Bank switching
Encyclopedia
Bank switching is a technique to increase the amount of usable memory beyond the amount directly addressable by the processor
. It can be used to configure a system differently at different times; for example, a ROM
required to start a system
from diskette could be switched out when no longer needed.
Bank switching originated in minicomputer
systems. Many modern microcontroller
s and microprocessor
s use bank switching to manage random-access memory
, non-volatile memory, input-output devices and system management registers in small embedded system
s. The technique was common in 8-bit
microcomputer
systems. Bank-switching may also be used to work around limitations in address bus width, where some hardware constraint prevents straightforward addition of more address lines. Some control-oriented microprocessors use a bank-switching technique to access internal I/O and control registers, which limits the number of register address bits that must be used in every instruction.
Unlike memory management by "paging
", data is not exchanged with a mass storage device like disk memory. Data remains in quiescent storage in a memory area that is not currently accessible to the processor, (although it may be accessible to the video display, DMA controller
, or other subsystems of the computer).
The latch can be set or cleared by the processor in several ways; a particular memory address may be decoded and used to control the latch, or, in processors with separately-decoded I/O addresses, an output address may be decoded. Several bank-switching control bits could be gathered into a register, approximately doubling the available memory spaces with each additional bit in the register.
Because the external bank-selecting latch (or register) is not directly connected with the program counter
of the processor, it does not automatically change state when the program counter overflows; this cannot be detected by the external latch since the program counter is an internal register of the processor. The extra memory is not seamlessly available to programs. Internal registers of the processor remain at their original length, so the processor cannot directly span all of bank-switched memory by, for example, incrementing an internal register. Instead the processor must explicitly do a bank-switching operation to access large memory objects. There are other limitations. Generally a bank-switching system will have one block of program memory that is common to all banks; no matter which bank is currently active, for part of the address space only one set of memory locations will be used. This area would be used to hold code that manages the transitions between banks, and also to process interrupt
s.
Unlike a virtual memory
scheme, bank-switching must be explicitly managed by the running program or operating system; the processor hardware cannot automatically detect that data not currently mapped into the active bank is required. The application program must keep track of which memory bank holds a required piece of data, and then call the bank-switching routine to make that bank active. However, bank-switching can access data much faster than, for example, retrieving the data from disk.
addressing (Z80, 6502, 6809, etc.) commonly used in home computer
s 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, interrupt
s, 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.
and Intel introduced Expanded Memory
Specification (EMS) 3.0 for use in IBM PC compatible
computers running MS-DOS
. Microsoft
joined for versions 3.2 in 1986 and 4.0 in 1987 and the specification became known as Lotus-Intel-Microsoft EMS or LIM EMS. It is a form of bank switching technique that allows more than the 640 KB of RAM defined by the original IBM PC architecture, by letting it appear piecewise in a 64 KB "window" located in the Upper Memory Area
. The 64 KB is divided into four 16 KB "pages" which can each be independently switched. Some computer games made use of this, and though EMS is obsolete, the feature is nowadays emulated by later Microsoft Windows
operating system
s to provide backwards compatibility with those programs.
The later eXtended Memory Specification (XMS), also now obsolete, is a standard for, in principle, simulating bank switching for memory above 1 MB (called "extended memory
"), which is not directly addressable in the Real Mode
of x86 processors in which MS-DOS runs. XMS allows extended memory to be copied anywhere in conventional memory, so the boundaries of the "banks" are not fixed, but in every other way it works like the bank switching of EMS, from the perspective of a program that uses it. Later versions of MS-DOS (starting circa version 5.0) included the EMM386 driver, which simulates EMS memory using XMS, allowing programs to use extended memory even if they were written for EMS. Microsoft Windows
emulates XMS also, for those programs that require it.
s. The Atari 2600
, for instance, could only address 4 KB of ROM, so later 2600 game cartridges contained their own bank switching hardware in order to permit the use of more ROM and thus allow for more sophisticated games (via more program code and, equally important, larger amounts of game data such as graphics and different game stages). The Nintendo Entertainment System
contained a modified 6502
but its cartridges sometimes contained a megabit
or more of ROM, addressed via bank switching called a Multi-Memory Controller
. Game Boy
cartridges used a chip called MBC (Memory Bank Controller), which not only offered ROM bank switching, but also cartridge SRAM bank switching, and even access to such peripherals as infrared links or rumble motors. Bank switching was still being used on later game systems. Several Sega Mega Drive cartridges were over 4MB in size and required the use of this technique (4MB being the maximum address size).
