keil中所有库函数的帮助文件。 The C51 run-time library provides you with more than 100 predefined functions and macros to use in your 8051 C programs. This library makes embedded software development easier by providing you with routines that perform common programming tasks such as string and buffer manipulation, data conversion, and floating-point math operations. Typically, the routines in this library conform to the ANSI C Standard. However, some functions differ slightly in order to take advantage of the features found in the 8051 architecture. For example, the function isdigit returns a bit value as opposed to an int. Where possible, function return types and argument types are adjusted to use the smallest possible data type. In addition, unsigned data types are favored over signed types. These alterations to the standard library provide a maximum of performance while also reducing program size. All routines in this library are implemented to be independent of and to function using any register bank.

Keil uvision中的帮助文档，包含完整的英文解释，不仅包含目录，还可以点开查看解释和用法。

C51lib.chm 包含所有的函数，可以帮助你编写程序