We have 3 kinds of share memory items in BIOS as follows.

AGP aperture size 128MB ==> For external AGP share memory 
On chip video window size 128MB ==> For onboard AGP share memory 
On chip frame buffer size 8MB ==> share memory size under legacy mode In Windows operation system, system will depend on the memory size you’re using to decide how much share memory is enabled. This part was not controlled by BIOS. BIOS only controlls the share memory under legacy (DOS) mode.