Concepts about big-size terminals and bit Domains

Size end:

For data types such as char in C ++, it occupies a byte size and will not cause any problems. However, when the numeric type is int, In a 32-bit system, it takes up 4 bytes (32bit). In this case, the order of the four bytes in the register is generated. For example, int maxheight = 0x12345678, & maxheight = 0x0042ffc4. How can I store it? At this time, we need to understand the principle of computer size.

Big end: (big-Endian) Place the high byte of the value on the low address of the memory, and place the value's position byte on the high address of the memory.

Small End: (little-Endian) is to put the high byte of the number on the high address, the low byte on the low address.

The commonly used X86 architecture is the small-end mode, while most DSPs and arm are also the small-end mode, but some arm can choose the large-size-end mode. Therefore, the above maxheight should be stored in the small-end mode. For details, see the following two tables.

Address	0x0042ffc4	0x0042ffc5	0x0042ffc6	0x0042ffc7
Value	0x78	0x56	0x34	0x12

Figure (1) shows the small-end mode. 

Address	0x0042ffc4	0x0042ffc5	0x0042ffc6	0x0042ffc7
Value	0x12	0x34	0x56	0x78

Figure (2) shows the big end mode.

Through the above table, we can see the differences in the size of the end. Here we cannot discuss the better way. I personally think the big end mode is more in line with my habits. (Note: Here I also want to say that there is no data type in the computer memory, such as Char or Int. The role of this type in code is to let the compiler know the number of data records that should be read from that address each time and assign values to the corresponding variables .)

Bit domain:

In a computer, binary 0 and 1 are used to represent data. Each 0 or 1 occupies 1 bit of storage space and 8 bytes constitute a byte ), is the smallest unit of data type in a computer. For example, char occupies one byte in a 32-bit system. However, as we know, sometimes the data in the program may not need such bytes. For example, if the status of a switch is enabled or disabled, it can be expressed by replacing 1 with 0. In this case, only one bucket is required for the switch status. If one byte is used for storage, another 7-bit storage space is obviously wasted. Therefore, in C language, we have the concept of bit segments (some are also called bit domains. The specific syntax is to add the colon (:) and the number of digits of the specified bucket after the variable name. The specific definition syntax is as follows:

 

Struct bit segment name
{
Bit segment data type bit segment variable name: Bit segment length;
.......
}

// Instance
Struct Node
{
Char A: 2;
Double I;
Int C: 4;
} Node;

In fact, the definition is very simple. The meaning of the above example is to define a char variable A, which occupies 2-bit storage space, a double variable I, and an int variable C that occupies 4-bit storage. Note that the variable used byte is changed here. It is not a commonly defined int variable that occupies 4 bytes, and a char variable occupies 1 byte. In the actual running environment, sizeof (node) = 24 is obtained because the memory bytes are aligned. The memory byte alignment has been discussed in the previous article.