The GP2X
handheld from Gamepark Holdings uses bank switching in order to control the start address (or memory offset) for the second processor.
in many 16-bit systems, which in turn gave way to paging
memory management unit
s. In embedded systems, however, bank switching is still often used, for its simplicity, low cost, and often better adaptation to those contexts than to general purpose computing.
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
. It can be used to configure a system differently at different times; for example, a ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
required to start a system
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
from diskette could be switched out when no longer needed.
Bank switching originated in minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
systems. Many modern microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
s and microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
s use bank switching to manage random-access memory
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
, non-volatile memory, input-output devices and system management registers in small embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
s. The technique was common in 8-bit
8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
systems. Bank-switching may also be used to work around limitations in address bus width, where some hardware constraint prevents straightforward addition of more address lines. Some control-oriented microprocessors use a bank-switching technique to access internal I/O and control registers, which limits the number of register address bits that must be used in every instruction.
Unlike memory management by "paging
Paging
In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...
", data is not exchanged with a mass storage device like disk memory. Data remains in quiescent storage in a memory area that is not currently accessible to the processor, (although it may be accessible to the video display, DMA controller
Direct memory access
Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....
, or other subsystems of the computer).
Technique
Bank switching can be considered a way of extending the address bus of a processor with some external register. For example, a processor with a 16-bit external address bus can only address 216 = 65536 memory locations. If an external latch was added to the system, it could be used to control which of two sets of memory devices, each with 65536 addresses, could be accessed. The processor could change which set is in current use by setting or clearing the latch bit.The latch can be set or cleared by the processor in several ways; a particular memory address may be decoded and used to control the latch, or, in processors with separately-decoded I/O addresses, an output address may be decoded. Several bank-switching control bits could be gathered into a register, approximately doubling the available memory spaces with each additional bit in the register.
Because the external bank-selecting latch (or register) is not directly connected with the program counter
Program counter
The program counter , commonly called the instruction pointer in Intel x86 microprocessors, and sometimes called the instruction address register, or just part of the instruction sequencer in some computers, is a processor register that indicates where the computer is in its instruction sequence...
of the processor, it does not automatically change state when the program counter overflows; this cannot be detected by the external latch since the program counter is an internal register of the processor. The extra memory is not seamlessly available to programs. Internal registers of the processor remain at their original length, so the processor cannot directly span all of bank-switched memory by, for example, incrementing an internal register. Instead the processor must explicitly do a bank-switching operation to access large memory objects. There are other limitations. Generally a bank-switching system will have one block of program memory that is common to all banks; no matter which bank is currently active, for part of the address space only one set of memory locations will be used. This area would be used to hold code that manages the transitions between banks, and also to process interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s.
Unlike a virtual memory
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
scheme, bank-switching must be explicitly managed by the running program or operating system; the processor hardware cannot automatically detect that data not currently mapped into the active bank is required. The application program must keep track of which memory bank holds a required piece of data, and then call the bank-switching routine to make that bank active. However, bank-switching can access data much faster than, for example, retrieving the data from disk.
Microcomputer use
Processors with 16-bit16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...
addressing (Z80, 6502, 6809, etc.) commonly used in home computer
Home computer
Home computers were a class of microcomputers entering the market in 1977, and becoming increasingly common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single nontechnical user...
s can directly address only 64 KB
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
. 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
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
calls, interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s, the machine stack
Stack
-Mathematics:* Stack , general category-theoretical concept to formalise "pull-back" operations in geometry and algebra* Algebraic stack, a generalisation of scheme and algebraic space in algebraic geometry; a specific type of the above-Computers:...
