Hello, I would like to know if there's any way I can write C code that will use the assembly instructions RLC and ACC.x efficiently. I would like to some C code that will result in: MOV A,byBuffer MOV c,bInput RLC A MOV byBuffer,A The closest I could get was this: byBuffer=(byBuffer<<1)&bInput; Which results in: MOV A,byBuffer ADD A,ACC MOV R7,A MOV C,bInput CLR A RLC A ANL A,R7 MOV byBuffer,A The same thing happens when I want to use a specific bit from a byte. I have never seen the C-compiler use the ACC.x instruction. It uses solutions like this: MOV A,R6 MOV R0,AR5 INC R0 SJMP ?C0078 ?C0077: CLR C RLC A ?C0078: DJNZ R0,?C0077 Any ideas ????
Sorry, forgot to use the pre and /pre.... //--------------------------------------// Hello, I would like to know if there's any way I can write C code that will use the assembly instructions RLC and ACC.x efficiently. I would like to some C code that will result in:
MOV A,byBuffer MOV c,bInput RLC A MOV byBuffer,A
byBuffer=(byBuffer<<1)&bInput;
MOV A,byBuffer ADD A,ACC MOV R7,A MOV C,bInput CLR A RLC A ANL A,R7 MOV byBuffer,A
MOV A,R6 MOV R0,AR5 INC R0 SJMP ?C0078 ?C0077: CLR C RLC A ?C0078: DJNZ R0,?C0077
I don't think that it is quite what you are looking for, but there are a number of intrinsic functions provided by Keil and detailed in the C51 Compiler Manual. Most of these intrinsic functions roll a 1, 2 or 4-byte variable n places left or right. Although not perfect, the code for 1 and 2 byte variables is put in-line (rather than a function call) and they make use of RLC type instructions so that they are reasonably fast when shifting by a small number of bits. In spite of the claims of the manual, the rolls for 4-byte variables are actually implemented by library calls. Also, if I recall correctly from the last time that I looked, the compiler always codes a loop even when the roll is know to be just one bit. For a faster way to roll long variables, see: http://www.keil.com/forum/docs/thread1252.asp C can access any bit of a variable by various means. See http://www.keil.com/forum/docs/thread554.asp for some examples The most portable method it to use masks, but I don't think that that will generate an ACC.x type reference. The most obvious method is to use a structure containing fields with a width of just one bit, this will cause ACC.x type instructions to be generated, but Keil does not seem to make very efficient use of them (see http://www.keil.com/forum/docs/thread1291.asp ). In the end, if there are just a few very specific functions that you want to perform then your best bet is going to be to write an assembly language sub-routine that you can call from a C function.
First of all, your C code fragment doesn't do what the ASM did. The truly equivalent C fragment would have been:
byBuffer=(byBuffer<<1) | bInput;
MOV A, byBuffer RL A MOV ACC.7, bInput MOV byBuffer, A