"Encouraging" the MSP430 PC v4.0.0 C/C++ compiler to use BIC/BIS instructions

Fred Carter

In the documentation for many of the MSP430 peripherals the reader is advised:

"it is recommended to set or clear [this/these] bit(s) using BIS.B or BIC.B instructions,
rather than MOV.B or CLR.B instructions"

The C/C++ code: P6DIR |= 0x30u;
generates: OR.B #48,&P6DIR+0
rather than: BIC.B #48,&P6DIR+0

Is there some way of encouraging the compiler to use the BIS/BIC
instructions whenever possible?

Tool Versions: Code Composer Studio 5.1.0.09000

MSP430 Compiler 4.0.0

over 14 years ago

0 Archaeologist over 14 years ago

TI__Guru* 84285 points

"OR" is just a pseudo-op for "BIS". If you look at the disassembly, you'll see that it doesn't matter whether you use OR or BIS, as they both end up as a BIS instruction.

0 Fred Carter over 14 years ago in reply to Archaeologist

Intellectual 515 points

Dear Archaeologist,

That just leaves AND and BIC. The compiler has an intrinsic (_bic_SR_register) for the Status Register
but I haven't found a general way for memory mapped address other than inline asm statements.

Regards,

Fred

0 Archaeologist over 14 years ago in reply to Fred Carter

TI__Guru* 84285 points

Fred Carter said:

That just leaves AND and BIC.

AND and BIC are distinct opcodes, because they aren't quite the same. AND seems perfectly fine to use; the peripheral user's guide is proscribing MOV and CLR, not absolutely requiring BIS and BIC. Do you see a problem with the use of AND?

Fred Carter said:

The compiler has an intrinsic (_bic_SR_register) for the Status Register but I haven't found a general way for memory mapped address other than inline asm statements.

As I understand it, this is generally done by including MSP-specific header files provided in CCS which define macros for these registers, so that you can then refer to them directly in the C code. See http://processors.wiki.ti.com/index.php/MSP430_LaunchPad_Low_Power_Mode and related pages.

0 JP Norair over 13 years ago in reply to Archaeologist

Intellectual 630 points

I've found that this style of C code will cause the compiler to use BIC rather than inverting and anding.

dst &= ~var;

0 Fred Carter over 13 years ago in reply to JP Norair

Intellectual 515 points

Hi JP,

Usually I am using constants (eg P1OUT &= ~BIT0) which usually generates a BIC instruction, while P1OUT &= ~0xA5 doesn't. The only difference I can see is that the compiler is distinguishing between clearing a single bit versus multiple bits. Thank you for the information when variables are used.

Regards,

Fred

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"Encouraging" the MSP430 PC v4.0.0 C/C++ compiler to use BIC/BIS instructions