was for a time a good trade-off for many practical uses.
https://en.wikipedia.org/wiki/Architecture_of_Windows_NT
"The Windows NT 3.x series of releases had placed the GDI component in the user-mode Client/Server Runtime Subsystem, but this was moved into kernel mode with Windows NT 4.0 to improve graphics performance.[23]"
Note NT 4.0 appeared a year after Windows 95. The programmers had the benefit of using Windows NT even before 95 appeared, but the advantages of 95 were clear for the "normal users" who didn't think they needed the "protection" aspects: the most popular software of the time (e.g. Microsoft Word) was unstable enough to, from time to time, crash even on NT. The more stable Word versions appeared only after the famous Gates' 2002 memo started to be applied to the Microsoft Office products:
https://en.wikipedia.org/wiki/Trustworthy_computing#Microsof...
Windows 95 was for a time a good trade-off for many practical uses.
"It is now safe to turn off your computer"
The days before ACPI.
The days before ACPI.
That OS/2 2.0 debacle is one of my favorite topics.
Good post, but believe me, you can write a post without ending every paragraph with an exclamation mark. It's possible, but only if you try!
Intel made the paragraph size of the x86 16 bytes, or four bits of the physical address. The documentation I read at the time claimed that this was to make sharing pieces of segments easier (or perhaps they wanted to encourage people to make zillions of very small segments, e.g., one per procedure).
I think that was simply done without much thought and then post-facto justified by the hardware engineers. They couldn't afford another 4 pins of physical address space, so they effectively made that pin limitation architectural.
If the paragraph size had been 256 bits (just shift another four bits) then the x86 would have had a logical address space of 16MB, just like the 68K at the time. This probably would have changed the face of the personal computing industry (no 640K limit, for starters).
I think that was simply done without much thought and then post-facto justified by the hardware engineers. They couldn't afford another 4 pins of physical address space, so they effectively made that pin limitation architectural.
If the paragraph size had been 256 bits (just shift another four bits) then the x86 would have had a logical address space of 16MB, just like the 68K at the time. This probably would have changed the face of the personal computing industry (no 640K limit, for starters).
I wasn't there, but building entire systems from zillions of small procedures sounds familiar. Wasn't this a guiding principle behind languages such as SmallTalk and Forth? The first system to be implemented on 8086 was Forth with its structure of loading code from 1K blocks.
But I can see the chicken-and-egg here. Did languages at the time think in terms of small contained blocks of code because the hardware they ran on was limited? Or did CPU engineers restrict their designs to match the segmented demands of the programming languages?
But I can see the chicken-and-egg here. Did languages at the time think in terms of small contained blocks of code because the hardware they ran on was limited? Or did CPU engineers restrict their designs to match the segmented demands of the programming languages?
I've wondered if the 16 byte paragraph size came from prejudices of engineers who were involved with the iAPX 432, an "object oriented" processor that was canceled after Intel realized it had significant, architecture-level performance flaws.
I can't take someone seriously who writes "Of course, PCs were much more powerful than, say, an Amiga 500."
The Amiga had a MC68000 - essentially a 32bit instruction set with a 16-bit implementation. None of these segmented memory issues and 640K or 16M limitations existed on that superior processor of the day. The whole post is about inferior features of x86.
The Amiga had a MC68000 - essentially a 32bit instruction set with a 16-bit implementation. None of these segmented memory issues and 640K or 16M limitations existed on that superior processor of the day. The whole post is about inferior features of x86.
> None of these segmented memory issues and 640K or 16M limitations existed on that superior processor of the day.
Uh... the 68000 had, in fact, exactly 16MB of addressible space (no MMU and a 24 bit address bus).
And, I dunno, arguing about the benefits of a flat memory model on a device that shipped with an non-upgradable 512kb of memory is a little spun.
The 68k was a more forward-looking ISA for sure. In consumer computers of 1987, that didn't buy it much. Look to the '020 machines Sun and others were shipping for examples that make it more attractive.
Uh... the 68000 had, in fact, exactly 16MB of addressible space (no MMU and a 24 bit address bus).
And, I dunno, arguing about the benefits of a flat memory model on a device that shipped with an non-upgradable 512kb of memory is a little spun.
