I guess it's kinda solved.
In msconfig.exe, the maximum memory option on Boot tab was set to 2048 MB. After unchecking that and restarting the OS, I got 5.48 GB usable now.
I'm assuming that the other 0.52 GB is...
I'm using Asus K43TK notebook, with specification:
- CPU: AMD A6-3420M
- Graphics: Radeon HD 6520M/7670M
- RAM bawaan: Hynix DDR3 2GB PC12800
- OS: Win7 Ultimate SP1 64-bit
Next, I upgrade the RAM with Corsair DDR3...