Seems quite clear to me. Your report shows this:
"System Memory 512 MB (PC133 SDRAM)". Got it?
And, it shows this:
[ Memory Modules / A0 ]
Memory Module Properties:
Socket Designation A0
Type DIMM, SDRAM
Speed 60 ns
Installed Size 256 MB
Enabled Size 256 MB
[ Memory Modules / A1 ]
Memory Module Properties:
Socket Designation A1
Type DIMM, SDRAM
Speed 60 ns
Installed Size 256 MB
Enabled Size 256 MB
[ Memory Modules / A2 ]
Memory Module Properties:
Socket Designation A2
Type DIMM, SDRAM
Speed 60 ns
Installed Size 256 MB
Enabled Size 256 MB
I interpret that to say you have 3 - 256MB memory modules installed. So, you have 768 MB. What part of my explanation do you not understand?
