16-bit computing

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In computer architecture, 16-bit integers, memory addresses, or other data units are those that are 16 bits (2 octets) wide. Also, 16-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers, address buses, or data buses of that size. 16-bit microcomputers are microcomputers that use 16-bit microprocessors.

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A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two most common representations, the range is 0 through 65,535 (216 − 1) for representation as an (unsigned) binary number, and −32,768 (−1 × 215) through 32,767 (215 − 1) for representation as two's complement. Since 216 is 65,536, a processor with 16-bit memory addresses can directly access 64 KB (65,536 bytes) of byte-addressable memory. If a system uses segmentation with 16-bit segment offsets, more can be accessed.

16-bit architecture

The MIT Whirlwind (c. 1951) [1] [2] was quite possibly the first-ever 16-bit computer. It was an unusual word size for the era; most systems used six-bit character code and used a word length of some multiple of 6-bits. This changed with the effort to introduce ASCII, which used a 7-bit code and naturally led to the use of an 8-bit multiple which could store a single ASCII character or two binary coded decimal digits.

The 16-bit word length thus became more common in the 1960s, especially on minicomputer systems. Early 16-bit computers (c. 1965–70) include the IBM 1130, [3] the HP 2100, [4] the Data General Nova, [5] and the DEC PDP-11. [6] Early 16-bit microprocessors, often modeled on one of the mini platforms, began to appear in the 1970s. Examples (c. 1973–76) include the five-chip National Semiconductor IMP-16 (1973), [7] the two-chip NEC μCOM-16 (1974), [8] [7] the three-chip Western Digital MCP-1600 (1975), and the five-chip Toshiba T-3412 (1976). [7]

Early single-chip 16-bit microprocessors (c. 1975–76) include the Panafacom MN1610 (1975), [9] [10] [7] National Semiconductor PACE (1975), General Instrument CP1600 (1975), Texas Instruments TMS9900 (1976), [7] Ferranti F100-L, and the HP BPC. Other notable 16-bit processors include the Intel 8086, the Intel 80286, the WDC 65C816, and the Zilog Z8000. The Intel 8088 was binary compatible with the Intel 8086, and was 16-bit in that its registers were 16 bits wide, and arithmetic instructions could operate on 16-bit quantities, even though its external bus was 8 bits wide.

16-bit processors have been almost entirely supplanted in the personal computer industry, and are used less than 32-bit (or 8-bit) CPUs in embedded applications.

16/32-bit Motorola 68000 and Intel 386SX

The Motorola 68000 is sometimes called 16-bit because of the way it handles basic arithmetic. The instruction set was based on 32-bit numbers and the internal registers were 32 bits wide, so by common definitions, the 68000 is a 32-bit design. Internally, 32-bit arithmetic is performed using two 16-bit operations, and this leads to some descriptions of the system as 16-bit, or "16/32".

Such solutions have a long history in the computer field, with various designs performing math even one bit at a time, known as "serial arithmetic", while most designs by the 1970s processed at least a few bits at a time. A common example is the Data General Nova, which was a 16-bit design that performed 16-bit math as a series of four 4-bit operations. 4-bits was the word size of a widely available single-chip ALU and thus allowed for inexpensive implementation. Using the definition being applied to the 68000, the Nova would be a 4-bit computer, or 4/16. Not long after the introduction of the Nova, a second version was introduced, the SuperNova, which included four of the 4-bit ALUs running in parallel to perform math 16 bits at a time and therefore offer higher performance. This was invisible to the user and the programs, which always used 16-bit instructions and data. In a similar fashion, later 68000-family members, like the Motorola 68020, had 32-bit ALUs.

One may also see references to systems being, or not being, 16-bit based on some other measure. One common one is when the address space is not the same size of bits as the internal registers. Most 8-bit CPUs of the 1970s fall into this category; the MOS 6502, Intel 8080, Zilog Z80 and most others had 16-bit address space which provided 64 KB of address space. This also meant address manipulation required two instruction cycles. For this reason, most processors had special 8-bit addressing modes, the zero page, improving speed. This sort of difference between internal register size and external address size remained in the 1980s, although often reversed, as memory costs of the era made a machine with 32-bit addressing, 2 or 4 GB, a practical impossibility. For example, the 68000 exposed only 24 bits of addressing on the DIP, limiting it to a still huge (for the era) 16 MB. [11]

A similar analysis applies to Intel's 80286 CPU replacement, called the 386SX, which is a 32-bit processor with 32-bit ALU and internal 32-bit data paths with a 16-bit external bus and 24-bit addressing of the processor it replaced.

16-bit application

In the context of IBM PC compatible and Wintel platforms, a 16-bit application is any software written for MS-DOS, OS/2 1.x or early versions of Microsoft Windows which originally ran on the 16-bit Intel 8088 and Intel 80286 microprocessors. Such applications used a 20-bit or 24-bit segment or selector-offset address representation to extend the range of addressable memory locations beyond what was possible using only 16-bit addresses. Programs containing more than 216 bytes (65,536 bytes) of instructions and data therefore required special instructions to switch between their 64-kilobyte segments, increasing the complexity of programming 16-bit applications.

List of 16-bit CPUs

See also

Related Research Articles

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<span class="mw-page-title-main">Intel 80286</span> Microprocessor model

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<span class="mw-page-title-main">Intel 8086</span> 16-bit microprocessor

The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-bit data bus, and is notable as the processor used in the original IBM PC design.

<span class="mw-page-title-main">Intel 8088</span> Intel microprocessor model

The Intel 8088 microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range are unchanged, however. In fact, according to the Intel documentation, the 8086 and 8088 have the same execution unit (EU)—only the bus interface unit (BIU) is different. The 8088 was used in the original IBM PC and in IBM PC compatible clones.

