Why does a motherboard have a maximum RAM limit? Why cant I just insert bigger DIMMs as they become available?

Trying to upgrade RAM on a "senior citizen" laptop. I can find the relevant chips (256 MB 133MHz SDRAM) in the grey market, but the laptop docs say I am only allowed a maximum of 128 MB.|||To annoy you is not the main reason.





The CPU has number of physical lines (ie, pins on the chip) that it can control, and which are used as memory address lines. Modern chips have 32, or 64, or these; older chips had 16. Short of memory management trickery, that means that there will be only 2^16 (64KB) worth of addresses, or 2^32, or 2^64 which can ever be distinguished. There's an upper limit there.





As well, the chipset for your computer (consolidated 'glue' circuitry connecting CPU, mmeory, I/O circuitry, etc) will have its own upper limits on numbers of individually distinguishable addresses. So your CPU can amange 2^32 addresses on its addressing lines, but the chipset can only manager 2^24 addresses, say. You won;t be able to install any more than that.





And the board layout matters. Let's say that the two DIM memory slots on some motherboard are arragned so that high address are stored in one and low addresses in the other. If there are only two clots, and the chipset only supports 1GB of address space, then the largest DIMM you can install in each slot is 512MB.





There are multiple other constraints on the memory type and size you can use in your machine. Several different sorts of timing issues, for instance.





Consult the manufacturer's documentation you got with the board, or the manufacturers' web site. Note carefully any limitations on the type speed and size of the memory you can install. Or talk to one o fhte memory vendors -- at least two of their Web sites contain databases organized by motherboard maker and model, listing what's compatible.





I would suggest that, when you order memory, err genrously on the high side. You can always not fully use memory, but if you need it (because some vital program is greedy), you;ll regret not having it.|||You cannot increase the size of the random access memory|||I liked fewofmanys explaination, that is correct.|||Some "bigger" chips are not compatable. I think it's trial and error. I have a Toshiba that was rated for 128MB max and I'm running 192MB. If compatable, you may have trouble with the motherboard "recognizing" the additional RAM. It may also be that at the time of manufacture, that was the max RAM available. Try emailing the customer tech support.|||It's because the motherboard is designed with a maximum RAM capacity depending on the chipset and processor architecture. Speaking from an electrical engineering point of view, it is not possible to make an infinitely modular Random Access Memory configuration, because the board would literally run out of memory addresses. 64 (and soon to be 128) bit processors do all but eliminate the limit on the amount of RAM one can have. But again, with older boards it's a matter of how many addresses are available to the chipset.|||becoz they didnt think of it then lol


tell the old bloke to buy new lappy