question: What's the difference between 32 and 64bit computer systems, and why aren't there, say 128bit systems.
comment: Firstly, there’s never any danger running 32 on 64. Ok, so you want an ELI5. Try this. Two people with different sized mouths. The guy with the small mouth can speak perfectly well and perfectly loudly. The guy with the big mouth can do everything the other can but he can also wiggle his tongue independently while talking normally. What a neat trick. In other words, big mouth guy can, potentially, carry more information for each word he speaks. Then the analogy breaks down pretty darn quickly.
passages: Some supercomputer architectures of the 1970s and 1980s, such as the Cray-1, used registers up to 64 bits wide, and supported 64-bit integer arithmetic, although they did not support 64-bit addressing. In the mid-1980s, Intel i860 development began culminating in a (too late for Windows NT) 1989 release; the i860 had 32-bit integer registers and 32-bit addressing, so it was not a fully 64-bit processor, although its graphics unit supported 64-bit integer arithmetic. However, 32 bits remained the norm until the early 1990s, when the continual reductions in the cost of memory led to installations with amounts of RAM approaching 4 GB, and the use of virtual memory spaces exceeding the 4 GB ceiling became desirable for handling certain types of problems. In response, MIPS and DEC developed 64-bit microprocessor architectures, initially for high-end workstation and server machines. By the mid-1990s, HAL Computer Systems, Sun Microsystems, IBM, Silicon Graphics, and Hewlett Packard had developed 64-bit architectures for their workstation and server systems. A notable exception to this trend were mainframes from IBM, which then used 32-bit data and 31-bit address sizes; the IBM mainframes did not include 64-bit processors until 2000. During the 1990s, several low-cost 64-bit microprocessors were used in consumer electronics and embedded applications. Notably, the Nintendo 64 and the PlayStation 2 had 64-bit microprocessors before their introduction in personal computers. High-end printers, network equipment, and industrial computers, also used 64-bit microprocessors, such as the Quantum Effect Devices R5000. 64-bit computing started to trickle down to the personal computer desktop from 2003 onward, when some models in Apple's Macintosh lines switched to PowerPC 970 processors (termed "G5" by Apple), and AMD released its first 64-bit x86-64 processor.
   Despite having a 32-bit ALU and word-length, processors based on ARM architecture version 1 and 2 had only a 26-bit PC and address bus, and were consequently limited to 64 MiB of addressable memory. This was still a vast amount of memory at the time, but because of this limitation, architectures since have included various steps away from the original 26-bit design.
   In 1995, Sun Microsystems introduced the 64-bit UltraSPARC microprocessor. As Sun was the dominant SPARC vendor, the majority of the market followed and began the migration to 64-bit SPARC. As a result, the 32-bit SPARC market began to decline.
   Section::::History.:Early ARM processors.
  
  In the ARM processor architecture, 26-bit refers to the design used in the original ARM processors where the Program Counter (PC) and Processor Status Register (PSR) were combined into one 32-bit register (R15), the status flags filling the high 6 bits and the Program Counter taking up the lower 26 bits.
   26-bit
  
  In computer architecture, 26-bit integers, memory addresses, or other data units are those that are 26 bits wide, and thus can represent values up to 64 mega (base 2). Two examples of computer processors that featured 26-bit memory addressing are certain second generation IBM System/370 mainframe computer models introduced in 1981 (and several subsequent models), which had 26-bit physical addresses but had only the same 24-bit virtual addresses as earlier models, and the first generations of ARM processors.
  
  Section::::History.
  
  Section::::History.:IBM System/370.
   Memory, as well as other digital circuits and wiring, was expensive during the first decades of 32-bit architectures (the 1960s to the 1980s). Older 32-bit processor families (or simpler, cheaper variants thereof) could therefore have many compromises and limitations in order to cut costs. This could be a 16-bit ALU, for instance, or external (or internal) buses narrower than 32 bits, limiting memory size or demanding more cycles for instruction fetch, execution or write back.
   BULLET::::- 2014: Google releases Android Lollipop, the first version of the Android operating system with support for 64-bit processors.
  
  BULLET::::- 2017: Apple releases iOS 11, supporting only machines with AArch64 processors. It has a 64-bit kernel that only supports 64-bit applications. 32-bit applications are no longer compatible.
  
  BULLET::::- 2019: Apple releases macOS 10.15 "Catalina", dropping support for Intel 32-bit applications.
  
  Section::::64-bit applications.
  
  Section::::64-bit applications.:32-bit vs 64-bit.
   At launch, the "T414" was the 32-bit offering. Originally, the first 32-bit variant was to be the "T424", but fabrication difficulties meant that this was redesigned as the T414 with 2 KB on-board RAM instead of the intended 4 KB. The T414 was available in 15 and 20 MHz varieties.
   BULLET::::- HP-Intel's IA-64 architecture:
  
  BULLET::::- Intel's Itanium processors
  
  BULLET::::- MIPS Technologies' MIPS64 architecture
  
  BULLET::::- ARM Holdings' AArch64 architecture
  
  BULLET::::- Elbrus architecture:
  
  BULLET::::- Elbrus-8S
  
  BULLET::::- NEC SX architecture
  
  BULLET::::- SX-Aurora TSUBASA
  
  BULLET::::- RISC-V
  
  Most architectures of 64 bits that are derived from the same architecture of 32 bits can execute code written for the 32-bit versions natively, with no performance penalty. This kind of support is commonly called "bi-arch support" or more generally "multi-arch support".
  
