ARM Basics: Length of a "word" in ARM-EEWORLD

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Description: ARM Basics: The length of a "word" in ARM.

(I won’t discuss the issue of one “Chinese character” representing two “bytes” here, as this almost bothered me.)

When learning assembly, I used LDRH R2, [R1]; to read the half-word data at memory address R1 into register R2, and clear the upper 16 bits of R2. I had never heard of the concept of word before, so I was very confused. This aroused my curiosity to find out the length of "word". How many bytes are there in a word? This is a very obscure question, especially the following explanation:

"In computing, a word is the term used to represent the natural unit of data for a particular computer design. In this particular computer, a word is a fixed-length group of bits that is used to process transactions at one time. The number of bits in a word (the word length) is an important characteristic in the architecture of a computer system."

I just don't understand, but the result of the LDRH R2, [R1] experiment above is that the 16 bits of the memory address pointed to by R1 are all given to R2, indicating that the "word" is 32 bits. But when I used the calculator on Win7, I found another situation. As shown in the figure below, a "word" is given 16 bits.

ARM Basics: Length of a "word" in ARM

This irritated me even more, and I had to figure it out. I found the answer on Wikipedia when I googled it, and it turns out that this "word" is not fixed, it is 16 bits for X86 and 32 bits for ARM. (Link here)

The word length statistics are made for each architecture, let’s take a look.

This shows that no one is wrong, the calculator that comes with Win7 is based on the x86 standard. If you want to use this calculator to calculate words in ARM, you need to use the "double word" here to represent a "word" in ARM.

Word length table

years	Computer architecture	Word length	Integer length	Floating point length	Length instruction	Addressing unit	Character length
1941	Samsung Z3	22 b	–	In	8 b	In	–
1942	ABC	50 b	In	–	–	–	–
1944	Harvard Mark I	23 d	In	–	24 b	–	–
1946 (1948) {1953}	ENIAC (w/Panel #16) {w/Panel #26}	10 d	w, 2w (w) {w}	–	– (2d, 4d, 6d, 8d)	- - {In}	–
1951	UNIVAC I	12 d	In	–	½w	In	1 d
1952	IAS machine	40 b	In	–	½w	In	5 b
1952	IBM 701	36 b	½ in, in	–	½w	½ in, in	6 b
1952	UNIVAC 60	n d	1d, ... 10d	–	–	–	2d, 3d
1953	IBM 702	n d	0d, ... 511d	–	5d	d	1 d
1953	UNIVAC 120	n d	1d, ... 10d	–	–	–	2d, 3d
1954 (1955)	IBM 650 (w/IBM 653)	10 d	In	- (In)	In	In	2 d
1954	IBM 704	36 b	In	In	In	In	6 b
1954	IBM 705	n d	0d, ... 255d	–	5d	d	1 d
1954	IBM NORC	16 d	In	w, 2w	In	In	–
1956	IBM 305	n d	1d, ... 100d	–	10d	d	1 d
1958	UNIVAC II	12 d	In	–	½w	In	1 d
1958	SAGE	32 b	½w	–	In	In	6 b
1958	Autonetics Recomp II	40 b	w, 79 b, 8d, 15d	2w	½w	½ in, in	5 b
1959	IBM 1401	n d	1d, ...	–	d, 2d, 4d, 5d, 7d, 8d	d	1 d
1959 (TBD)	IBM 1620	n d	2d, ...	– (4d, ... 102d)	12d	d	2 d
1960	LARC	12 d	w, 2w	w, 2w	In	In	2 d
1960	IBM 1410	n d	1d, ...	–	d, 2d, 6d, 7d, 11d, 12d	d	1 d
1960	IBM 7070	10 d	In	In	In	w, d	2 d
1960	PDP-1	18 b	In	–	In	In	6 b
1961	IBM 7030 (Stretch)	64 b	1b, ... 64b, 1d, ... 16d	In	½ in, in	b, ½w, w	1 b, ... 8 b
1961	IBM 7080	n d	0d, ... 255d	–	5d	d	1 d
1962	UNIVAC III	25 b, 6 d	w, 2w, 3w, 4w	–	In	In	6 b
1962	UNIVAC 1107	36 b	1/6 in, ⅓ in, ½ in, in	In	In	In	6 b
1962	IBM 7010	n d	1d, ...	–	d, 2d, 6d, 7d, 11d, 12d	d	1 d
1962	IBM 7094	36 b	In	w, 2w	In	In	6 b
1963	Gemini Guidance Computer	39 b	26 b	–	13 b	13 b, 26 b	–
1963 (1966)	Apollo Guidance Computer	15 b	In	–	w, 2w	In	–
1964	CDC 6600	60 b	In	In	¼w, ½w	In	6 b
1965	IBM 360	32 b	½w, w, 1d, ... 16d	w, 2w	½ in, in, 1½ in	8 b	8 b
1965	UNIVAC 1108	36 b	1/6w, ¼w, ⅓w, ½w, w, 2w	w, 2w	In	In	6 b, 9 b
1965	PDP-8	12 b	In	–	In	In	8 b
1970	PDP-11	16 b	In	2w, 4w	w, 2w, 3w	8 b	8 b
1971	Intel 4004	4 b	w, d	–	2w, 4w	In	–
1972	Intel 8008	8 b	w, 2d	–	w, 2w, 3w	In	8 b
1972	Calcomp 900	9 b	In	–	w, 2w	In	8 b
1974	Intel 8080	8 b	w, 2w, 2d	–	w, 2w, 3w	In	8 b
1975	Cray-1	64 b	24 b, w	In	¼w, ½w	In	8 b
1975	Motorola 6800	8 b	w, 2d	–	w, 2w, 3w	In	8 b
1975	MOS Tech. 6501 MOS Tech. 6502	8 b	w, 2d	–	w, 2w, 3w	In	8 b
1976	Zilog Z80	8 b	w, 2w, 2d	–	w, 2w, 3w, 4w, 5w	In	8 b
1978 (1980)	Intel 8086 (w/Intel 8087)	16 b	½w, w, 2d (w, 2w, 4w)	– (2w, 4w, 5w, 17d)	½ in, in, ... 7 in	8 b	8 b
1978	VAX-11/780	32 b	¼w, ½w, w, 1d, ... 31d, 1b, ... 32b	w, 2w	¼w, ... 14¼w	8 b	8 b
1979	Motorola 68000	32 b	¼w, ½w, w, 2d	–	½ in, in, ... 7½ in	8 b	8 b
1982 (1983)	Motorola 68020 (w/Motorola 68881)	32 b	¼w, ½w, w, 2d	– (w, 2w, 2½w)	½ in, in, ... 7½ in	8 b	8 b
1985	ARM1	32 b	In	–	In	8 b	8 b
1985	MIPS	32 b	¼in, ½in, in	w, 2w	In	8 b	8 b
1989	Intel 80486	16 b	½w, w, 2dw , 2w, 4w	2w, 4w, 5w, 17d	½ in, in, ... 7 in	8 b	8 b
1989	Motorola 68040	32 b	¼w, ½w, w, 2d	w, 2w, 2½w	½ in, in, ... 7½ in	8 b	8 b
1991	PowerPC	32 b	¼in, ½in, in	w, 2w	In	8 b	8 b
2000	IA-64	64 b	8 b, ¼w, ½w, w	½ in, in	41 b	8 b	8 b
2002	XScale	32 b	In	w, 2w	½ in, in	8 b	8 b
Description: b: bit, d: decimal number, w: word length of the structure, n: variable size

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