The 68k was a more forward-looking ISA for sure. In consumer computers of 1987, that didn't buy it much. Look to the '020 machines Sun and others were shipping for examples that make it more attractive.
You could upgrade the Amiga 500 to 1 meg, using the trapdoor expansion. This was a very popular configuration.
I had a SCSI and RAM expansion for my Amiga 500. It gave me 3 megs and a whopping 30 meg hard drive!
The Amiga was so far ahead of early DOS machines, it was pathetic. Of course, by the time 486, SVGA, etc. came along it was pretty long in the tooth...
I had a SCSI and RAM expansion for my Amiga 500. It gave me 3 megs and a whopping 30 meg hard drive!
The Amiga was so far ahead of early DOS machines, it was pathetic. Of course, by the time 486, SVGA, etc. came along it was pretty long in the tooth...
An Amiga 500 was around $600. (edit: $700) A Sun 3 color workstation could easily be $20,000 or more.[1] In today's money that Sun would be more like $40,000.
Also, I don't believe you're correct about the memory being non-upgradable. A cursory Googling suggests at least 8MB could be added, maybe more.
[1]http://textfiles.com/bitsavers/pdf/sun/Sun_Price_List_Dec88....
Also, I don't believe you're correct about the memory being non-upgradable. A cursory Googling suggests at least 8MB could be added, maybe more.
[1]http://textfiles.com/bitsavers/pdf/sun/Sun_Price_List_Dec88....
Cue Germanglish rambling about how the PC was effectively just a CPU (and not a very good one) with slow peripherals attached while the Amiga is a perfect jewel of cooperating subsystems with an ISA that's like poetry from the angels.
I just created an account to say: Yes, as an Amiga user back in those days I don‘t remember having to deal with something similar to autoexec nor config to run a game. It just worked out of the box. I never understood my friends who traded this simplicity for some svga graphics.
I started getting into PC programming around 1995 after spending time on the c64 and Amiga, and it was right at that test that DOS extenders became available.
DOS4GW was part of the Watcom C compiler, but I seem to recall that Borland Pascal also had some protected mode going on. I'm pleased I've never had to spend too much time with the segmented memory model.
DOS4GW was part of the Watcom C compiler, but I seem to recall that Borland Pascal also had some protected mode going on. I'm pleased I've never had to spend too much time with the segmented memory model.
DOS extenders were available in the later 1980s.
But xe is nonetheless right that DOS/4GW was part of Watcom C/C++. That's what the "W" at the end means. The full DOS extender was DOS/4G. And Watcom C was available from the middle 1980s, too.
Zortech C++ came with both a 286 extender and a 386 extender.
I don't know off the top of my head whether Watcom C/C++ ever targetted the 16-bit Phar Lap or Rational Systems extenders. It certainly targetted the 32-bit ones, and others besides.
Yep, Borland Pascal 7.0 supported DPMI, but with a 16-bit segmented memory model. I often wonder how the C hegemony would have been affected if BP had a optimizing 32-bit compiler for DOS. IIRC, Doom's use of Watcom C validated that you could write performant games without using very much assembler, but BP wasn't up to the task.
Lots of clever ideas were born due to limitations!
Back in a days i was writing "virtual machines" on C, emulating x86 instruction set including memory tracking.
Was fun way to detect maliciously acting programs and even detecting new/unknown computer viruses - including ways to recover files from infections.
Back in a days i was writing "virtual machines" on C, emulating x86 instruction set including memory tracking.
Was fun way to detect maliciously acting programs and even detecting new/unknown computer viruses - including ways to recover files from infections.
It's amazing to think how hamstrung the early PC was by stuff like the 640 KiB limit.
The contemporary Tandy 2000 could be kitted out with up to 768 KiB of memory (896 KiB with an aftermarket add-on). Because it loaded its BIOS from disk instead of having it in ROM, and because there were no hardware restrictions on contiguous access to memory except perhaps the video framebuffer, the Tandy 2000 could use nearly all that RAM, and was for a brief shining moment preferable to the IBM PC, XT, or AT for memory-intensive jobs like Lotus spreadsheets. That and its hi-res color display made it sought after as an inexpensive CAD workstation.
It was not 100% PC compatible, but PC business apps often needed only a few small patches to work on the 2000, so official and unofficial ports were available.
