recordsdata portray

At the starting of the ninety’s, the PC platform became once on the total mocked by its rivals. Of route, PCs were rather more famous than, articulate, an Amiga 500. But the Amiga supplied a flat memory contend with, while a DOS program would possibly per chance maybe well most attention-grabbing get entry to memory the usage of cumbersome sixty four KiB segments. And to add insult to hurt, there became once this unparalleled 640 KiB memory limitation. No matter how noteworthy bodily memory you needed to your field, the say most well-known Aged Memory became once runt to 640 KiB!

The Narrative teaches us that Invoice Gates once declared that “640 KB must always unexcited be ample for somebody“, then designed MS-DOS to enforce this limitation.

In fact needless to inform reasonably of more refined than that.


Segmented memory

The first “IBM PC”, numbered 5150, became once designed around the Intel 8088, a castrated model of the 8086. As a sixteen bit CPU, it will most attention-grabbing manipulate addresses up to sixty four KiB in its registers. Thus, in picture to get entry to more memory, a greater contend with space is split into multiple segments.

The kind the 8086 generates addresses from segments is de facto unparalleled. As a replace of the usage of a register to title a section and yet one more one to say an contend with interior it, a brand new section begins every sixteen bytes. As segments are sixty four KiB lengthy and there is no longer any memory protection, it manner that segments overlap! Any bodily contend with is also accessed by up to 4096 section+offset combos! This moreover caps the most addressable memory to around 1 MiB, linked to the 20 bit contend with bus of the 8086/8088.

x86 Actual Mode Segmentation

x86 segmented memory mannequin is slightly unparalleled: segments can overlap!

The 640K ceiling

With its 8088, the IBM PC would possibly per chance maybe well contend with up to 1 MiB of memory. A gigantic amount in 1981!

This contend with space needed to be shared between the RAM, the video memory, and the quite loads of peripherals. IBM made up our minds that the decrease 640 KiB would be reserved for the (person accessible) RAM, while the greater 384 KiB would be ancient to contend with the video memory, the BIOS and the peripherals. These two zones must always unexcited be to be big ample to meet their motive…

And Microsoft? It never imposed any limitation!
Take hang of for event the practically forgotten Apricot PC, which became once yet one more MS-DOS machine of the time, albeit incompatible with the IBM PC. It puts the frontier in numerous locations and the person accessible RAM is in all probability to be sized up to 768 KiB!

So the atrocious 640K memory ceiling became once attributable to IBM! And moreover reasonably to Intel…


In 1984, IBM commercialized the IBM PC/AT (“Developed Expertise”!) in step with an Intel 80286. The 80286 is needless to inform fully effectively matched with the 8086. It is unexcited a sixteen bit CPU counting on segmented memory, however it absolutely can get entry to to up to sixteen MiB of memory! And it presents some originate of memory protection!


In picture to wait on compatibility with the applications and OSes written for the 8086, all these shining parts are most attention-grabbing accessible when working in the newly presented Real Mode. When operating in Actual Mode (a.ok.a “Legacy Mode”), the 286 manages memory upright treasure a (sooner) 8086! No memory protection and no get entry to to more than 1 MiB of memory!

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The Actual Mode is the default operating mode of the 286 when starting off. Once loaded, the OS can switch to Real Mode and let the person reap the advantages. Alas MS-DOS, along with its drivers and its existing applications, had been written when no PC got here with the relaxation than a 8086 or a 8088. Sadly, their builders made some mettlesome assumptions on that matter. Editing MS-DOS to neutral fully in Real Mode would were an intensive work and would own damaged the compatibility with most applications and drivers!

A resolution would possibly per chance maybe were to replace from Real Mode. But coming into Real Mode on the 286 is a a technique ticket: at the same time as you occur to must always replace motivate to Actual Mode it would possibly be famous to reset the CPU! Invoice Gates, who understood very early all the implications, is declared to own called the 286 a “mind ineffective chip” for that cause.

Fortunately, some (slightly hacky) schemes were devised to allow DOS applications having access to more memory. Otherwise, the PC as a computing platform would own failed…

Working out these schemes requires some naming conventions.

