RAM limit

In electronic digital computers, there are different limitations on the usable memory address space. Even if a microprocessor supports, for example, 32-bit addressing, the integrated circuit package may only allow external access to a lower number of address bits, restricting the memory that can be installed. In modern personal computers, some limits are due to the design of processor, others due to the design of chipsets, BIOS and other hardware and related electrical limitations. Operating system and application software on a hardware platform may not have the capacity to use the full address space physically available.

For performance reasons, all the parallel address lines of an address bus must be valid at the same time, otherwise access to memory would be delayed and performance would be seriously reduced. Integrated circuit packages may have a limit on the number of pins available to provide the memory bus. Different versions of a CPU architecture, in different-sized IC packages, can be designed, trading off reduced package size for reduced pin count and address space. A trade-off might be made between address pins and other functions, restricting the memory physically available to an architecture even if it inherently has a higher capacity. On the other hand, segmented or bank switching designs provide more memory address space than is available in an internal memory address register.

As integrated circuit memory became less costly, it was feasible to design systems with larger and larger physical memory spaces.

Microcontroller devices with integrated I/O and memory on-chip sometimes had no, or a small, address bus available for external devices. For example, a microcontroller family available with a 2 kilobyte address space might have a variant that brought out an 11 line address bus for an external ROM; this could be done by reassigning I/O pins as address bus pins. Some general-purpose processors with integrated ROM split a 16-bit address space between internal ROM and an external 15-bit memory bus.

Some very early computers also had CPUs with fewer than 16 address pins: The MOS Technology 6507 (a reduced pin count version of the 6502) was used in the Atari 2600 and was limited to a 13 line address bus.

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