The whole point of this is research is to free up as much conventional memory as possible on machines that are not compatible with other memory managers, such as UMBPCI, while remaining in real mode.

I'm using HIMEMX since it appears to be 100% compatible with HIMEM.SYS (is it?) and it frees as much as 16K more than HIMEM.SYS. I don't know how it does it, but it loads the system somewhere else, because there's no UMBs or EMS. Version 3.34 frees 64 bytes more than 3.38 so I'm sticking to it.

The setting "BootGUI" in MSDOS.SYS determines if the system boots directly into Windows (BootGUI=1) or if it stops at the command prompt (BootGUI=0). In the latter case, if you manually start Windows with WIN.COM, you'll have a second instance of COMMAND.COM loaded into conventional memory (why?). That takes about 7K.

Here are some numbers taken from MEM/C/P with the default settings. The only thing I added to CONFIG.SYS is LASTDRIVE=E, since that frees 4K. I tried other options such as FILES, BUFFERS, NOAUTO, but they were always counterproductive.

The first column is the conventional memory and the second column is the largest executable program size.

1BootGUI=1
2Inside a DOS box       - 602,544 (588K) / 602,528 (588K)
3Restart in MS-DOS mode - 620,224 (606K) / 610,288 (596K)
4
5BootGUI=0
6Inside a DOS box       - 606,832 (593K) / 606,816 (593K) 
7Restart in MS-DOS mode - 613,936 (600K) / 613,920 (600K)

BootGUI=1 should free more memory since it doesn't load an extra COMMAND.COM. It does provide more conventional memory only if you "Restart in MS-DOS mode" (+6,288 bytes), but the largest executable size is smaller (-3,632 bytes).

BootGUI=0 always creates a second instance of COMMAND.COM. That takes about 7K of conventional memory. However it results in more memory for executables in all scenarios... Can anyone explain why? Isn't it counterintuitive or am I crazy?

For the record these are the numbers for pure DOS mode (BootGUI=0 before WIN.COM):

1Pure DOS               - 624,512 (610K) / 624,496 (610K)

Any other tweaks or suggestions are welcome.