Memory Type: SDRAM, PC133, SDRAM (non-ECC)
Maximum Memory: 512MB
ThinkPad A20 and A21 contain two memory sockets, one of which can be upgraded by the user. To upgrade the standard memory, extensive disassembly may be required and should be taken to an authorized service center to upgrade. Each socket can use a 256MB.
How much memory your Windows OS will recognize depends on which version of Windows you are running. 32-bit versions of Windows will see (and utilize) only 3GB or 3.5GB. To utilize more memory, install a 64-bit version of your OS. More information about OS memory maximums can be found here.
Memory or DRAM is the "working" memory of the computer. It's used to store data for programs (sequences of instructions) on a temporary basis.
video - installing memory