Bank Switching - Microcomputer Use

Microcomputer Use

Processors with 16-bit addressing (Z80, 6502, 6809, etc.) commonly used in early video game consoles and home computers can directly address only 64 KB. Systems with more memory had to divide the address space into a number of blocks that could be dynamically mapped into parts of a larger address space. Blocks of various sizes were switched in and out via bank select registers or similar mechanisms. Some blocks typically were always enabled. Some caution was required in order not to corrupt the handling of subroutine calls, interrupts, the machine stack, and so on. While the contents of memory temporarily switched out from the CPU was inaccessible to the processor, it could be used by other hardware, such as video display, DMA, I/O devices, etc. The last version of CP/M released in 1982 supported bank switching to allow use of more than the 64K of memory that the 8080 or Z80 processor could address.

Bank switching allowed extra memory and functions to be added to a computer design without the expense and incompatibility of switching to a processor with a wider address bus. For example, the C64 used bank switching to allow for a full 64KB of RAM and still provide for ROM and memory-mapped I/O as well. The Atari 130XE could allow its two processors (the 6502 and the ANTIC) to access separate RAM banks, allowing programmers on both machines to make large playfields and other graphic objects without using up the memory visible to the CPU.