The contemporary Tandy 2000 could be kitted out with up to 768 KiB of memory (896 KiB with an aftermarket add-on). Because it loaded its BIOS from disk instead of having it in ROM, and because there were no hardware restrictions on contiguous access to memory except perhaps the video framebuffer, the Tandy 2000 could use nearly all that RAM, and was for a brief shining moment preferable to the IBM PC, XT, or AT for memory-intensive jobs like Lotus spreadsheets. That and its hi-res color display made it sought after as an inexpensive CAD workstation.
It was not 100% PC compatible, but PC business apps often needed only a few small patches to work on the 2000, so official and unofficial ports were available.
The effort that so many went to in order to grab a few K of memory here and there in the DOS world to get some game to load or to otherwise cram something into a limited address space.
In a way, it was even worse than that because the DOS COM file format had even more limits around the 64K segments.
In a way, it was even worse than that because the DOS COM file format had even more limits around the 64K segments.
The EXE format has none of those limitations and was available from day 1.
I dunno, this kind of dance always seems unfair to me.
The 8086 shipped in 1978 into a market where the only successful 32 bit architectures were mainframes (the VAX broke open the 32 bit world in minicomputers the same year).
It wasn't supposed to be a world-beater in the late 80's, it was designed to be a better/cleaner and much cheaper PDP 11. And it was!
The 8086 shipped in 1978 into a market where the only successful 32 bit architectures were mainframes (the VAX broke open the 32 bit world in minicomputers the same year).
It wasn't supposed to be a world-beater in the late 80's, it was designed to be a better/cleaner and much cheaper PDP 11. And it was!
I was there, and as near as I can tell it was meant to be something closer in concept to an enterprise-level Apple ][ that could run VisiCalc.
That's where IBM positioned the 5150 it in the market in 1981. I'm talking about where Intel aimed the architecture in 1976-78.
No one really wants to hear this, but in the world of 16 bit architectures the 8086 was clean, straightforward, easy to code for, easy to optimize, and vastly cheaper than the minicomputers competing in the same space at only moderately higher performance levels. It wasn't until the mid-80's and the mess that was 286 protected mode[1] that people started to sour on it.
[1] They had a bunch of new transistors to spend, and could pick any two of "performance", "address stretch" and "memory protection". They did the MMU before the stretch, and paid dearly. The 68k made, in some sense, the opposite choice and had fewer growing pains in the late 80's. By 1990 no one cared anymore.
No one really wants to hear this, but in the world of 16 bit architectures the 8086 was clean, straightforward, easy to code for, easy to optimize, and vastly cheaper than the minicomputers competing in the same space at only moderately higher performance levels. It wasn't until the mid-80's and the mess that was 286 protected mode[1] that people started to sour on it.
[1] They had a bunch of new transistors to spend, and could pick any two of "performance", "address stretch" and "memory protection". They did the MMU before the stretch, and paid dearly. The 68k made, in some sense, the opposite choice and had fewer growing pains in the late 80's. By 1990 no one cared anymore.
I concede. That makes a lot of sense to me. I did greatly enjoy the instruction set.
So a simpler, cheaper, smaller PDP it is then.
Getting Ultima VII to successfully run, which required its Voodoo Memory Manager [0], was probably how I became comfortable hacking computer settings.
[0] http://wiki.ultimacodex.com/wiki/Voodoo_Memory_Manager
[0] http://wiki.ultimacodex.com/wiki/Voodoo_Memory_Manager
Back in the day there was also DR-DOS which was very similar and to some extent, compatible and in some aspects, better. Until the arrival of Windows, that is.
Have not used it in decades, though, so I couldn't really provide details about it. In fact, back then a lot of people still used monochrome displays.
Have not used it in decades, though, so I couldn't really provide details about it. In fact, back then a lot of people still used monochrome displays.
I could either choose to load the mouse and MS Windows. Or I could choose to load the modem and soundcard and stick to running DOS. And now you know why I never felt like using Windows was necessary for me.
Incidentally, in my circle of friends on the BBSes I used to dial, programming was one of the things the cool kids knew. So I taught myself C using nothing but the K&R book. As a loner and an outsider, I didn't have any other resources available. The result of this -- cutting my teeth on C pointers with nothing but that book for help -- is that when people in job interviews ask me about the most difficult technical thing I've ever worked on, I don't even hesitate to tell them about those years learning that stuff about C. What a fun and extremely intellectually challenging time!
