The Compaq Portable was the first "real" IBM PC compatible, introduced in 1982.
The mainboard has 128KB RAM by default and was expandable to 256KB, enough for MS-DOS 1.1 (which was the version delivered with the computer) and the first MS-DOS applications. The Compaq Portable has expansion slots like the IBM PC, so you can upgrade it even with an SCSI controller and SCSI hard disk drives (if you want).
It's BIOS was a result of reverse engineering the original IBM PC BIOS, which gives ultimate compatibility. Even the first BASICA versions were running without problems.
My device have two full-height 40 track double density drives, not really exotic.
The weight is about 12Kg, not really portable (but the 9" green CRT is one reason).
I've "expanded" my Compaq Portable with a 256KB RAM card and a Central Point Option Board, running "Transcopy". To get the drives connected to the Option Board card, I had to exchange A and B (so drive A is now on the right side, which is unusual but works ... don't know what sort of cables Central Point offered for use with the Compaq Portable, but these must be very, very long).
Also, I had to repair the original Compaq Keyboard, the conductive mylar discs (per key) were already decomposed and had to be replaced first. This took a lot of time btw. :-(
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