51单片机资源扩展:扩展片外RAM

51单片机资源扩展:从片内ROM跳转到片外ROM 一文中扩展了单片机的程序存储器，4KB存储空间提升到64KB。其实，4K的代码空间还凑合，但是51自带的256B数据存储空间使用起来还真紧张，其中留给用户的连128B都不到，所以不得不扩展片外RAM。扩展RAM方法和扩展ROM差不多，都是占用P0/P2口做地址线，同时P0用锁存器74373分时复用地址和数据信号。

以前扩展RAM是用汇编语言访问存储器，好处是定位精准，指哪打哪，坏处就是：程序规模一大就有点难维护了，所以还得改用C实现。

对应于汇编语言用R1,R0/DPTR访问外部RAM，keil C扩展了存储类型，增加了如pdata(等同用movx @Rn访问方式)/xdata(等同于movx @DPTR访问方式)存储类型用于访问片外ram。同时，还提供了绝对地址访问的宏，如PBYTE/XBYTE，查看定义：

#define PBYTE((unsigned char volatile pdata*)0);#define XBYTE((unsigned char volatile xdata*)0);

其实，也还是定义相应存储类型的指针~。

上仿真图和代码前，整理一下keil c提供访问绝对地址的方法：

1._at_定义变量：

变量类型 [存储类型] 变量名 _at_ 常数，指定变量存放在常数所指定的ram位置，注意bit型变量不能使用_at_指定位置；

2.绝对地址访问：

头文件absacc.h提供了绝对地址访问宏，用于字节/字寻址，如

val=XBYTE[0x0000];读片外ram 0x00处内存值

XBYTE[0x0200]=val;写片外ram 0x0200处内存值

扩展ram仿真图：

上段去年用汇编写的访问ram的代码：

ORG  0SJMP  STARTORG  10HSTART:;1456是要访问的内存地址MOV  DPTR,#1456H;MOV  A,  #0FFH;;0xFF移入[1456]MOVX @DPTR,A;CLR  A;MOVX  A, @DPTR; ;对外部RAM 1456H读出数据到ANOPMOV  A, #0A0H;INC  DPLMOVX @DPTR,A; ;将0xA0写入外部RAM 1457HNOP;从外部RAM [1457]H读出数据到AMOVX  A, @DPTR; NOPSJMP STARTEND

c语言版本：

#include #include #define OFFRAMBASE 0x100unsigned char xdata offRamVal0 _at_ OFFRAMBASE*2;unsigned char xdata offRamVal1 _at_ 0x00;int main(){unsigned char idata index = 0;offRamVal0 = 0xAA;offRamVal1 = 0xCC;for(index=0;index<0x10;index++){XBYTE[index+OFFRAMBASE] = 0x55;	}		return 0;}

最后看下程序运行后6264内部存储的数据：

0000: CC 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................0010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................0020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................0090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................00A0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................00B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................00C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................00D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................00E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................00F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................0100: 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 | UUUUUUUUUUUUUUUU0110: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................0120: 00 00 00 00 00 00 00 00 00 00 0001E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................01F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................0200: AA 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................