Incidentally, in my circle of friends on the BBSes I used to dial, programming was one of the things the cool kids knew. So I taught myself C using nothing but the K&R book. As a loner and an outsider, I didn't have any other resources available. The result of this -- cutting my teeth on C pointers with nothing but that book for help -- is that when people in job interviews ask me about the most difficult technical thing I've ever worked on, I don't even hesitate to tell them about those years learning that stuff about C. What a fun and extremely intellectually challenging time!
What in the hell about this post got it downvoted? How bizarre.
> A solution might have been to switch back and forth from Protected Mode. But entering Protected Mode on the 286 is a one way ticket: if you want to switch back to Real Mode you have to reset the CPU! Bill Gates, who understood very early all the implications, is said to have called the 286 a “brain dead chip” for that reason.
Even if you never wanted to go back to real mode, the 80286 was brain dead because of the way they laid out the bits in selectors. One small change to the layout would have prevented so much pain for programmers.
In protected mode, like in real mode, the 286 used a segmented addressing scheme.
In real mode, an address consists of two 16-bit parts:
In protected mode, an address looks like this:
There are two lookup tables available to the program at any given instant. The T bit selects which is used. If T is 0, it uses a table called the Global Descriptor Table (GDT). If T is 1, it uses a table called the Local Descriptor Table (LDT).
The GDT, as the name suggests, is global. There is a single GDT shared by all code on the system. Typically it is used to map the segments containing the operating system code and data, and os protected so that user mode code cannot access it.
Each process has its own LDT.
The RL bits specify the Requested Privilege Level (RPL). The 286 has four privilege levels, 0-3, with lower numbers being higher privilege. To determine if access is allowed, the processor looks at RPL, the provilege level given for the segment in the descriptor (DPL), and the privilege level the processor is currently running at (CPL). The check is max(CPL, RPL) <= DPL. It's OK for user mode code to set RPL to whatever it wants, because it runs at CPL == 3, so RPL has no effect.
Let's look at this from the point of view of a user mode C program running on a protected mode operating system. The address space as seen by the C program, viewed as a 32-bit unsigned value, looks like this:
To prevent making all programs incur the performance hit from that, the compiler writers gave us several different memory models. If you compiled a program with "small model" [1], the compiler would limit it to a single 64k code segment and a single 64k data segments. It could load the CS and DS segment registers once at launch, and treat thee address space as a 16-bit flat space. In "large model" it would do multiple code and data segments, and incur the overhead of dealing with pointer arithmetic crossing segment boundaries. There were mixed models, that had multiple code segments but only one data segment, or vice versa.
The C compilers for 286 protected mode added new keywords, "near", "far", and "huge" that could modify pointer declarations. If you declared a point "near", it was just an offset within a segment. It had no selector. A "far" pointer had a selector and offset, but the selector would NOT be changed by pointer arithmetic--incrementing past the end of a segment wrapped to the beginning of the segment. A "huge" pointer had a selector and offset, and would include the selector in pointer arithmetic, so that you could increment through to the next segment.
Now throw in needing to link to libraries that might have been compiled using a different model from the model your code is compiled with, and that might be expecting different kinds (near, far, huge) pointers than you are using, and all in all it was a big freaking pain in the ass.
They could have avoided all that pain with a simple change. Instead of addresses in protected mode looking like this:
Why didn't they do this?
I've heard one theory. The descriptors in the GDT and LDT were 8 bytes long, so the physical address of the descriptor is 8 * selector + base_of_descriptor_table. Note that the selector in the segment register:
My CPU designer friends tell me that this is unlikely. They tell me that when you have an input to some unit (e.g., the selector input to the MMU) that is always going to be shifted a fixed amount, you can build that in essentially for free.
Note also if they had done it this way:
[1] I might be mixing up the names of the various memory models.
Even if you never wanted to go back to real mode, the 80286 was brain dead because of the way they laid out the bits in selectors. One small change to the layout would have prevented so much pain for programmers.
In protected mode, like in real mode, the 286 used a segmented addressing scheme.
