中国DOS联盟

Understanding DOS Memory

When the DOS operating system was initially designed, the hardware system of the PC only supported a 1M-byte addressing space, so DOS could only manage a continuous memory space of up to 1M byte. Among this 1M memory, only 640K was reserved for application programs, which are called conventional memory or base memory, and the other 384K is called upper memory, which is reserved for video display, BIOS, etc. In 1982, 640K of memory was more than enough for microcomputers, and people even thought that 640K of memory could be used to do anything. Now it seems a bit funny, but at that time, that was indeed the case.

Now, even if your computer has several megabytes or tens of megabytes of memory, if you use the DOS operating system, then you only have 640K of memory that can be directly used, and memory above 1M needs to be used through some memory management tools. Fortunately, Windows 95 has no restrictions on conventional memory, and all your memory, whether 8M or 128M, can be directly used.

Under DOS, there are the following four types of memory in the system:
- Conventional Memory;
- Upper Memory;
- Expanded Memory;
- Extended Memory.

Conventional memory refers to the memory area from 0 to 640K. Under DOS, general application programs can only use the system's conventional memory, so they are all limited by 640KB of memory. Moreover, because DOS itself and device drivers installed in the config.sys file and memory-resident programs executed in the autoexec.bat file all occupy some conventional memory, the conventional memory that application programs can use is less than 640K. Many times, we have to try every means to organize memory to leave enough conventional memory for some application programs with a relatively large "appetite", which must be the most familiar thing to many computer enthusiasts in the DOS era.

Upper memory refers to the 384K of memory above conventional memory. Programs generally cannot use this memory area, but EMM386.exe can activate part of the upper memory, and it allows users to load certain device drivers and user programs into the upper memory using Devicehigh or LH (that is, loadhigh). dos=high,umb also loads part of DOS into the upper memory. Here, umb is the abbreviation of Upper Memory Block.

Expanded memory is an early standard for increasing memory, which can be expanded up to 32M at most. To use expanded memory, a special expanded memory board must be installed in the computer, and a management program for the expanded memory board must also be installed. Since expanded memory was introduced before extended memory, most programs are designed to be able to use expanded memory, but not extended memory. Because expanded memory is more troublesome to use, it was eliminated soon after extended memory appeared.

Extended memory can only be used on 80286 or higher-end machines. Currently, almost all memory above 1M on machines using DOS is extended memory. Extended memory also cannot be directly used by DOS. After DOS 5.0, Himem.sys, an extended memory management program, is provided, and we can manage extended memory through it. EMM386.exe can simulate extended memory (XMS) as expanded memory (EMS) to meet the requirements of some programs that use expanded memory.

Finally, let's emphasize again that no matter how large expanded memory or extended memory is, DOS application programs can only run under conventional memory. Some programs can make the CPU enter protected mode through a DOS extender (such as DOS4GW.exe and other programs), so as to directly access extended memory; but it should be noted that after entering protected mode, the computer is no longer in the DOS state.

Reprinted from "Learning Information"

The statement is somewhat inaccurate. It is not that "after entering protected mode, the computer is out of the DOS state." Instead, after entering the DOS state of protected mode, the computer is out of the usual DOS state of real mode.

Completely agree with the moderator's comment

Can the poster specifically explain the differences between high and umb in dos=high,umb? I know that actually they can be written separately, which shows that the two are different. Also, I don't understand the concepts of upper memory and DOS reserved area. Can you explain them?

The function of dos=high,umb is to hand over the control of UMB to DOS. As for the difference between high and umb, I can't explain it clearly.

MS-DOS was originally an operating system designed for 8086/8088 CPUs and can use at most 1MB of memory. The first 640KB of this 1MB memory is the aforementioned conventional memory. As for the remaining 384KB behind, it is reserved for BIOS ROM, ROM on hardware interface cards (such as VGA, hard disk control cards) and VIDEO BUFFER.

The address space of Reserved Memory (Reserved Memory) is between 640~1024KB, totaling 384KB, which is an address space reserved for external use. Reserved memory should not actually be called memory, but should be called reserved memory space, because no physical RAM is installed in this address space.

The so-called reserved memory actually only has an address space of 384KB, not really 384KB of RAM memory available for storing data.

In the 384KB address space of reserved memory (between 640~1024), in addition to being used by video buffers, BIOS ROM and other hardware expansion cards, there are still many gaps. We can use the paging (Paging) ability of 386/486 CPUs to map extended memory over to fill these gaps. Because these gaps are still within the 1MB range of DOS, they couldn't be used before because there was no installed memory. After configuring memory with paging technology, they then become memory available for DOS to use. We call this memory above 640KB address "Upper Memory (Upper Memory)". Upper memory may be divided into several blocks due to the installation of external cards, so it is also called Upper Memory Blocks, abbreviated as UMB.

Yes, the function of dos=high,umb is to hand over the control of UMB to DOS, but the two have their own functions. High means placing the DOS kernel in the upper memory; while UMB is to associate DOS with UMB, so that the upper memory managed by EMM386.EXE can be used by some programs. So usually they are written together, which can not only place the DOS kernel in UMB, but also let other programs use UMB. Of course, if there is no EMM386.EXE, the parameter UMB will be meaningless. This is my understanding, and I hope to discuss it with you.

high is to make DOS use HMA, and HMA is the first 64KB of memory above 1MB. It is not UMB.

Please don't just say "Okay". Thanks