HIMEM


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AcronymDefinition
HIMEMHigh Memory
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Since our PCs cannot address extended memory without HIMEM.SYS, it makes sense that HIMEM.SYS would have to be loaded before we could load drivers into upper memory or into a special area of memory called the High Memory Area.
This block of memory is special because HIMEM.SYS, by toggling an address line on and off, can use these 65,520 bytes of memory without shifting into protected mode.(1) Best of all, once HIMEM.SYS has been loaded, we can free up about 47,000 bytes of conventional memory by loading DOS into the High Memory Area.
Although HIMEM.SYS is usually the first executable line in the CONFIG.SYS file (see sample file in Figure 1), it is not mandatory that HIMEM.SYS appear in the first line of the CONFIG.SYS file; in fact, if our PC has a device driver that controls a SCSI hard disk, the hard disk driver must appear first in the CONFIG.SYS file.
Like HIMEM.SYS, the EMM386.EXE driver must be loaded into conventional memory, so we cannot use DEVICEHIGH with this command.
Using HIMEM.SYS and EMM386.SYS and loading MS-DOS into upper memory yielded 613 KB of usable conventional memory and 5,460 KB of extended or expanded memory as needed.
For example, HIMEM.SYS, Microsoft's extended memory manager, cannot be loaded using DEVICEHIGH or LOADHIGH because, until it is loaded, DOS does not even know extended memory exists.
The HIMEM.SYS and EMM386.EXE programs discussed earlier are the first essential step in activating extended and upper memory.
(Just remember that the HIMEM.SYS should always precede the EMM386 and the DOS=HIGH,UMB lines in the CONFIG.SYS.) There are a number of command parameters that can be used during loading that can precisely determine the address ranges we wish to include or exclude in this process.
HIMEM.SYS is of course fully compatible with the Extended Memory Specification, or XMS, not to be confused with EMS, the Expanded Memory Specification.
You may need to include one or two switches on the line that loads HIMEM.SYS.
So the good news is that you can greatly increase the amount of available conventional RAM in your computer by using DOS 5 and its memory management programs, HIMEM.SYS and EMM386.EXE.