Ian,
Just from a purely AVR architecture point of view, the
AVR_HAVE_RAMPD #define indicates that the AVR chip in question
supports the "RAMP" paging register, described as follows:
RAMPD
Register concatenated with the Z-register enabling direct
addressing of the whole data space on MCUs
with more than 64KB data space.
Eventually the originally tiny AVR chips' addressable memory got
so big that we needed a "paging" register. (That's a repeating
theme with every 8-bit/16-bit microcontroller family.)
So the "RAMPD" register needs to be saved along with the X, Y and
Z "special function" registers, if it is present.
Best regards,
Bruce
On 12/16/2021 3:46 PM, Ian Molton wrote:
Browsing the GCC source, I found this in gcc/config/avr/avr.c
if (AVR_HAVE_RAMPZ
&& TEST_HARD_REG_BIT (set, REG_Z)
&& TEST_HARD_REG_BIT (set, REG_Z + 1))
{
emit_push_sfr (rampz_rtx, false /* frame */, AVR_HAVE_RAMPD,
treg);
}
I wont pretend to fully understand this part of the compiler, but that
AVR_HAVE_RAMPD looks shady to me?
Anyone with deeper knowledge want to have a look?
-Ian