, 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
Direct memory access
Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....
, I/O devices, etc. The last version of CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
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
Address bus
An address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...
. For example, the C64
Commodore 64
The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...
used bank switching to allow for a full 64KB of RAM and still provide for ROM and memory-mapped I/O
Memory-mapped I/O
Memory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...
as well. The Atari 130XE
Atari 8-bit family
The Atari 8-bit family is a series of 8-bit home computers manufactured from 1979 to 1992. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips...
could allow its two processors (the 6502 and the ANTIC
ANTIC
Alphanumeric Television Interface Controller is an early video system chip used in the Atari 8-bit family of microcomputers as well as the Atari 5200 in the 1980s. The chip was patented by Atari, Inc. in 1981...
) 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.
The IBM PC
In 1985 the companies LotusLotus Software
Lotus Software is a software company with headquarters in Westford, Massachusetts...
and Intel introduced Expanded Memory
Expanded memory
In DOS memory management, expanded memory is a system of bank switching introduced April 24, 1985 that provided additional memory to DOS programs beyond the limit of conventional memory. Expanded memory uses parts of the address space normally dedicated to communication with peripherals for program...
Specification (EMS) 3.0 for use in IBM PC compatible
IBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
computers running MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
. Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
joined for versions 3.2 in 1986 and 4.0 in 1987 and the specification became known as Lotus-Intel-Microsoft EMS or LIM EMS. It is a form of bank switching technique that allows more than the 640 KB of RAM defined by the original IBM PC architecture, by letting it appear piecewise in a 64 KB "window" located in the Upper Memory Area
Upper Memory Area
In DOS memory management, the upper memory area refers to memory between the addresses of 640 KB and 1024 KB in an IBM PC or compatible. IBM reserved the uppermost 384 KB of the 8088 CPU's 1024 KB address space for ROM, RAM on peripherals, and memory-mapped input/output...
. The 64 KB is divided into four 16 KB "pages" which can each be independently switched. Some computer games made use of this, and though EMS is obsolete, the feature is nowadays emulated by later Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s to provide backwards compatibility with those programs.
The later eXtended Memory Specification (XMS), also now obsolete, is a standard for, in principle, simulating bank switching for memory above 1 MB (called "extended memory
Extended memory
In DOS memory management, extended memory refers to memory above the first megabyte of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems...
"), which is not directly addressable in the Real Mode
Real mode
Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...
of x86 processors in which MS-DOS runs. XMS allows extended memory to be copied anywhere in conventional memory, so the boundaries of the "banks" are not fixed, but in every other way it works like the bank switching of EMS, from the perspective of a program that uses it. Later versions of MS-DOS (starting circa version 5.0) included the EMM386 driver, which simulates EMS memory using XMS, allowing programs to use extended memory even if they were written for EMS. Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
emulates XMS also, for those programs that require it.
Video game consoles
Bank switching was also utilized in some video game consoleVideo game console
A video game console is an interactive entertainment computer or customized computer system that produces a video display signal which can be used with a display device to display a video game...
s. The Atari 2600
Atari 2600
The Atari 2600 is a video game console released in October 1977 by Atari, Inc. It is credited with popularizing the use of microprocessor-based hardware and cartridges containing game code, instead of having non-microprocessor dedicated hardware with all games built in...
, for instance, could only address 4 KB of ROM, so later 2600 game cartridges contained their own bank switching hardware in order to permit the use of more ROM and thus allow for more sophisticated games (via more program code and, equally important, larger amounts of game data such as graphics and different game stages). The Nintendo Entertainment System
Nintendo Entertainment System
The Nintendo Entertainment System is an 8-bit video game console that was released by Nintendo in North America during 1985, in Europe during 1986 and Australia in 1987...
contained a modified 6502
MOS Technology 6502
The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...
but its cartridges sometimes contained a megabit
Megabit
The megabit is a multiple of the unit bit for digital information or computer storage. The prefix mega is defined in the International System of Units as a multiplier of 106 , and therefore...
or more of ROM, addressed via bank switching called a Multi-Memory Controller
Multi-Memory Controller
Multi-Memory Controllers or Memory Management Controllers are different kinds of special chips designed by various video game developers for use in Nintendo Entertainment System cartridges...