In real mode, an address consists of two 16-bit parts:
+----------------+ +----------------+
| segment | | offset |
+----------------+ +----------------+
In protected mode, an address looks like this:
+-------------+-+--+ +----------------+
| selector |T|RL| | offset |
+-------------+-+--+ +----------------+
There are two lookup tables available to the program at any given instant. The T bit selects which is used. If T is 0, it uses a table called the Global Descriptor Table (GDT). If T is 1, it uses a table called the Local Descriptor Table (LDT).
The GDT, as the name suggests, is global. There is a single GDT shared by all code on the system. Typically it is used to map the segments containing the operating system code and data, and os protected so that user mode code cannot access it.
Each process has its own LDT.
The RL bits specify the Requested Privilege Level (RPL). The 286 has four privilege levels, 0-3, with lower numbers being higher privilege. To determine if access is allowed, the processor looks at RPL, the provilege level given for the segment in the descriptor (DPL), and the privilege level the processor is currently running at (CPL). The check is max(CPL, RPL) <= DPL. It's OK for user mode code to set RPL to whatever it wants, because it runs at CPL == 3, so RPL has no effect.
Let's look at this from the point of view of a user mode C program running on a protected mode operating system. The address space as seen by the C program, viewed as a 32-bit unsigned value, looks like this:
00040000 - 0004FFFF first 64k
000C0000 - 000CFFFF second 64k
00140000 - 0014FFFF third 64k
...
To prevent making all programs incur the performance hit from that, the compiler writers gave us several different memory models. If you compiled a program with "small model" [1], the compiler would limit it to a single 64k code segment and a single 64k data segments. It could load the CS and DS segment registers once at launch, and treat thee address space as a 16-bit flat space. In "large model" it would do multiple code and data segments, and incur the overhead of dealing with pointer arithmetic crossing segment boundaries. There were mixed models, that had multiple code segments but only one data segment, or vice versa.
The C compilers for 286 protected mode added new keywords, "near", "far", and "huge" that could modify pointer declarations. If you declared a point "near", it was just an offset within a segment. It had no selector. A "far" pointer had a selector and offset, but the selector would NOT be changed by pointer arithmetic--incrementing past the end of a segment wrapped to the beginning of the segment. A "huge" pointer had a selector and offset, and would include the selector in pointer arithmetic, so that you could increment through to the next segment.
Now throw in needing to link to libraries that might have been compiled using a different model from the model your code is compiled with, and that might be expecting different kinds (near, far, huge) pointers than you are using, and all in all it was a big freaking pain in the ass.
They could have avoided all that pain with a simple change. Instead of addresses in protected mode looking like this:
+-------------+-+--+ +----------------+
| selector |T|RL| | offset |
+-------------+-+--+ +----------------+
+-+--+-------------+ +----------------+
|T|RL| selector | | offset |
+-+--+-------------+ +----------------+
80000000 - 9FFFFFFF 524288k flat address space
00000000 - 1FFFFFFF 525288k flat address space
Why didn't they do this?
I've heard one theory. The descriptors in the GDT and LDT were 8 bytes long, so the physical address of the descriptor is 8 * selector + base_of_descriptor_table. Note that the selector in the segment register:
+-------------+-+--+
| selector |T|RL|
+-------------+-+--+
My CPU designer friends tell me that this is unlikely. They tell me that when you have an input to some unit (e.g., the selector input to the MMU) that is always going to be shifted a fixed amount, you can build that in essentially for free.
Note also if they had done it this way:
+-+--+-------------+
|T|RL| selector |
+-+--+-------------+
[1] I might be mixing up the names of the various memory models.
Your CPU designer friends are right, and your "emulator" is pretty much how real mode and v8086 protected mode addressing were actually architected. The descriptor parts of segment registers effectively had fixed contents derived from what was loaded into the selector parts, rather than from descriptor tables. Yes, this indeed allowed the 80386 to run real mode programs. (-:
This, combined with doing the IRQ ISA slot shuffle makes me very glad for modern PC architecture and OSes.
LOADHIGH and DEVICEHIGH - Now we're in business! 595 KiB - Awesome!
Oh no. My dad's coming home and he won't allow me to mess with the config files. What did I change...
Then I learned about boot disks. And with DOS4/GW all problems were gone.
Never knew the technical reasons for the limitations but this article brought back a lot of memories.