The 777s I fly have a push-button switch just to the left of the lower centre screne that allows switching between true and mag. Fromthe FM:
"Pushing alternately selects the heading reference for the PFDs, NDs, AFDS, and
FMCs.
NORM –
• normally references magnetic north
• automatically references true north when north of 82°N or south of 82°S
latitude or within the vicinity of the magnetic poles (PFDs, NDs, and
FMCs)
• provides no reference for AFDS roll modes other than LNAV when north
of 82°N or south of 82°S latitude or in the vicinity of the magnetic poles.
TRUE – references true north regardless of latitude.
When the AFDS roll mode is HDG SEL, switching the heading reference switch
from NORM to TRUE or TRUE to NORM engages the HDG HOLD mode.
When the AFDS roll mode is TRK SEL, switching the heading reference switch
from NORM to TRUE or TRUE to NORM engages the TRK HOLD mode."