assemblyavr
How do I perform a multiplication by 2 of the unsigned 32-bit quantity in registers r18:r19:r20:r21 where r18 is the most significant byte?
My answer is this: rol r18; rol r19; rol r20; rol r21.
My thought process is that since this is an unsigned number, we can exclude the use of ASL as there is no carry out in MSB. But I didn't get it right; may I know which part is wrong?
You can use AVRA (AVR Assembler) on Linux to develop your assembly language program on Linux. I'm on Linux and free both as in beer & freedom of use.
Plan to use AVRDUDE (also free - both above contexts) to burn in your object code. I have used it in Linux.
Plan to use another (cheap - free as in opensource) Arduino like as a programmer. I have used one here and documented. In my article, I download a bootloader to a new ATmegaxx, instead you can burn in your own program. I did it all through USB (as you asked).
You definitely can use an USB based Arduino or RBBB+USB cable to program your target Arduino using USB. RBBB is indeed a utility Arduino that I have used in that role.
Overall, to use my solution, you would have to use another programmer or another cheap Arduino like RBBB. I also have reviewed RBBB in my other post.
Another way to program your Arduino with another Arduino.
Programming your Arduino using a third party USB programmer.
You have the op code for ANDI wrong, it's 0111 KKKK dddd KKKK. Using the assembler, I get 0x7120 as the machine code for ANDI r18, $10, which is 0111 0001 0010 0000.
0010 for dddd is 2, which represents r18, because it's the third register from r16 to r31. Only the upper 16 registers are valid for the ADDI instruction. 0000 is r16, 0001 is r17, and 0010 is r18.
Best Answer
To do this correctly for multiplying a 4-byte value by two you have to ensure that the low bit of the low byte gets a '0' shifted into it. In the AVR instruction set this can be achieved with the following sequence of instructions assuming a value in r18:r19:r20:r21.
Note the need to shift first the low byte and then to propagate bits through carry for the rest of the shifts. The first CLC instruction takes care of assuring that the LSB of R21 ends up being '0'.