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  • in CS
  • 6 min read

x86's ill-timed WORD

The word size is the computer's preferred size for moving units of information around; technically it's the width of your processor's registers. It reflects the amount of data that can be transmitted between memory and the processor in one chunk. Likewise, it may reflect the size of data that can be manipulated by the CPU's ALU in one cycle.

Whereas, in the universe of x86, word continues to designate a 16-bit quantity. Microsoft's Windows API maintains the programming language definition of WORD as 16 bits, despite the fact that the API may be used on a 32- or 64-bit x86 processor.

  • in CS
  • 6 min read

Machine Word

In computing, a word is the natural unit of data used by a particular processor design. The term word refers to the standard number of bits that are manipulated as a unit by any particular CPU.

The word size is the computer's preferred size for moving units of information around; technically it's the width of your processor's registers. It reflects the amount of data that can be transmitted between memory and the processor in one chunk. Likewise, it may reflect the size of data that can be manipulated by the CPU's ALU in one cycle.

  • in CS
  • 7 min read

Data Models

In 32-bit programs, pointers and data types such as integers generally have the same length. This is not necessarily true on 64-bit machines. Mixing data types in programming languages such as C and its descendants such as C++ and Objective-C may thus work on 32-bit implementations but not on 64-bit implementations.

  • in CS
  • 14 min read

Byte Order(Endianess)

在 Binary —— Bitset & Bytes 中，我们通过打印 bitset 和 byte array，直观感受了二进制的位模式（bit pattern & binary representation）。然后，遗留下了三个问题：

  1. hex(2010) 输出十六进制是 0x7da，hexdump(2010) 输出内存中的字节数组是 {0xda, 0x7}，字节顺序为什么不同？
  2. 每2个byte组合而成的短整型 short array [0] 为什么是 0x3130，而非 0x3031？
  3. 程序 test-conversion-narrow-down.c 输出结果是怎样的？

这三个问题的答案都涉及到字节序（Byte Order）问题。

  • in CS, c, cpp
  • 8 min read

Binary —— Bitset & Bytes

所谓数制是指计数的方法。对于有10根手指的人来来说，使用十进制表示法是很自然的事情，现代计算机则采用的是二进制系统，存储和处理的信息以二值信号表示。

一串二进制数码按照固定长度（8）组合出有意义的基本存储单位——字节，多个字节（1,2,4,8）可以组合出基本算术单元 char,short,int/float,long/double，或复合类型和用户自定义类型。

在计算机内存或磁盘上，指令和数据没有任何区别，都是二进制信息。CPU 在工作的时候把有的信息解析为指令，有的信息解读为数据，为同样的信息赋予了不同的意义。

这涉及到 Abstract State Machine：

  • a value : what state are we in
  • the type : what this state represents
  • the representation : how state is distinguished
  • in CS
  • 4 min read

Data Type

The type defines the operations(access or manipulate) that can be done on the data, the meaning of the data, and the way values of that type can be stored.