  Section::::See also.
  
  BULLET::::- Computer memory
  
  Section::::External links.
  
  BULLET::::- 64-bit Transition Guide, Mac Developer Library
   However, the opposite is often true for newer 32-bit designs. For example, the Pentium Pro processor is a 32-bit machine, with 32-bit registers and instructions that manipulate 32-bit quantities, but the external address bus is 36 bits wide, giving a larger address space than 4 GB, and the external data bus is 64 bits wide, primarily in order to permit a more efficient prefetch of instructions and data.
  
  Section::::Architectures.
   Furthermore, chips are often not available in order to implement the fastest rates. AMD, for instance, does not support the 32-bit HyperTransport interface on any CPU it has shipped as of the end of 2009. Additionally, WiMAX service providers in the US typically support only up to 4 Mbit/s as of the end of 2009.
   BULLET::::- 2003: Apple releases its Mac OS X 10.3 "Panther" operating system which adds support for native 64-bit integer arithmetic on PowerPC 970 processors. Several Linux distributions release with support for AMD64. FreeBSD releases with support for AMD64.
   Section::::Images.
  
  In digital images/pictures, 32-bit usually refers to RGBA color space; that is, 24-bit truecolor images with an additional 8-bit alpha channel. Other image formats also specify 32 bits per pixel, such as RGBE.
   On 64-bit hardware with x86-64 architecture (AMD64), most 32-bit operating systems and applications can run with no compatibility issues. While the larger address space of 64-bit architectures makes working with large data sets in applications such as digital video, scientific computing, and large databases easier, there has been considerable debate on whether they or their 32-bit compatibility modes will be faster than comparably priced 32-bit systems for other tasks.
   BULLET::::- Intel's K1OM architecture, a variant of "Intel 64" with no CMOV, MMX, and SSE instructions, used in Xeon Phi coprocessors, binary incompatible with x86-64 programs
  
  BULLET::::- VIA Technologies' 64-bit extensions, used in the VIA Nano processors
  
  BULLET::::- IBM's PowerPC/Power ISA:
  
  BULLET::::- IBM's POWER4, POWER5, POWER6, POWER7, POWER8, POWER9, and IBM A2 processors
  
  BULLET::::- SPARC V9 architecture:
  
  BULLET::::- Oracle's M8 and S7 processors
  
  BULLET::::- Fujitsu's SPARC64 XII and SPARC64 XIfx processors
  
  BULLET::::- IBM's z/Architecture, a 64-bit version of the ESA/390 architecture, used in IBM's eServer zSeries and System z mainframes:
  
  BULLET::::- IBM z13 and z14
  
  BULLET::::- Hitachi AP8000E
   Section::::History.:Development.
   Section::::Reception and legacy.
   BULLET::::- 1961: IBM delivers the IBM 7030 Stretch supercomputer, which uses 64-bit data words and 32- or 64-bit instruction words.
  
  BULLET::::- 1974: Control Data Corporation launches the CDC Star-100 vector supercomputer, which uses a 64-bit word architecture (prior CDC systems were based on a 60-bit architecture).
  
  BULLET::::- 1976: Cray Research delivers the first Cray-1 supercomputer, which is based on a 64-bit word architecture and will form the basis for later Cray vector supercomputers.
  
  BULLET::::- 1983: Elxsi launches the Elxsi 6400 parallel minisupercomputer. The Elxsi architecture has 64-bit data registers but a 32-bit address space.
   BULLET::::- 2006: Microsoft releases Windows Vista, including a 64-bit version for AMD64/EM64T processors that retains 32-bit compatibility. In the 64-bit version, all Windows applications and components are 64-bit, although many also have their 32-bit versions included for compatibility with plug-ins.
  
  BULLET::::- 2007: Apple releases Mac OS X 10.5 "Leopard", which fully supports 64-bit applications on machines with PowerPC 970 or EM64T processors.
   A common misconception is that 64-bit architectures are no better than 32-bit architectures unless the computer has more than 4 GiB of random-access memory. This is not entirely true:
   Section::::History.:Sega Neptune.
   In 2003, AMD shipped its 64-bit based microprocessor line for desktop computers, Opteron and Athlon 64. Also in 2003, IBM released the 64-bit based PowerPC 970 for Apple's high-end Power Mac G5 systems. Intel, in 2004, reacted to AMD's success with 64-bit based processors, releasing updated versions of their Xeon and Pentium 4 lines. 64-bit processors were first common in high end systems, servers and workstations, and then gradually replaced 32-bit processors in consumer desktop and laptop systems since about 2005.
  
  Section::::1990s and 2000s.:Lenovo.
   The x32 ABI for Linux (which defines an environment for programs with 32-bit addresses but running the processor in 64-bit mode) uses a 64-bit codice_4. Since it was a new environment, there was no need for special compatibility precautions.
   Section::::Game library.
   BULLET::::- 1991: MIPS Computer Systems produces the first 64-bit microprocessor, the R4000, which implements the MIPS III architecture, the third revision of its MIPS architecture. The CPU is used in SGI graphics workstations starting with the IRIS Crimson. Kendall Square Research deliver their first KSR1 supercomputer, based on a proprietary 64-bit RISC processor architecture running OSF/1.
  
  BULLET::::- 1992: Digital Equipment Corporation (DEC) introduces the pure 64-bit Alpha architecture which was born from the Prism project.
  