To read from a physical address, use the !db, !dc, !dd, !dp, !du, and !dw extension commands.
To write to a physical address, use the !eb and !ed extension commands.
The fp (Fill Physical Memory) command writes a pattern to a physical memory range, repeating it until the range is full.
When you are using WinDbg in kernel mode, you can also read or write to physical memory directly from the Memory window.
To search physical memory for a piece of data or a range of data, use the !search extension command.
Also, for more information about physical addresses, see Converting Virtual Addresses to Physical Addresses.