About code, idata, xdata in C51-EEWORLD

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In terms of data storage type, the 8051 series has on-chip and off-chip program memory, on-chip and off-chip data memory. The on-chip program memory is also divided into direct addressing area and indirect addressing type, corresponding to code, data, xdata, idata and pdata type set according to the characteristics of the 51 series. Using different memories will make the program execution efficiency different. When writing C51 programs, it is best to specify the storage type of variables, which will help improve the program execution efficiency (this issue will be specifically discussed later). Slightly different from ANSI-C, it is only divided into SAMLL, COMPACT, and LARGE modes. Different modes correspond to different actual hardware systems and will also have different compilation results.

The difference between data, idata, xdata, and pdata in the 51 series

Data: Fixedly refers to the first 128 RAMs from 0x00 to 0x7f, which can be read and written directly using acc, with the fastest speed and the smallest generated code.

idata: Fixedly refers to the first 256 RAM from 0x00 to 0xff, of which the first 128 are exactly the same as the 128 of data, but the access method is different. idata is accessed in a pointer-like manner similar to C. The statement in the assembly is: mox ACC, @Rx. (Unimportant supplement: pointer-style access to idata in C works very well)

xdata: external extended RAM, generally refers to the external 0x0000-0xffff space, accessed using DPTR.

pdata: The lower 256 bytes of the external expansion RAM, read and write when the address appears on A0-A7, use movx ACC, @Rx to read and write. This is quite special, and C51 seems to have a bug for this, so it is recommended to use it less. But it also has its advantages. The specific usage belongs to the intermediate problem, so it will not be mentioned here.

Keywords：C51 code idata xdata Reference address：About code, idata, xdata in C51

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