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http://users.ece.cmu.edu/~koopman/stack_computers/sec4_1.html
" 4.1 CHARACTERISTICS OF 16-BIT DESIGNS
The systems discussed here are 16 bits wide because that is the smallest configuration that makes sense for most commercial stack processor applications.
4.1.1 Good fit with the traditional Forth model
The primary motivating factor for making Forth machines 16 bits wide is that the Forth programming model has traditionally been 16 bits. This is consistent with average Forth program sizes of less than 32K bytes and the implementation of most of the first Forth compilers on microprocessors with 64K byte address ranges.
4.1.2 Smallest interesting width
A major reason that Forth has historically been a 16-bit language is that 8 bits is too small for general purpose computations and addressing data structures. While 12 bits was tried in some of the earliest minicomputers, 16 bits seems to be the smallest integer size that is truly useful. Forth traditionally has not used more than a 16-bit computing model because it was developed before 32-bit microprocessors were available.
16-Bit machines are capable of addressing 64K of memory, which for a stack machine is a rather large program memory. 16-Bit machines have single precision integers in the range of -32 768 to +32 767 which is large enough for most computations. Using double precision (32-bit integers), a 16-bit machine can represent integers in the range of -2 147 483 648 to +2 147 483 647, which is large enough for all but the most demanding applications.
Of course, a machine with a 4-bit or 8-bit data path can be made to emulate a 16-bit machine. The result is generally unsatisfactory performance, because an 8-bit machine can be expected to be about half as fast as a 16-bit machine when manipulating 16-bit data. Since the machines discussed in this chapter are all designed for high speed processing, all have 16-bit internal data paths.
4.1.3 Small size allows integrated embedded system
The three Forth chips discussed in this chapter (the M17, NC4016, and RTX 2000) are all targeted at the embedded applications market. Embedded applications require a small processor with a small amount of program memory to satisfy demanding power, weight, size, and cost considerations. A 16-bit processor is often a good compromise that provides higher levels of performance than an 8-bit processor, which probably would need to spend a lot of time synthesizing 16-bit arithmetic operations, and a 32-bit processor, which is overkill for many applications. "
" 5.1 WHY 32-BIT SYSTEMS?
The 16-bit processors described in Chapter 4 are sufficiently powerful for a wide variety of applications, especially in an embedded control environment. But, there are some applications that require the added power of a 32-bit processor. These applications involve extensive use of 32-bit integer arithmetic, large memory address spaces, or floating point arithmetic.
One of the difficult technical challenges that arises when designing a 32-bit stack processor is the management of the stacks. A brute force approach is to have separate off-chip stack memories in the manner of the NC4016. Unfortunately, on a 32-bit design this requires having 64 extra pins for just the data bits, making the approach unpractical for cost-sensitive applications. The FRISC 3 solves this problem by maintaining two automatically managed top-of-stack buffers on-chip, and using the normal RAM data pins to spill individual stack elements to and from program memory. The RTX 32P simply allocates a large amount of chip space to on-chip stacks and performs block moves of stack elements to and from memory for stack spilling. Chapter 6 goes into more detail about the tradeoffs involved with these approaches. "
" 8.2 16-BIT VERSUS 32-BIT HARDWARE
8.2.1 16-Bit hardware often best
16-bit stack processors in general have lower costs than 32-bit processors. Their internal data paths are narrower, so they use fewer transistors and cost less to manufacture. They only need 16-bit paths to external memory, so they have half as many memory bus data pins as 32-bit processors. System costs are also lower, since a minimum configuration 16-bit processor only needs to have half the number of memory chips as a 32-bit processor for a single bank of memory. ... 16-Bit processors should always be evaluated for an application, then rejected in favor of 32-bit processors only if there is a clear benefit for the change.
8.2.2 32-Bit hardware is sometimes required
... 32-Bit stack processors should be used instead of 16-bit processors only in cases where the application requires high efficiency at one or more of the following: 32-bit integer calculations, access to large amounts of memory, or floating point arithmetic.
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-- http://users.ece.cmu.edu/~koopman/stack_computers/sec8_2.html
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apparently a lot of systems have 4k pages.
http://makezine.com/magazine/make-36-boards/which-board-is-right-for-me/
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"
Jim TurleyAbout 60% of all processors sold are 8-bitters; 4-bitters and 16-bitters each make up about 15%, and 32-bit CPUs are the smallest category, at about 9%. (The portions are a bit smaller if you count DSPs.) Of the 32-bit processors, about 75% of them go into embedded systems. And these are new processors. The installed base is even more skewed toward low-end chips.
Embedded processors outsell PC processors by orders of magnitude, and always have. Remember, microprocessors were invented for embedded systems, not for computers.
This information is in my January column in Embedded Systems Programming. You can verify the data from the usual sources, such as WSTS or SIA. " -- http://www.embedded.com/electronics-blogs/shop-talk/4024514/Do-processors-and-instruction-sets-matter-
" "Unlike the declining 16-bit market, the 8-bit is still holding the fort because of its price and software simplicity," he said. Overall, customers are trying to standardize their software across a single platform to minimize the software development costs and to enable easy portability across low to high-end platforms, he added. " -- http://www.eetimes.com/document.asp?doc_id=1323578
" Microcontrollers are classified on the basis of internal bus width into 4/8-bit microcontrollers, 16-bit microcontrollers and 32-bit microcontrollers. The 32-bit microcontrollers segment dominated the market in terms of revenue share and accounted for over 34% of the market revenue in 2013. It is also expected to be the fastest growing segment over the next six years. 32-bit microcontrollers have witnessed increased popularity owing to reduction in unit prices. They are embedded in devices requiring higher processing capabilities as they are adept at performing complex control operations. The 16-bit microcontrollers segment followed the 32-bit microcontrollers segment in terms of market share in 2013 accounting for close to 30% of total market revenue in 2013. They are used in applications requiring faster processing speeds and greater power requirements such as automotive, air conditioning, refrigeration, factory automation medical applications and others. " -- http://www.grandviewresearch.com/industry-analysis/microcontroller-market
" April 25, 2013 MCU Market on Migration Path to 32-bit and ARM-based Devices
Historically, the 8-bit segment has dominated MCU sales, but as shown in Figure 2, the 32-bit segment became the leading segment of the MCU market in 2010. The difference between 4-/8- and 32-bit MCU sales was just $235 million in 2010, but the size gap has grown much wider since then. The 32-bit microcontroller segment is forecast to reach nearly $6.9 billion in 2013, 57% larger than the size of the 4 /8-bit MCU market.
In terms of unit shipments, 16-bit microcontrollers became the largest volume MCU category in 2011, overtaking 8-bit devices for the first time that year
" -- http://www.icinsights.com/news/bulletins/MCU-Market-On-Migration-Path-To-32bit-And-ARMbased-Devices/
