Is there a more efficient way to swap two bytes?

LOL The only homework this 50 year old has is sweeping. I cut my teeth on a 6502 (OSI Challenger 2P, VIC-20, etc.) but have recently gotten back into assembly language for fun. There doesn't seem to be a Rodnay Zaks out there for the Cortex-M. Any recommendations for a practical (non-text-book) Cortex-M assembly language/patterns book?

And thanks for the homework :-). I'll see if I can't get it down to 8 instructions. I appreciate the challenge.

LOL The only homework this 50 year old has is sweeping. I cut my teeth on a 6502 (OSI Challenger 2P, VIC-20, etc.) but have recently gotten back into assembly language for fun. There doesn't seem to be a Rodnay Zaks out there for the Cortex-M. Any recommendations for a practical (non-text-book) Cortex-M assembly language/patterns book?

And thanks for the homework :-). I'll see if I can't get it down to 8 instructions. I appreciate the challenge.

Still on 6502 myself (https://www.pouet.net/prod.php?which=85609) :-)
No "tips and tricks" book AFAIK for Cortex-M, but Cortex-M0 is really simple as there are only 50 or so instructions.

Important advantage over 6502: Some instruction take three arguments.

Checkout "REV" ;-)

Anyway, I can always recommend this book:
https://www.sciencedirect.com/book/9780124080829/the-definitive-guide-to-arm-cortex-m3-and-cortex-m4-processors

I think there is also one for Cortex-M0.

Also, checkout this forum for "multiply" for some tricks.

Edit:
www.sciencedirect.com/.../the-definitive-guide-to-the-arm-cortex-m0

2ndEdit:
community.arm.com/.../34063

42Bastian Schick said:

I can always recommend this book ...

Edit:
www.sciencedirect.com/.../the-definitive-guide-to-the-arm-cortex-m0

Absolutely!

See also: http://www.keil.com/books/

specifically: http://www.keil.com/books/armbooks.asp

Thank you! The reason I decided to go with the Cortex-M0 is because of that simplicity of learning. I appreciate the links to the books, will check them out, and thanks for the pointer to REV. Glad you didn't provide an answer... it's much more fun to figure things out.

And so cool you are making demos! Amazing what you can do in 248 bytes.

Thank you for that link! Will check out the books there.

Sensei, I was able to get it to 6 instructions using REV :-) Thank you!

LDR R0, =0xAABBCCDD
LDR R1, =0x00FFFF00
REV R2, R0  // R2 0xDDCCBBAA
ANDS R2, R1 // R2 0x00CCBB00
BICS R0, R1 // R0 0xAA0000DD
ORRS R0, R2 // R0 0xAACCBBDD

Neat. Don't think it can be shorter.

Here is my version, which avoids the literal loading:

.text
	.thumb
	.thumb_func

	ldr	r0,=0xaabbccdd
	bl	swap

swap:
	lsls	r1,r0,#8	// r1 = bbccdd00
	lsrs	r1,r1,#16	// r1 = 00bbccdd
	lsls	r1,r1,#8	// r1 = 00bbcc00
	eors	r0,r1		// r0 = aa0000dd
	rev16	r1,r1		// r1 = bb0000cc
	movs	r2,#16
	rors	r1,r2		// r1 = 00ccbb00
	orrs	r0,r1		// r0 = aaccbbdd
	bx	lr