  • Aged Memory is the dear first 640 KiB of addressable memory. In MS-DOS the instructions of a program shall reside in it. That in part explains why some latter applications / games required most of veteran memory to be at their disposal. And that’s why freeing a mere KiB of it became once an excellent fight!
  • Upper Memory Predicament refers to the set up that became once reserved by IBM for the video adapter and the quite loads of peripherals. Show that this set up became once most attention-grabbing reserved and became once no longer entirely consumed!
  • Extended Memory is the memory out of attain of for a 8086/8088 (previous the first MiB). It is most attention-grabbing naturally accessible on latter Intel CPUs. Right here is the memory pool which get entry to became once required for more developed computing capabilities.
IBM PC Memory map

IBM PC Memory map (by Wtshymanski, CC BY-SA three.0)

HMA (High Memory Predicament)

In Actual Mode, memory segments open every sixteen bytes and are 64KiB lengthy. So the closing section exceeds the 1 MiB memory limit by practically sixty four KiB!

On a 8086 here’s a non-concern. Indeed, its 20 pin contend with bus makes unattainable to get entry to more than 1 MiB of memory. All generated addresses that exceed 1 MiB are aliased to the very first addresses.
But the 286 is equipped wit a 24 pin contend with bus. So it will get entry to this memory block, even in Actual Mode!

This very closing section is called HMA (High Memory Predicament). This triggered compatibility problems with some 8086 applications that ancient the closing section, ready for it to loop motivate to the first addresses. But it moreover allowed HIMEM.SYS to load some formula of MS-DOS 5 to HMA on a 286/386/486, freeing up to forty six KiB of precious veteran memory!

EMS (Expanded Memory Specification)

In 1985 Lotus, Intel and Microsoft presented the EMS customary, allowing a standardized get entry to to prolonged memory.

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A sixty four KiB unused and contiguous section of Upper Memory is identified and split into four sixteen KiB segments. A hardware assisted bank switching mechanism and an relevant DOS driver are then ancient to get entry to any role in prolonged memory when requiring get entry to to those four segments!

It doubtlessly presents the applications get entry to to the total sixteen MiB contend with space, even in true mode!  Though, programmers must always manually configure the four sixteen KiB “views” by job of the motive force.

EMS Illustrated

EMS Illustrated: a free 64K block from Upper Memory presents a “stumble on” to four 16K pages from Extended Memory.

Gargantuan! Is there any scheme back? It requires devoted hardware give a elevate to on the motherboard and the utilize of a driver ingesting a pair of KiB of outdated veteran memory. And programmers can no longer without problems manipulate recordsdata structures greater than sixteen KiB.

Quoting Invoice Gates himself:
“It’s garbage! It’s a kludge! … But we’re going to attain it!”

XMS (Extended Memory Specification)

Yet every other map exists to get entry to prolonged memory: XMS. The premise: switch the CPU into Real Mode and duplicate segments recordsdata from/to prolonged memory to/from unused segments of greater memory, then to replace motivate to Actual Mode to allow a DOS program to get entry to it.

The advantages? It does no longer require any devoted hardware. It moreover frees the programmer from being caped to sixteen KiB of continuous memory.
The scheme back? The 286 requires a reset to replace motivate to Actual Mode, making the utilize of XMS on a 286 extremely late! Thus it became once infrequently ever ever ancient, till later…

EMS and XMS each require free blocks in greater memory. If there are no longer ample free blocks obtainable for each to be obtainable, the person has to build which customary he’ll enable in the boot configuration files. His judgment will in all probability be in step with the applications he intends to open. Configuring MS-DOS on a 286 became once no longer for the faint hearted!


When it designed the 80386, Intel now understood effectively that 8086 and DOS compatibility would be its predominant asset. Ultimate parts were added.

  • An instruction to hasty switch between Real Mode and Actual Mode on the hump
  • The Virtual 8086 mode, a sub-mode of Real Mode, by which Actual Mode program runs in a more or much less digital machine. The video display itself operates in capable mode so developed memory usage reminiscent of paging and isolation are imaginable!
  • The 80386 unexcited depends on a segmented memory mannequin. But being a 32 bit CPU, the section is also 4 GiB lengthy, giving the same companies as a flat memory mannequin.