. Game Boy
Game Boy
The , is an 8-bit handheld video game device developed and manufactured by Nintendo. It was released in Japan on , in North America in , and in Europe on...
cartridges used a chip called MBC (Memory Bank Controller), which not only offered ROM bank switching, but also cartridge SRAM bank switching, and even access to such peripherals as infrared links or rumble motors. Bank switching was still being used on later game systems. Several Sega Mega Drive cartridges were over 4MB in size and required the use of this technique (4MB being the maximum address size).
The GP2X
GP2X
The GP2X is an open-source, Linux-based handheld video game console and portable media player developed by South Korean company GamePark Holdings. It was released on November 10, 2005, in South Korea only....
handheld from Gamepark Holdings uses bank switching in order to control the start address (or memory offset) for the second processor.
Alternative and successor techniques
Bank switching was later supplanted by segmentationMemory segment
x86 memory segmentation refers to the implementation of memory segmentation on the x86 architecture. Memory is divided into portions that may be addressed by a single index register without changing a 16-bit segment selector. In real mode or V86 mode, a segment is always 64 kilobytes in size . In...
in many 16-bit systems, which in turn gave way to paging
Paging
In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...
memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
s. In embedded systems, however, bank switching is still often used, for its simplicity, low cost, and often better adaptation to those contexts than to general purpose computing.
Video processing
In some types of computer video displays, the related technique of double buffering may be used to improve video performance. In this case, while the processor is updating the contents of one set of physical memory locations, the video generation hardware is accessing and displaying the contents of a second set. When the processor has completed its update, it can signal to the video display hardware to swap active banks, so that the transition visible on screen is free of artifacts or distortion. In this case, the processor may have access to all the memory at once, but the video display hardware is bank-switched between parts of the video memory. If the two(or more) banks of video memory contain slightly different images, rapidly cycling (page-flipping) between them can create animation or other visual effects that the processor might otherwise be too slow to carry out directly.Computers using bank switching
- Apollo Guidance ComputerApollo Guidance ComputerThe Apollo Guidance Computer provided onboard computation and control for guidance, navigation, and control of the Command Module and Lunar Module spacecraft of the Apollo program...
- TRS-80 Model 4
- Amstrad CPCAmstrad CPCThe Amstrad CPC is a series of 8-bit home computers produced by Amstrad between 1984 and 1990. It was designed to compete in the mid-1980s home computer market dominated by the Commodore 64 and the Sinclair ZX Spectrum, where it successfully established itself primarily in the United Kingdom,...
- Amstrad PCWAmstrad PCWThe Amstrad PCW series was a range of personal computers produced by British company Amstrad from 1985 to 1998, and also sold under licence in Europe as the "Joyce" by the German electronics company Schneider in the early years of the series' life. When it was launched, the cost of a PCW system was...
- Apple II seriesApple II seriesThe Apple II series is a set of 8-bit home computers, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977 with the original Apple II...
- Apple IIIApple IIIThe Apple III is a business-oriented personal computer produced and released by Apple Computer that was intended as the successor to the Apple II series, but largely considered a failure in the market. Development work on the Apple III started in late 1978 under the guidance of Dr. Wendell Sander...
- Atari 130XE
- Commodore 64Commodore 64The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...
- Commodore 128Commodore 128The Commodore 128 home/personal computer was the last 8-bit machine commercially released by Commodore Business Machines...
- MSXMSXMSX was the name of a standardized home computer architecture in the 1980s conceived by Kazuhiko Nishi, then Vice-president at Microsoft Japan and Director at ASCII Corporation...
- PIC microcontrollerPIC microcontrollerPIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division...
- ZX Spectrum 128 models