JH> What exactly is a GADD driver anyway?

GRADD, not GADD.  "GRaphics Adapter Device Driver".

GRADD was a new graphics subsystem that was invented for PowerPC OS/2.  IBM
had to invent one because the "base video handler" subsystems on Intel OS/2
(BVHVGA.DLL and so forth) were strongly tied to the PC architecture and the
Intel CPU, which of course PowerPC machines do not have.  (-:  So GRADD was
invented.

GRADD is a much better design than the old one (which has its roots in
Microsoft OS/2 version 1.1, believe it or not), and it was reverse-ported to
Intel OS/2.  It's designed to provide generic support with device driver
"chains", giving the potential for flexibility.  It is designed to support
seamless Windows, text mode, and PM with a single underlying device driver,
rather than having (as is the case pre-GRADD on Intel OS/2) multiple
co-operating but independent display driver subsystems, one for each.

From the OEM's point of view, also, it allows the portion of the display
device driver that has to be written by the OEM to be smaller, since it
provides IBM-supplied fallback functions (in SOFTDRAW.DLL) that will emulate
in software most drawing primitives that are required by Presentation
Manager's Graphics Engine (PMGRE), allowing an OEM to produce a fully
functional (but unaccelerated) display driver with a minimum of coding effort.

 ¯ JdeBP ®

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