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的代码空间还将就，可是51自带的256B数据存储空间运用起来还真严重，其间留给用户的连128B都不到，所以不得不扩展片外RAM。扩展RAM办法和扩展ROM差不多，都是占用P0/P2口做地址线，一起P0用锁存器74373分时复用地址和数据信号。
曾经扩展RAM是用汇编言语拜访存储器，优点是定位精准，指哪打哪，害处便是：程序规划一大就有点难保护了，所以还得改用C完成。
对应于汇编言语用R1,R0/DPTR拜访外部RAM，keil C扩展了存储类型，添加了如pdata(等同用movx @Rn拜访办法)/xdata(等同于movx @DPTR拜访办法)存储类型用于拜访片外ram。一起，还供给了肯定地址拜访的宏，如PBYTE/XBYTE，检查界说：
#define PBYTE((unsigned char volatile pdata*)0);#define XBYTE((unsigned char volatile xdata*)0);
其实，也仍是界说相应存储类型的指针~。
上仿真图和代码前，收拾一下keil c供给拜访肯定地址的办法：
1._at_界说变量：
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方位，留意bit型变量不能运用_at_指定方位；
2.肯定地址拜访：
头文件absacc.h供给了肯定地址拜访宏，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：
上段上一年用汇编写的拜访ram的代码：
ORG 0SJMP STARTORG 10HSTART:;1456是要拜访的内存地址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言语版别：
#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;}
最终看下程序运转后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 | ................
