ASROCK M3A785GMH/128M 設定共享內顯記憶體問題

請問各位, 為何我在BIOS裡設定shared memory 為128MB時....cpuz內只會顯示192MB display ram
但正常應是256MB display ram(128side port memory + 128 shared memory)
但在bios中設定shared memory 為256時則正常, cpuz內會顯示368MB (128side port memory + 256 shared memory)
謝謝
系統配置
AMD Althon II x4 620
Adata-DDR3-1333 2gb x2
Seagate 500GB 16 MB ST3500418AS
ASROCK M3A785GMH/128M
LG 4163b ATA DVD writer