Figuring out size of Device Drivers and where they are loaded in High Memory

Question

I'm setting up a 486 PC (aiming for around 1994 or so) and I have the following device drivers set up in DOS 6.22:

HIMEM.SYS
EMM386.EXE
ANSI.SYS
SBIDE.SYS
CTMOUSE.EXE
SMARTDRV.EXE
DOSKEY.COM
MSCDEX.EXE

I'm loading all these devices (including part of DOS itself) into High Memory and the MEM command is showing that the largest executable program size is 618 K, which I think means that DOS was successful in allocating all the drivers into upper memory, but I'm not sure.

If I understand loading things into high memory correctly, there are only a certain number of "blocks" available to do this, correct? And the order in which the drivers are loaded high (in AUTOEXEC.BAT and CONFIG.SYS) matters, because once a block is taken, the extra space is wasted within that block.

How do I know what the optimal order of loading these drivers should be?

Answer 1

You can see the details of everything loaded in memory using

MEM /D /P

including device drivers etc. If you have Microsoft’s Manifest tool, you can use that to get a better idea of memory use too; many other tools exist to explore your system’s configuration.

IIRC 618 K is quite a good result with MS-DOS, and does indeed suggest that all drivers and TSRs are loaded high. To improve on that, you’d have to move some DOS data structures high, using a tool such as DOSMAX (or QEMM’s DOS-Up). MGDx has a nice page on the subject.

To optimise the memory footprint, you typically need to vary the load order, with the aim of loading larger programs first — larger not necessarily in terms of resident size, but in terms of load size, since the loaded device driver or TSR needs to fit entirely in an upper memory block before it goes resident. In most cases, for .SYS or .COM files (strictly speaking, for non-MZ files), the memory required to load them is the size of the file; for MZ files (.EXE usually), it’s often the size of the file but not necessarily.

There are other considerations. Some device drivers and TSRs can move themselves to upper memory, and should be allowed to do so (i.e. loaded with DEVICE instead of DEVICEHIGH, and without LOADHIGH for TSRs). If your upper memory is split into multiple blocks, you can tell DEVICEHIGH and LOADHIGH which block to use with the /L parameter (/L:1,10240 will load into region 1 if it has at least 10240 bytes available, IIRC). In some situations, you might want to load a TSR earlier than AUTOEXEC.BAT, using INSTALLHIGH in CONFIG.SYS — this allows a large TSR to be loaded before certain device drivers.

As Raffzahn says, it’s often a case of trial-and-error — at least with DOS 6 you can skip your boot files if things get too messed up for the system to boot. Since you’re running DOS 6.22, you can use MEMMAKER to do a lot of this for you — it will try various combinations and place device drivers and TSRs into the appropriate blocks if necessary.

Another angle to investigate is to look for device drivers or TSRs providing equivalent functionality, but using less memory. You’re already using a small mouse driver; you might like SHSUCD instead of MSCDEX, and NNANSI instead of ANSI.SYS. Using 4DOS instead of COMMAND.COM will provide better control over upper memory use, and allow you to drop DOSKEY (along with all the nice command-line features of 4DOS).

Answer 2

MEM /C /P

Should show you what has been loaded into UMBs and what not.

If not all of your programs you think that should run in upper memory actually do, you can try permutating the load order in AUTOEXEC.BAT and CONFIG.SYS to find an order that may fit better.

Back in the days, I used to have a program UMBINFO.EXE that showed more information about occupied UMBs (and possibly wasted space), but I can't find it anywhere.