Lpc2000

Sorry for typo error in my tutorial. Pleae replace WATCHDOG with
WDFEED.  E&OE comes in handy at times like this :)


--- In lpc2000@yahoogroups.com, "jayasooriah" <jayasooriah@...> wrote:
>
> This is an add-on to Danish Ali's contribution to AH's original
question.
> 
> I hope it will show how one can use GCC's inline assembly features in
> an effective way to produce code that is both readable *and* efficient.
> 
> I am not sure if I could have handcrafted the assembler code to be
> that efficient, considering the three primitives I created in the
> header file are not specific to the WATCHDOG or the LPC.
> 
> Sorry it is longish but striking a balance between being brief and
> clear is not always easy considering the nature of this audience.
> 
> Enjoy!
> 
> Jaya
> 
> PS:  All you need to know about inline assembly for purposes of
> understanding this tutorial can be found at:
> 
>   http://www.ethernut.de/en/documents/arm-inline-asm.html
> 
> I am happy to explain finer points if anyone wants to know more.
> 
> --- In lpc2000@yahoogroups.com, "Danish Ali" <danish@> wrote:
> 
> > Attached is an example of something that gcc likes (as
> > supplied with Rowley Crossworks).
> 
> > void Watchdog_Feed(void) { /* NOT fiq safe! */
> > 	int irqStat;
> > 	asm volatile (
> > 		" mrs r0, cpsr\n"
> > 		" mov %0,r0\n" : "=r" (irqStat) : : "r0" );
> > 	if (128 & ~irqStat)
> > 		__ARMLIB_disableIRQ();
> > 	asm volatile (
> > 		"WDFEED = 0xE0000008\n"
> > 		" LDR r0, =WDFEED\n"
> > 		" MOV r1,#0xAA\n"
> > 		" MOV r2,#0x55\n"
> > 		" STRB r1,[r0]\n"
> > 		" STRB r2,[r0]\n" : : : "r0", "r1", "r2");
> > 	if (128 & ~irqStat)
> > 		__ARMLIB_enableIRQ();
> > }
> 
> I wrote a header file, foo.h with three primitives to do what the
> above code does.  This is included at the end of this post E&OE.
> 
> I wrote foo.c which includes foo.h (and other header files), and in
> which I invoked the equivalent of the Watchdog_Feed() function as
follows:
> 
> >  feedSeq(&WATCHDOG);
> 
> I used GCC to generate the optimised assembler source file as follows:
> 
> > arm-esdk-gcc -Wall -Werror -O3 -S foo.c
> 
> The assembler output for the feedSeq() call I got is as follows
> (comments added)
> 
> > 	mov	r0, #128		@ 0x80 (IRQ BIT)
> > 	mov	lr, #-536870912		@ 0xe0000000 (&WATCHDOG)
> > 	mrs	ip, cpsr		@ save copy of CPSR
> > 	orr	r0, r0, ip		@ set IRQ bit in temp
> > 	msr	cpsr, r0		@ disable interrupts
> > 	mov	r0, #170		@ 0xAA (feed byte #1)
> > 	mov	r3, #85			@ 0x55 (feed byte #2)
> > 	str	r0, [lr, #0]		@ WATCHDOG FEED #1
> > 	str	r3, [lr, #0]		@ WATCHDOG FEED #2
> > 	msr	cpsr, ip		@ restore interrupts
> 
> 
> The header file where I make use of inline functions and inline
> assembly to achieve the above result follows.  Note that I do not
> refer to any register by name, but let the compiler tell me what
> register to use.
> 
> The name of the game here is not to override the compiler with your
> choices, but to tell the compiler (in the nicest way possible) what
> you want it to do for you ... to produce the code shown above.
> 
> > // ===========================================================
> > // 'irqMask'
> > 
> > // mask either irq or fiq bit
> > inline int
> > irqMask(mask)
> > {
> > 	int mode;
> > 
> > 	// set mask bit
> > 	asm volatile
> > 	(
> > 		"mrs\t%0, cpsr"		"\n\t"
> > 		"orr\t%1, %1, %0"	"\n\t"
> > 		"msr\tcpsr, %1"	
> > 		: "=&r" (mode)
> > 		: "r" (mask)
> > 	);
> > 
> > 	// previous mode
> > 	return (mode);
> > 
> > } // irqMask()
> > 
> > // End of 'irqMask'
> > // ===========================================================
> > 
> > 
> > // ===========================================================
> > // 'irqMode'
> > 
> > // restore mode and irq bits
> > inline void
> > irqMode(int mode)
> > {
> > 
> > 	// restore CPSR
> > 	asm volatile ("msr\tcpsr, %0" : : "r" (mode));
> > 
> > } // irqMode()
> > 
> > // End of 'irqMode'
> > // ===========================================================
> > 
> > 
> > // ===========================================================
> > // 'feedSeq'
> > 
> > // atomic feed sequence
> > inline void
> > feedSeq(int *loc)
> > {
> > 	int mode;
> > 
> > 	// enter critical section
> > 	mode = irqMask(IRQ_BIT);
> > 
> > 	// atomic feed sequence
> > 	asm volatile
> > 	(
> > 		"str\t%1, [%0, #0]"	"\n\t"
> > 		"str\t%2, [%0, #0]"
> > 		: 
> > 		: "r" (loc), "r" (0xaa), "r" (0x55)
> > 		: "r1", "r2"
> > 	);
> > 
> > 	// leave critical section
> > 	irqMode(mode);
> > 
> > } // feedSeq()
> > 
> > // End of 'feedSeq'
> > // ===========================================================
>