> they describe ExecSG as sitting on top of a HAL [...]. I'm pretty sure that this didn't
> eventually happen, and ExecSG had to be adapted for every new piece of hardware.

Whether it's really there or not, the existence of a HAL in ExecSG wouldn't mean that ExecSG wouldn't have "to be adapted for every new piece of hardware". Of course it would have to, namely the (smaller) HAL part of it. What wouldn't have to be adapted is the (bigger) part of ExecSG that sits atop the HAL. That would be the whole point of a HAL.

Btw, Ben Hermans in November 2002:

"Exec SG now stands at 42000 lines of code, 4000 of which are PPC ASM located in the HAL."
http://anna.amigazeux.org/comments2.php?show=1036519888&number=63#comment