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I would like to to use the bit addressable memory range (bdata) with an index variable as in the following example:
#define SETBIT(I) WAIT_4_MUTEX; flags^I=1; FREE_MUTEX; unsigned char bdata flags; func () { unsigned char i; for (i=0; i < MAX_CELL; i++) { // do interesting stuff SETBIT(i); } }
This construction of coarse gives a lval error on the use of the SETBIT macro.
My current fall back are the macro's:
#define SETBIT(I) base |= ((unsigned char) (1 << I)) #define CLEARBIT(I) base &= ~((unsigned char) (1 << I))
But still have the feeling that I am wasting cycles on the shift operators in these two macro's and because this is within a mutex region the timing might become an issue.
Is there a better way to set/reset the i-th bit of a bit addressable variable ?
But still have the feeling that I am wasting cycles on the shift operators in these two macro's
Why would you worry about feelings, when you can so easily replace them by certainty? Check the generated code! List files exist for a reason.
If "I" is constant, probably not. It'll optimize to two simple operations, as in
mov a, #mask ; computed from (1 << I) orl base, a
For variable arguments, a table look-up instead of the shift would probably be better --- the 8051's shift opcodes are really rather limited, so a run-time computed shift-by-(n) is quite inefficient.
And of course, if all else fails, there's always Duff's Device. If you haven't heard of that before, let me put like this: you want to have seen it. And once you have, you may wish you hadn't... ;-)
Well, there is a suggestion, here is the listing for setting the second bit of the Hs_st1 bitfield:
C:0x2D4F 122FB9 LCALL OS_WAIT(C:2FB9) C:0x2D52 432202 ORL Hs_st1(0x22),#0x02 C:0x2D55 7F09 MOV R7,#0x09 C:0x2D57 123A72 LCALL OS_SEND_TOKEN(C:3A72)
The shifts apparently have been taken care of outside the critical region, probably even in the preproc's macro expansion. Although for that, in hindsight slightly embarrassing reason, this is what I wanted.
Thanks everybody for the contributions,
"In peace my mind withdraws"