Your Amstrad PC1640 has only 640Kb. No area above 640k on this machine. Even if you install an EMS card you probably won't get high memory.

Himem.sys needs a 80286, and EMM386 a 80386. Remember that you have a 8086... they won't work.

I'd recommend you to use MS-DOS 3.x (Amstrad PC1640 system disks had MS-DOS 3.2) to maximize your free memory.

Maybe we had a memory expansion card, like this one
https://monotech.fwscart.com/MicroRAM_640K_UM … 4_19914752.aspx and loaded some EMS pages into the ROM area above 640k.
Guess that's the only explanation, then

In my XT I have an EGA board installed, I also installed a clone Hercules board, with USE! UMB with DOS 5.0 leaves me free 32k of high UMB memory that takes them from the hercules board, there with DOSMAX I upload files, fcbs, command shell com, buffers, etc. to the UMB. + drivers mouse, doskey, etc and it leaves me free 595936 bytes which is quite good, the memory of the hercules board is slow, but in this case as they are drivers that I upload it works perfectly.

perhenden wrote on 2020-05-10, 18:21:

According to my memory, we had more than 600k free under MS-DOS on our Amstrad PC1640, as reported by the mem command.
Now, I can't seem to get more than 561k.

Am I wrong when I recall having 600k available under MS-DOS 6.22?

Quite likely you are wrong if you combine MS-DOS 6.22 and the PC1640. You can have 600k available under MS-DOS 6.22 easily with DOS=HIGH,UMB (likely even with just DOS=HIGH), but this requires a 286 computer with more than 640KB of RAM. With just conventional memory, you are stuck where you are. On the other hand, remember that DOS 6.22 came out around 1994 (if memory serves me right), and the typical time to use an XT (at least in Western Europe and the US) was around 1986-1991. Many XT computers were used with DOS 3.3, which is a good fit for the capabilities of that computer. DOS 3.3 is considerably smaller than DOS 6.22, and it is quite likely you can get more than 600k free when you boot DOS 3.3 on the Amstrad PC1640.