Bank switching allows computers to access more memory by setting up separate banks for the processor to use. It originated in the 1980s to extend the life of older systems and is implemented through a latch technique. Bank switching was used in video game consoles and is still used in modern embedded systems.
The bank switching technique provides a way for computer systems to access more memory than they would otherwise be able to. When a computer’s processor is limited to a specific amount of addressable memory space, additional banks of memory can be set up for the processor to use. These separate banks can then be used to leave code that is no longer used, such as read-only memory (ROM) used when the computer started, and open memory banks for more users on the system or store memory for other devices on the system .
Switching the bank originated as a cost-effective way to keep computers up and running in the 1980s without having to replace the processor. It found good use on older 8-bit computer systems, extending the useful life of a computer by simply adding more memory. As newer systems were developed, they also implemented bank switching methods so that programs created on older systems could still work.
The way bank switching works is by implementing what is called a latch technique. The latch is really just a kind of switch that switches the address space used by the computer’s processor. For example, 8-bit computers use a 16-bit address space, meaning they are only able to work with 64K, or 65,536, individual memory locations at any given time. When a latch was added, whether by software or hardware, it could then switch between multiple memory banks.
The latch is set separate from the processor, leaving bank switching in the hands of an external operation. In some cases, it simply hides in the upper register of memory addresses and is triggered as needed by the computer’s operating system or some other software. As memory fills up, the processor can check the top bit and move to another bank. Other latch decoding methods involved bit-addressable gates that allowed access to another memory bank.
Bank switching also made its way into a number of video game consoles of the time. ROM cartridges would come with additional hardware built in that would expand the console beyond its limited available memory space, allowing for better graphics in games and longer play through additional stages. As technology and techniques improved, however, the method fell out of use. Some modern operating systems can still emulate switching banks to run older software. Many modern embedded computer systems, those computer systems that are integrated into some other device or system and typically designed to perform a single task, still use bank switching due to its convenience and ease of use.
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