以前擴(kuò)展RAM是用匯編語(yǔ)言訪問(wèn)存儲(chǔ)器，好處是定位精準(zhǔn)，指哪打哪，壞處就是：程序規(guī)模一大就有點(diǎn)難維護(hù)了，所以還得改用C實(shí)現(xiàn)。

對(duì)應(yīng)于匯編語(yǔ)言用R1,R0/DPTR訪問(wèn)外部RAM，keil C擴(kuò)展了存儲(chǔ)類型，增加了如pdata(等同用movx @Rn訪問(wèn)方式)/xdata(等同于movx @DPTR訪問(wèn)方式)存儲(chǔ)類型用于訪問(wèn)片外ram。同時(shí)，還提供了絕對(duì)地址訪問(wèn)的宏，如PBYTE/XBYTE，查看定義：

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

上仿真圖和代碼前，整理一下keil c提供訪問(wèn)絕對(duì)地址的方法：

1._at_定義變量：

變量類型 [存儲(chǔ)類型] 變量名 _at_ 常數(shù)，指定變量存放在常數(shù)所指定的ram位置，注意bit型變量不能使用_at_指定位置；

2.絕對(duì)地址訪問(wèn)：

頭文件absacc.h提供了絕對(duì)地址訪問(wèn)宏，用于字節(jié)/字尋址，如

val=XBYTE[0x0000];讀片外ram 0x00處內(nèi)存值

XBYTE[0x0200]=val;寫片外ram 0x0200處內(nèi)存值

擴(kuò)展ram仿真圖：

上段去年用匯編寫的訪問(wèn)ram的代碼：

ORG  0SJMP  STARTORG  10HSTART:;1456是要訪問(wèn)的內(nèi)存地址MOV  DPTR,#1456H;MOV  A,  #0FFH;;0xFF移入[1456]MOVX @DPTR,A;CLR  A;MOVX  A, @DPTR; ;對(duì)外部RAM 1456H讀出數(shù)據(jù)到ANOPMOV  A, #0A0H;INC  DPLMOVX @DPTR,A; ;將0xA0寫入外部RAM 1457HNOP;從外部RAM [1457]H讀出數(shù)據(jù)到AMOVX  A, @DPTR; NOPSJMP STARTEND

#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;}

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 | ................