Hi,
Taking the code snippet below,
char* ptr = (char*)0x0000; short size = 0x100; short sum = 0x00; do { sum += *ptr++; } while (size--)
This will cause C51 to allocate "ptr" and "size" in RAM.
MOV R3,#00H MOV R2,#00H MOV R1,#00H MOV DPTR,#pre?1251 MOVX @DPTR,PR0 MOV DPTR,#size?1252 MOV A,#01H MOVX @DPTR,A INC DPTR CLR A MOVX @DPTR,A
and everytime "size" is decremented, or "ptr" is incremented, the values are loaded back into the RAM.
Is there a way to force C51 to not use RAM but use the registers only as "ptr" and "size" will never be used anymore after that. What I mean is, is there a way to force C51 to compile to something like this,
mov dptr, #WORD0(00h) mov r4, #01h ; r4:r5 = size mov r5, #00h mov r6, #00h ; r6:r7 = sum mov r7, #00h loop: movx A, @dptr+#000h addw 006h, A ; r6:r7 += A inc dptr subw 004h, 001h ; r4:r5-- jnc loop ; add code to store r6:r7 into RAM
Any advice would be greatly appreciated. Thanks in advance.
when I learned 'C' back in the 90's was there not a 'C' thing called 'register'?
Erik
"when I learned 'C' back in the 90's was there not a 'C' thing called 'register'?"
It's still there - But it's still only a hint to the compiler.
You probably mean this thing: http://www.keil.com/support/man/docs/c51/c51_le_register.htm
Ignored by the compiler.
The starting text in that link said "should be stored".
But it was just added as a hint, letting a developer put some extra priority on the decisions of what variables to allocate to registers.
It was only applicable when compilers were dumb and variables lived in memory or in registers for the full lifetime of a function.
Todays compilers performs huge amounts of lifetime analysis and other optimization steps, so a variable may sometimes be stored in a register, and at other times be stored in RAM. It's just a question of what the compiler have found to be the most efficient alternative for the different parts of the code.
Most newer compilers sees it as a dummy keyword to 100% ignore. We have the volatile keyword to force memory accesses. And we have the optimization flags to tell how much work we want the compiler to spend on tweaking the actual code generated.
Most newer compilers sees it as a dummy keyword to 100% ignore.
I'm quite sure that's untrue because otherwise most newer compilers would be defective. register still has ssome required semantic effect that compiler writers are not at liberty to ignore: it forbids taking address of any object so qualified, and code violating that rule has to be diagnosed. To do that, the compiler must heed the register keyword.
Not exactly. Suppose you specify 10 register variables? How many get used depends on the number available. Even worse you only specify 1 register variables. The compiler uses them all then what? An Error? You move some code around an now it is OK. The register keyword never guaranteed that a variable would be in a register. Only that it would be nice if it were.
Is it ANSI required?
Thank you everyone for the information. I have tried using the 'register' keyword before posting the question here but so far it is ignored by the compiler. By manually writing in assembly using registers, I manage to speed up the task by 50%. However it would be tedious to do so as the reason to use C is to simplify coding. I really hope there is a way to ask the compiler do what is needed.
"Is it ANSI required?"
The keyword must be processed according to the standard, which means it does mean something gramatically. But most new compilers totally ignore it for the code generation phase.
But does the taks actually need to be speeded up by 50% - or, indeed, at all?
"However it would be tedious"
So, unless it is really essential, don't do it!
As has already been suggested, if the performance is adequate, there is no point in messing about trying to optimise it!
By manually writing in assembly using registers, I manage to speed up the task by 50%. However it would be tedious to do so as the reason to use C is to simplify coding. while wrinting small routines in asm for speed reasons is a very valid approach, it you have to write large amounts of code in asm to achieve your goal, this is the wrong approach unless your product is to ship in millions. Wrinting small routines in asm for speed reasons is hardly 'tedious' and, typically, for the very reason they are small it will be write, debug and forget, so the 'tedium' is just once.