Saturday, 1 February 2014

Farewell to 64bit Arms - Assembly, Loops, 64 Arms Part 2

So we had finished ten loops. Now we moved on to 30 for both x86 and aarch64.
From there, we had to move on to a loop of 30 while also splitting loop iterations greater than 9 into two separate digits to be inserted into the byte, and printed, and (whoops, unbeknown-st to use at the time) then remove leading zeros (eg print out "9" instead of "09").

This required a few, but not difficult changes to both x86 and aarch64 code.
Divide the loop iteration by 10, convert the 1's column of the resulting quotient (by adding zero as before), and writing it to the low half of the specified register, and while the iteration is less than 10, put a '0' in the ten's column, and compare whether or not the register holding the 10's column is holding a zero with (in our case) hex 0. If it is equal, we used je (jump if equal) to not write, or skip writing to the 10's column. If the 10's column is not '0', convert it to ascii, move it to the write byte index in our string message register.

       cmp $0x00, %rax /* if the 10's column is 0 */
       je skip_10s /* don't write it */

      mov $len, %rdx /*print out the string with the new byte*/
      mov $msg, %rsi
      mov $1, %rdi
      mov $1, %rax

From there on the code is mostly the same, and by this point, making the 'skip_10's' its own little function was the most work aside from keeping track of safe registers remaining (we always seem to get caught up on the little stuff). The aarch64 was, as it was before, done second and was mostly about converting between syntax. Why, when Assembly is this old, can newer versions of it not use syntax from existing variations? I mean really.

As for which I prefer of the two. x86_64 I feel has more straight forward, relatively speaking, I mean opcodes are only 3 characters long, and often times (regardless of platform syntax) have several different meanings attached to them. However, the aarch64 syntax does make keeping track of registers so much easier, with each simply having a numeric value/name from rzr to 30, and subdividing them into groups that are relatively easy to remember (and simply change letter denominations for different memory length). With that said, msub is one of those 'love to hate' opcodes that will take time to warm to after this experience. It was, with the help of Starter Kit relatively easy to write the code, compared to some other languages (C, C++), but isn't quite at the code-in-English level of say, COBOL. With more practical practice, I can see myself quite enjoying writing/working with Assembler (of either variety for this classes purpose)

Here is a link to the first teration of the code (10 iteration loop x86), and here is aarch64 version.
The finalized versions are x86 here and aarch64 here

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