16-bit

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In computer architecture, 16-bit integers, memory addresses, or other data units are those that are at most 16 bits (2 octets) wide. Also, 16-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 16-bit is also a term given to a generation of computers in which 16-bit processors were the norm.

[edit] 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 bits long and arithmetic instructions, even though its external bus was 8 bits wide. Other notable 16-bit processors include the Texas Instruments TMS9900 and the Zilog Z8000.

A 16-bit integer can store 2¹⁶ (or 65536) unique values. In an unsigned representation, these values are the integers between 0 and 65535; using two's complement, possible values range from −32768 to 32767. Hence, a processor with 16-bit memory addresses can directly access 64 KB of byte-addressable memory.

16-bit processors have been almost entirely supplanted in the personal computer industry, but remain in use in a wide variety of embedded applications. For example the 16-bit XAP processor is used in many ASICs.

[edit] The 16/32-bit Motorola 68000 and Intel 386SX

The Motorola 68000 is sometimes called "16-bit" because its internal and external data buses were 16 bits wide, however it was 32-bit in that the general purpose registers were 32 bits wide and most arithmetic instructions supported 32-bit arithmetic. The MC68000 was a microcoded processor with three internal 16-bit ALU units. Only 24-bits of the Program Counter were available on original DIP packages, with up to 16 megabytes of addressable RAM. MC68000 software is 32-bit in nature, and forwards-compatible with other 32-bit processors. ^[1] The MC68008 was a version of the 68000 with 8-bit external data path and 1 megabyte addressing. Several Apple Inc. Macintosh models, e.g. LC series, used 32-bit MC68020 and MC68030 processors on a 16-bit data bus to save cost.

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.

The 68000 processor of the Sega Mega Drive/Genesis was a highly advertised feature of the video game system. Due to the saturation of this advertising, the 1988-1995 era (fourth generation) of video game consoles is often called "the 16-bit era".

[edit] 16-bit file format

A 16-bit file format is a binary file format for which each data element is defined on 16 bits (or 2 Bytes). An example of such a format is UTF-16 and the Windows Metafile Format.

[edit] 16-bit memory models

Similar to 64-bit's data models, the 16-bit Intel architecture allows for different memory models—ways to access a particular memory location. The reason for using a specific memory model is the size of the assembler instructions or required storage for pointers. Compilers of the 16-bit era generally had the following typewidth characteristic:

“Tiny”

Code and data will be in the same segment (especially, the registers CS,DS,ES,SS will point to the same segment); “near” pointers are always used. Code, data and stack together cannot exceed 64K.

“Small”

Code and data will be in different segments, and “near” pointers are always used. There will be 64K of space for code and 64K for data/stack.

“Medium”

Code pointers will use “far” pointers, enabling access to 1MB. Data pointers remain to be of the “near” type.

“Compact”

Data pointers will use “far” and code will use “near” pointers.

“Large”/“Huge”

Code and data pointers will be “far”.^[2]

[edit] References

1. ^ http://cache.freescale.com/files/archives/doc/ref_manual/M68000PRM.pdf?fsrch=1&WT_TYPE=Reference%20Manuals&WT_VENDOR=FREESCALE&WT_FILE_FORMAT=pdf&WT_ASSET=Documentation
2. ^ Borland Turbo C++ 1.01 in-program manual