Keil C51 8051汇编函数参数和返回值放在哪里，C语言调用汇编函数时汇编函数如何处理参数？

作者	将狼才鲸
创建日期	2024-07-21

参考网址：
- 4课:第一个单片机小程序该文章前半部分有汇编原始工程下载链接
- C51中汇编的使用及参数传递与数据返回
- 试把如下c函数改写成汇编语言函数,用51汇编完整写一个函数
Keil C51 C语言中调用汇编函数时，最多使用三个参数，默认第一个参数从R7开始放，第二个参数从R5开始放，第三个参数从R3开始放，如果是2字节的int，那么遵循8051的大端模式，高字节放在R2、R4、R6，低字节放在R3、R5、R7；如果是指针参数，无论是第一第二第三个参数，都放在R1~R3，存储类型是R3，指针值是0xR2R1（注意此时是小端模式存储）；超过三个的参数请用外部RAM来实现；
汇编函数将返回值返回给C语言时，返回值需要放在R7开始的位置，char就放在R7，int是0xR6R7，long和float是0xR4R5R6R7，指针是类型在R3，指针值0xR2R1；
汇编里的函数如果要给C语言用，那么函数名（标号）前要叫下划线，例如 _LOOP: NOP; RET;，C语言调用时去掉下划线，如：LOOP();
- 其实加下划线是代表有参数调用，但是无参数的汇编函数你也这么加并没有问题；
想找原文的，在Keil安装目录的C:\Keil_v5\C51\Hlp\c51.chm，在里面搜索Parameter Passing或者Passing in Registers，原文摘抄如下：

Passing in Registers

C functions may pass parameters in registers and fixed memory locations. A maximum of 3 parameters may be passed in registers. All other parameters are passed using fixed memory locations. The following tables define which registers are used for passing parameters.

Arg Number	char, 1-byte ptr	int, 2-byte ptr	long, float	generic ptr
1	R7	R6 & R7 (MSB in R6,LSB in R7)	R4---R7	R1---R3 (Mem type in R3, MSB in R2, LSB in R1)
2	R5	R4 & R5 (MSB in R4,LSB in R5)	R4---R7	R1---R3 (Mem type in R3, MSB in R2, LSB in R1)
3	R3	R2 & R3 (MSB in R2,LSB in R3)		R1---R3 (Mem type in R3, MSB in R2, LSB in R1)

The following examples clarify how registers are selected for parameter passing.

Declaration	Description
func1 ( int a)	The first and only argument, a, is passed in registers R6 and R7.
*func2 ( int b, int c, int d)**	The first argument, b , is passed in registers R6 and R7. The second argument, c , is passed in registers R4 and R5. The third argument, d, is passed in registers R1, R2, and R3.
func3 ( long e, long f)	The first argument, e , is passed in registers R4, R5, R6, and R7. The second argument, f, cannot be located in registers since those available for a second parameter with a type of long are already used by the first argument. This parameter is passed using fixed memory locations.
func4 ( float g, char h)	The first argument, g , passed in registers R4, R5, R6, and R7. The second parameter, h, cannot be passed in registers and is passed in fixed memory locations.