<span class="mw-page-title-main">Microprocessor</span> Computer processor contained on an integrated-circuit chip

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<span class="mw-page-title-main">Motorola 68000</span> Microprocessor

The Motorola 68000 is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Sector.

<span class="mw-page-title-main">Motorola 68020</span> 32-bit microprocessor

The Motorola 68020 is a 32-bit microprocessor from Motorola, released in 1984. A lower-cost version was also made available, known as the 68EC020. In keeping with naming practices common to Motorola designs, the 68020 is usually referred to as the "020", pronounced "oh-two-oh" or "oh-twenty".

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<span class="mw-page-title-main">Zilog Z80</span> 8-bit microprocessor

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The NS32000, sometimes known as the 32k, is a series of microprocessors produced by National Semiconductor. The first member of the family came to market in 1982, briefly known as the 16032 before becoming the 32016. It was the first general-purpose microprocessor on the market that used 32-bit data throughout: the Motorola 68000 used 32-bit data but had a 16-bit ALU and thus took twice as long perform many operations. However, the 32016 contained many bugs and often could not be run at its rated speed. These problems, and the presence of the otherwise similar 68000 which had been available since 1980, led to little use in the market.

In computer architecture, 8-bit integers or other data units are those that are 8 bits wide. Also, 8-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers or data buses of that size. Memory addresses for 8-bit CPUs are generally larger than 8-bit, usually 16-bit. 8-bit microcomputers are microcomputers that use 8-bit microprocessors.

The 88000 is a RISC instruction set architecture developed by Motorola during the 1980s. The MC88100 arrived on the market in 1988, some two years after the competing SPARC and MIPS. Due to the late start and extensive delays releasing the second-generation MC88110, the m88k achieved very limited success outside of the MVME platform and embedded controller environments. When Motorola joined the AIM alliance in 1991 to develop the PowerPC, further development of the 88000 ended.

The Motorola 68000 series is a family of 32-bit complex instruction set computer (CISC) microprocessors. During the 1980s and early 1990s, they were popular in personal computers and workstations and were the primary competitors of Intel's x86 microprocessors. They were best known as the processors used in the early Apple Macintosh, the Sharp X68000, the Commodore Amiga, the Sinclair QL, the Atari ST and Falcon, the Atari Jaguar, the Sega Genesis, the Phillips CD-i, the Capcom System I (Arcade), the AT&T UNIX PC, the Tandy Model 16/16B/6000, the Sun Microsystems Sun-1, Sun-2 and Sun-3, the NeXT Computer, NeXTcube, NeXTstation, and NeXTcube Turbo, early Silicon Graphics IRIS workstations, computers from MASSCOMP, the Texas Instruments TI-89/TI-92 calculators, the Palm Pilot, the Control Data Corporation CDCNET Device Interface, and the Space Shuttle. Although no modern desktop computers are based on processors in the 680x0 series, derivative processors are still widely used in embedded systems.

In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32-bit units. Compared to smaller bit widths, 32-bit computers can perform large calculations more efficiently and process more data per clock cycle. Typical 32-bit personal computers also have a 32-bit address bus, permitting up to 4 GB of RAM to be accessed, far more than previous generations of system architecture allowed.

<span class="mw-page-title-main">Zilog Z8000</span> 16-bit microprocessor

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<span class="mw-page-title-main">Intel iAPX 432</span> Discontinued Intel microprocessor architecture

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4-bit computing is the use of computer architectures in which integers and other data units are 4 bits wide. 4-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers or data buses of that size. Memory addresses for 4-bit CPUs are generally much larger than 4-bit, such as 12-bit or more, while they could in theory be 8-bit.

In computer architecture, 24-bit integers, memory addresses, or other data units are those that are 24 bits wide. Also, 24-bit central processing unit (CPU) and arithmetic logic unit (ALU) architectures are those that are based on registers, address buses, or data buses of that size.

<span class="mw-page-title-main">History of general-purpose CPUs</span> History of processors used in general purpose computers

The history of general-purpose CPUs is a continuation of the earlier history of computing hardware.

References

  1. ↑ "Year 1951". Computer History Museum. (see also "Year 1943".).
  2. ↑ Digital Press, Digital at Work Archived 2013-07-02 at the Wayback Machine , Pearson, 1992, ISBN   1-55558-092-0, pp. 4, 23.
  3. ↑ "The IBM 1130 computing system". IBM Archives.
  4. ↑ "HP 2116". Computer History Museum.
  5. ↑ "Data General Nova minicomputer". Computer History Museum. Archived from the original on 2013-05-17. Retrieved 2012-06-11.
  6. ↑ Pearson, Jamie Parker (September 1992). Digital at work: snapshots from the first thirty-five years. Digital Press. pp. 58–61. ISBN   978-1-55558-092-6.
  7. 1 2 3 4 5 Belzer, Jack; Holzman, Albert G.; Kent, Allen (1978). Encyclopedia of Computer Science and Technology. Vol. 10 – Linear and Matrix Algebra to Microorganisms: Computer-Assisted Identification. CRC Press. p. 402. ISBN   9780824722609.
  8. ↑ "1970s: Development and evolution of microprocessors" (PDF). Semiconductor History Museum of Japan. Archived from the original (PDF) on 2019-06-27. Retrieved 2019-06-27.
  9. ↑ "16-bit Microprocessors". CPU Museum. Retrieved 5 October 2010.
  10. ↑ "History". PFU. Retrieved 5 October 2010.
  11. ↑ Motorola M68000 Family, Programmer's Reference Manual (PDF). Motorola, Inc. 1992. sec. 2.4, pp. 2–21. Retrieved 2023-06-05.
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