如何在不使用 C 的情况下使用 aarch64 打印数字

Question

As the question states, I want to print numbers, that are multiple digits, in aarch64.

Usually when solving proplems I python them first to see if they work, especially since I am very new to anything assembly. My pythonic solution was

number = 1234

while number != 0:
   digit = 1234 % 10         # always gives me the last digit
   print(digit, end='')      # to print the digit and not the next line
   number = number - digit   # to remove the digit I just got so the last digit is 0
   number = number / 10      # to remove said last digit and shorten number by 1

The way I tried to impement this in AARCH64:

printNumber:
        mov  x16, #10                  /* apparently need this for udiv/msub */
        udiv x14, x12, x16             /* x12 is the number I defined above, bc idk what registers are save to use (e.g. when syscall 64, print, happens, 0-8 are used) */
        msub x13, x17, x16, x12        /* thanks to: https://stackoverflow.com/questions/35351470/obtaining-remainder-using-single-aarch64-instruction */
        sub  x12, x12, x13             /* x13 is what above is digit, x12 is number */
        udiv x12, x12, x16
        add  x13, x13, #48             /* digit to string, possible error source 1 */

        mov  x0,  #1                   /* the print part */
        mov  x1,  x13                  /* the other part where I suspect a mistake */
        mov  x2,  #1
        mov  w8,  #64
        svc  #0

        cmp  x12, #0                   /* the loop part */
        beq  exit                      /* genereric exit method I left out */
        b    printNumber

State of the code: It compiles and runs without problems, though it prints nothing, I cannot debug since I Program this with an actual aarch64 device and am not using an emulator (though I am sure there is a way I don't know off)

I hope it is clear what I am trying to do, I am aware of some issues, like I should be using the stack for some of those things, but I can get that to work I hope. I also hope it is visible that I put some effort into this and have tried to look for a solution like this, but can not find anyone who does it like this, or any other non-c-lib way that prints numbers (note that I don't need the number as str, I really only want to print it)

Answer 1

As Jester points out, the reason you can't print is that the write system call expects a pointer to the data to be written in x1 , not the data itself. You need to store your character to some appropriate address in memory ( strb ) and pass that address in x1 .

One approach would be to use the stack; just subtract from the stack pointer (in multiples of 16, for alignment) to allocate some memory for yourself, and remember to put it back when you're done. Here's an example that should work; XXX are the lines I added or changed.

printNumberEntry:                      /* XXX */
        sub  sp, sp, #16               /* XXX allocate 16 bytes of stack space */
printNumber:
        mov  x16, #10                  /* apparently need this for udiv/msub */
        udiv x14, x12, x16             /* x12 is the number I defined above, bc idk what registers are save to use (e.g. when syscall 64, print, happens, 0-8 are used) */
        msub x13, x14, x16, x12        /* XXX fix unrelated bug */
        sub  x12, x12, x13             /* x13 is what above is digit, x12 is number */
        udiv x12, x12, x16
        add  x13, x13, #48             /* digit to string, possible error source 1 */

        strb w13, [sp]                 /* XXX Store the low byte of x13/w13 in memory at address sp */

        mov  x0,  #1                   /* the print part */
        mov  x1,  sp                   /* XXX x1 points to the byte to be written */
        mov  x2,  #1
        mov  w8,  #64
        svc  #0

        cmp  x12, #0                   /* the loop part */
        beq  exit                      /* genereric exit method I left out */
        b    printNumber

exit:                                  /* XXX */
        add  sp, sp, #16               /* XXX restore stack before returning */
        ret                            /* XXX */

I also fixed an unrelated bug: in your msub , x17 should be x14 , since that's where the quotient is.

Another approach would be to reserve a byte in static memory:

        .bss
dataToWrite:
        .resb 1
        .text
printNumberEntry:                      /* XXX */
        adr  x1, dataToWrite           /* XXX keep address in x1 throughout */
printNumber:
        mov  x16, #10                  /* apparently need this for udiv/msub */
        udiv x14, x12, x16             /* x12 is the number I defined above, bc idk what registers are save to use (e.g. when syscall 64, print, happens, 0-8 are used) */
        msub x13, x14, x16, x12        /* XXX fix unrelated bug */
        sub  x12, x12, x13             /* x13 is what above is digit, x12 is number */
        udiv x12, x12, x16
        add  x13, x13, #48             /* digit to string, possible error source 1 */

        strb w13, [x1]                 /* XXX Store the low byte of x13/w13 at dataToWrite */

        mov  x0,  #1                   /* the print part */
        /* x1 already contains the proper address */
        mov  x2,  #1
        mov  w8,  #64
        svc  #0

        cmp  x12, #0                   /* the loop part */
        beq  exit                      /* genereric exit method I left out */
        b    printNumber

exit:                                  /* XXX */
        ret                            /* XXX */
        /* your code */

The downside is this will make your function unusable for signal handlers, multiple threads, etc, as they will all try to use the same byte.

Other notes:

• The digits print in reverse order, of course. That's something I guess you will work on later.

• The sub and second udiv are unnecessary, because the first udiv already produces the rounded-down quotient (like Python's x14 = x12 // 10 ). So you could replace those two instructions with just mov x12, x14 .

• Several of your registers hold constant values throughout the whole function ( x16, x1, x2, x8 ), so you could initialize them outside the loop instead of redundantly redoing it on every iteration. (Note x0 is overwritten with the system call's return value, so you do need to reinitialize it each time.)

• You really want to find a way to be able to use a debugger. Otherwise, get ready for a lot more instances where you spend half an hour writing a StackOverflow question and waiting many more hours for an answer, for a bug that you could have found yourself in three minutes with a debugger. If you can't install gdb on your Android device, then consider setting up another Linux AArch64 box (say a Raspberry Pi 4, or a cloud server, or even a qemu emulator) where you can.