XMS is now usable on a 386!

EMM386.exe, the memory manager presented with MS-DOS 5 in 1991, locks DOS itself into a Virtual 8086 job. It moreover presents DOS with a driver emulating EMS, the usage of prolonged memory from Real Mode. No more devoted hardware is required!
It is moreover ready to load instrument drivers and TSRs into Upper Memory Predicament if ample free segments are obtainable. By the usage of LOADHIGH and DEVICEHIGH directives in autoexec.bat and config.sys, as an various of their older counterparts LOAD and DEVICE, it is imaginable to free dozen KiBs from the veteran memory!

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To be silent, those tactics were first presented by Quaterdeck in its QEMM memory manager, then by Digital Study in its DR-DOS 5, sooner than being mimicked by Microsoft in MS-DOS 5.


With EMS, XMS and strategies to free most of veteran memory, PCs with MS-DOS became excessive contenders in the video gaming platforms enviornment. Nevertheless, having access to EMS and XMS is a late route of. And, even on a 386, they don’t allow to eliminate the sixteen bit memory segmentation schemes.
Amiga users unexcited had many arguments to mock PC users… till the generalization of DOS Extenders, which in the extinguish allowed the PCs to hump. One in all the first games making utilize of a DOS Extender became once no-decrease than DOOM!

DOS4/GW when launching Doom

When you saw DOS4/GW, you knew you were in for some excessive gaming. When you own ever questioned what that DOS4/GW is, it’s a DOS Extender!

DPMI (DOS Real Mode Interface)

One predominant instrument editor impeded by the 640K memory limit became once Microsoft itself! Windows three.0 became once an ambitious mission, however it absolutely required more memory and some companies that Actual Mode became once unable to present. Thus Microsoft presented DPMI in 1989, sooner than giving its wait on watch over to an industry committee.

DPMI describes a chain of companies to be supplied by a DPMI host, which permits client applications, called DMPI purchasers, to bustle in capable mode while unexcited being ready to create utilize of DOS companies in a effectively-behaved and hardware-honest plan!
In picture to meet its role, a DPMI host has to get mode switching, allocate prolonged memory, allocate veteran memory, wait on watch over the interrupt subsystem, keep in touch with true mode applications, and browse or write particular CPU wait on watch over registers.
Working in Real Mode, some originate of multitasking between DPMI purchasers is imaginable while, for compatibility sake, Actual Mode TSRs are unexcited present alongside!

Though it is slightly imaginable to code a DPMI host on a 286, most of them require a 386. Thanks again to its speedily mode switching and its Virtual 8086 mode!

DOS Extenders

The DPMI specs allowed the apparition of many DOS Extenders, that are DPMI purchasers that load and take hang of wait on watch over of an application. The motive is to service them with an prolonged model of the APIs that DOS and the BIOS present by job of instrument interrupts (respectively 0x10 and 0x21). In picture to steer certain of switching motivate to Actual Mode too on the total and put efficiency, DOS Extenders moreover contend with frequent hardware interrupts, reminiscent of those generated by a mouse.

With a 32 bit DOS Extender, a 386 is willing to bustle qualified 32 bit applications and to allocate sizable blocks of memory! At closing!
This moreover ends the time when the veteran memory pool became once the fastest obtainable. Functions constructed on DOS Extender thus require a ways much less free Aged Memory, easing the painful route of of crafting customized autoexec.bat and config.sys in picture to bustle ones favourite games.

Games working in Real Mode

DOS Extenders in the extinguish allowed games to bustle in 32 bit Real Mode, making at closing qualified utilize of the famous Intel CPUs.

While unexcited reasonably of bit hacky, DOS extenders in the extinguish allowed PCs to create qualified utilize of their memory and tough CPUs. A pair of years later, Windows ninety five made those memory complications a ingredient of the previous as MS-DOS became progressively irrelevant.

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