Hello, BASE_ADDR = 0x8000 i=1 A)may I know what is #define COMMAND(i) (*((volatile char xdata *)(BASE_ADDR +i))) B)Is is better to address the "command register" of an external chip as XDATA of 8051 chip(using ALE,Port0,Port2,etc) or should I just manually arrange some I/O pins (of 8051) to do that? Regards, KC
Hello, Thank You Erik! I really need help on this --------------------------------------------------------------- BASE_ADDR = 0x8000 i=1 #define COMMAND(i) (*((volatile char xdata *)(BASE_ADDR +i))) --------------------------------------------------------------- this code seems to use casting to define COMMAND(i) to point to a specific xdata location 1) Is "COMMAND(i)" a "memory specific pointer"? 2) Does it take up any memory space (eg 2bytes as you would expect of a memory specific pointer)? 3) Is there a better way (or clearer code) to do this? 4) Is the "volatile" really necessary? (or should i do some settings to prevent optimization from affecting code) Regards, KC
1) Is "COMMAND(i)" a "memory specific pointer"? yes 2) Does it take up any memory space (eg 2bytes as you would expect of a memory specific pointer)? look in the generated assembly 3) Is there a better way (or clearer code) to do this? many ways, I would name the memory mapped I/Os individually and use #define SOME_COMMAND_ADDR (BASE_ADDR +nnn) xdata volatile SOME_COMMAND _at_ SOME_COMMAND_ADDR or something like that 4) Is the "volatile" really necessary? (or should i do some settings to prevent optimization from affecting code) Volatile works for this, why reinvent the wheel. Erik
1) Is "COMMAND(i)" a "memory specific pointer"? No. A Pointer is a variable - you are defining an expression 2) Does it take up any memory space (eg 2bytes as you would expect of a memory specific pointer)? No (apart from the code space necessary to evaluate the expression). COMMAND is a preprocessor definition - the macro is expanded by the preprocessor before the compiler ever sees it! 3) Is there a better way (or clearer code) to do this? How about _at_, as Erik suggested? Or Keil's predefined XBYTE? 4) Is the "volatile" really necessary? It's your system - only you can tell us that! ...should i do some settings to prevent optimization... No. This is precisely the situation for which volatile is provided - use it!